How Things Work Final Study Set
Water is flowing at constant velocity through a straight, horizontal pipe. The pressure at the entry end of the pipe is greater than the pressure at the exit end of the pipe. What would happen if those two pressures suddenly became equal?
(A) The water would stop flowing because of viscous interactions between the water and the pipe.
The body of your car is supported by springs. When you put a 100-pound box of books into the trunk, the body's average height above the ground decreases by 1 inch. If you add a second 100-pound box of books to the trunk, the body's average height above the ground will decrease by an additional
1 inch
Firefighters are battling a fire on the 12th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the steady stream of water up to the 10th floor. So they carry the end of the same fire hose up a 2-story-tall ladder and shoot water upward. The other end of the hose remains attached to the same water source as before. Now the steady stream of water rises to the
10th floor
A bottle in both thermal and phase equilibrium at 0 °C contains 1 kilogram of ice and 1 kilogram of liquid water. You move the bottle to a new location and allow it to reach equilibrium at -0.5 °C. The bottle now contains
2 kilograms of ice
Firefighters are battling a fire on the 10th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the water up to the 8th floor. So they carry the end of the same fire hose up a 2-story-tall ladder and shoot water upward. Now the water rises to the
8th floor.
Suppose a soccer ball is approaching your leg at 30 km/h and your leg is moving toward the soccer ball at 30 km/h. What is the fastest the soccer ball can possibly travel after it hits your leg? [You can assume perfect (ideal) bouncing behaviors for the ball and your leg.]
90 km/h
You are rearranging the game room in your mansion and are now pushing the pool table forward across the floor. The floor is exerting a backward frictional force of 200 newtons on the table as the table moves directly forward at a constant speed. What force are you exerting on the table?
A forward force of 200 newtons
A horizontal pipe bends toward the right. As water flows steadily through this pipe, what happens as the water starts to travel around the bend
A horizontal pipe bends toward the right. As water flows steadily through this pipe, what happens as the water starts to travel around the bend
The exposed air ducts in warehouses and gymnasiums are often several feet in diameter. If a wide duct and a narrow duct are delivering the same amount of air each second, the fan pushing air through the wide duct consumes less power than the fan pushing air through the narrow duct. Why?
Air in the wide duct moves slower, requiring less kinetic energy per liter, and it experiences weaker viscous interactions with the duct.
It is a beautiful summer day and the residents of a high-rise apartment building are eating dinner on their balconies. A resident accidentally knocks an empty glass off a balcony that is about 80 meters (260 feet) above the cement patio. The glass would have smashed on that patio after falling for 4 seconds, however, a quick-witted resident catches the glass after it has fallen for only 2 seconds. How far above the patio was the glass when it was caught?
Approximately 60 meters above the patio
How is a steam engine able to obtain work (ordered energy) from hot steam?
As heat flows from the hot steam to the colder environment, the steam engine is able to divert a fraction of that heat and convert it into work
You are at the gym, exercising on a step machine. You have one foot on each of the machine's pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push upward harder on your feet while moving downward than they do while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?
As that pedal moves downward
You inflate a rubber balloon with oxygen and then release it. The balloon flies around the room until it is empty and deflated. What pushed the balloon forward?
As the balloon pushed oxygen backward out of the balloon's opening that oxygen pushed the balloon forward
You are inflating a limp plastic balloon with helium. The pressure inside the balloon remains atmospheric pressure, even as more helium accumulates inside the balloon. How do the balloon's total weight and the buoyant force on the balloon change during this filling process?
Both forces increase, but the buoyant force increases more rapidly than the balloon's weight.
You are trying to set the world's record for drinking water through the tallest drinking straw. What could you do to have the best chance of setting this record?
Choose a good-weather day when the atmospheric pressure is as large as possible
An automobile engine "knocks" when the gaseous fuel-air mixture in its cylinder ignites spontaneously rather than waiting for the sparkplug to ignite it. What causes that premature ignition of the fuel-air mixture?
Compression of the fuel-air mixture causes its temperature to surge upward so that it ignites
It's a stormy day, and you're struggling to hold your umbrella steady. The wind is blowing toward the east and your motionless umbrella is pulling your hands upward and eastward. In which direction is the umbrella pushing on the air that passes it?
Downward and westward.
Why does a sprinter lean forward as she picks up speed at the start of a race?
If she stood upright as the ground pushed her feet forward, the inertia of her upper body would cause her to tip over backward
After heavy rains, the water flowing down a nearby river is turbulent, also known as "white water." During a drought, however, the water flowing down that same river is laminar; it appears smooth and clear. Explain the difference.
In the fast-moving water after the rains, inertia dominates the flow. In the slow-moving water during the drought, viscosity dominates the flow.
You string a zip-line (a taut metal cable) from the balcony of your mansion to the stables 500 meters away. A small trolley allows you to glide down that cable to visit your horses. On a windy day, you notice that the cable is vibrating slightly in its fundamental vibrational mode with a period (time to complete 1 full cycle of motion) of 1 second. What could you do to the cable to decrease (shorten) that period?
Increase the cable's tension, decrease the cable's mass, or shorten the cable
You roll a baseball off the top of a tall building and it hits the ground 4 seconds later. When the baseball has fallen for only 2 seconds, how far is it above the ground? [Note: neglect any effects due to the air.]
It is 3/4 of the building's height above the ground
Space-bound rockets are launched from ground-based launch pads and their torch-like exhaust plumes scorch the ground during the launches. If there were no ground or other horizontal surface beneath a rocket at launch, how would that change affect the upward thrust force on the rocket?
It would have almost no effect on the upward thrust force.
The oil and vinegar in a salad dressing tend to separate, with the oil floating on the vinegar. Suppose an herb in the dressing sits motionless at the interface where the oil and vinegar meet. Describe the average density of that herb
Its average density is less than the density of vinegar and greater than the density of oil
A juggler tosses a club straight up. Disregarding any effects due to the air, what force or forces are acting on the club while it is above the juggler's hands?
Its weight
You toss a basketball straight up. Disregarding any effects of due to the air, what force or forces are acting on the basketball while it is above your hands?
Its weight
You and your friend carry identical water bottles from the first floor to the fifth floor. You start side-by-side and you finish side-by-side, but one of you uses the stairs while the other climbs straight up a ladder. Which of you did the most work on your water bottle?
Neither. You both do the same work on the water bottle
A narrow boat, such as a canoe, kayak, or racing shell, is like a long cylinder that can rotate about a horizontal axis (see figure). When the boat's occupants are seated, its overall center of gravity is below the rotation axis and the boat is upright and stable. When someone stand up, however, the boat's center of gravity shifts above the rotation axis and the boat tips over. Why does this center gravity shift cause the boat to become unstable?
Once the center of gravity is above the rotation axis, tipping causes the boat's total potential energy to decrease
A birdfeeder hangs motionless from a tree branch on a long string. After a bird disturbs it, the birdfeeder swings back and forth below the branch before settling back to its original stable equilibrium position. Why is the birdfeeder's equilibrium position stable?
Since the birdfeeder's total potential energy always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium
You are watching an airshow at the local airport and an acrobatic airplane is flying upside down to impress the crowd. That airplane is traveling forward at a constant horizontal velocity, just 10 feet above the runway! In which direction is the overall force that the airplane exerts on the air
The airplane is pushing the air directly downward
An airplane is cruising at constant velocity, 30,000 feet in the air. Which of the following correctly describes the situation above the airplane's wing?
The airstream over the wing is bending downward, toward the wing, so the pressure just above the wing must be less than the local atmospheric pressure
You are moving into a loft apartment and are now dragging an old carpet across the floor in a straight line at a steady speed. Which of the following statements about the forces acting on the carpet is correct?
The amount of force that you're exerting on the carpet must be equal to amount of force that friction is exerting on it
As an orchestra warms up, the wind instruments (e.g., flutes, clarinets, oboes) and the air inside them actually become hotter. These instruments have openings and are not sealed. How does this warming process affect the average pressure and density of the air inside the wind instruments?
The average pressure does not change, but the average density decreases
You drop a bag of flour onto a trampoline and the bag rebounds upward almost to its original height. Compare the bag's energy and momentum just before it bounced on the trampoline with its energy and momentum just after bounced off the trampoline.
The bag's energy changed only a little, but its momentum changed significantly
A scuba tank is a metal bottle that can be filled with air and used to supply that air to a person who is underwater. Suppose you fill a scuba tank with air until the particle density in the tank is 100 times the particle density of the surrounding air. At room temperature, what are the density and pressure of the air inside that tank?
The density is about 100 times atmospheric density and the pressure is about 100 times atmospheric pressure
you heat your metal water bottle on a stovetop until it is filled with hot air at atmospheric pressure. You then seal the bottle and set it on the kitchen counter to cool. Assume the bottle doesn't change size (volume) throughout this process. During the cooling process, what happens to the density and pressure of the air in the bottle?
The density remains constant, but the pressure decreases
The air conditioner in your home is cooling the indoor air on a hot summer. As that electric air conditioner operates, what happens to the energy of the indoor air, the outdoor air, and the electric company?
The energies of the indoor air and the electric company decrease, and the energy of the outdoor air increases.
You are rearranging your room and are now sliding your desk across the floor at constant velocity. Which of the following statements about the forces acting on the desk is correct? [Consider only horizontal forces.]
The force that you're exerting on the desk must be equal in amount but opposite in direction to the force that the floor is exerting on it
You add a teaspoon of salt to a glass of pure water. The salt initially sits at the bottom of the water, but it slowly dissolves so that eventually you have a glass of salt water. Which has more entropy: the glass of pure water with salt sitting at its bottom or the glass of salt water?
The glass of salt water has more entropy
Suppose a musician is playing a wind instrument (e.g., a flute, clarinet, or oboe) and the density of the air suddenly decreases. There is no change in the air's pressure or in how the musician is playing the instrument. What happens to the instrument's sound?
The instrument's pitch (frequency) increases
A log is floating motionless on a calm lake, with about half the log located beneath the water level. It's a sunny day and the temperature of the air increases, although the air pressure remains constant. How does the air's increasing temperature affect the log?
The log descends slightly, so that more of it is located beneath the water level.
A mother lifts her toddler upward while that toddler clutches a toy rabbit. Who is doing work on whom?
The mother is doing work on the toddler and the toddler is doing work on the rabbit
You are pulling a wagon full of children up a hill at constant velocity. Which one of the following statements is true?
The net force on the wagon is zero and you are doing work on the wagon
The three nonflammable objects in your hand are identical except that one is black, one is white, and one is shiny. You place them in a roaring fire and soon they have the same temperature: 1800 °C. They remain solid. Which one is glowing most brightly?
The object that original appeared black
Doctors use a centrifuge to separate red blood cells from blood plasma. The centrifuge swings a container of blood at high speed in a circular path. The red bloods drift away from the center of the circle, leaving the blood plasma nearer the center. Describe the pressures and densities in the blood as the centrifuge swings it in a circle.
The pressure is highest farthest from the center of the circle and the red blood cells are denser than the blood plasma.
Suppose a rocket is launched vertically, so that its velocity always points vertically upward. By the time the rocket's fuel runs out, its altitude is 200 miles above the Earth's surface and its speed is approximately zero. What happens to the rocket after its fuel runs out?
The rocket falls, accelerating vertically downward because of its weight
Two identical rooms at identical temperatures contain identical collections of atoms. In one room that collection consists of a block of salt and a basin of fresh water. In the other room that collection consists of a basin of salt water. Which of the two rooms has the greater entropy (disorder)?
The room containing the salt water has the greater entropy
You drop an unopened metal can of soup and it lands bottom-first on the floor. When you inspect the can, you see that it has dented outward in places. Where did it dent outward and why?
The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom
You kick a soccer ball through the air. From the ball's perspective, air is flowing northward toward the ball. Describe the air pressures on the ball's south side and east side (see figure).
The south side pressure is greater than atmospheric pressure. The east side pressure is less than atmospheric pressure.
A pitcher throws a spinning baseball toward home plate and the baseball curves toward the pitcher's right. Why did the baseball curve?
The spinning baseball deflected the passing air toward the pitcher's left.
Water is flowing at constant velocity through a straight horizontal pipe. It is experiencing two forces: a forward force due to a difference in pressure at the two ends of the pipe and a backward force due to the water's viscosity and friction with the stationary pipe. How do the amounts of those two forces compare?
The two forces are equal in amount
Firefighters are battling a fire on the 10th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the water steadily up to the 8th floor. So they carry the nozzle end of the same fire hose to the top of a 4-story-tall ladder and again point the nozzle upward. How does the speed of the water as it emerges from the nozzle near the top of the ladder compare to its speed when it emerged from the nozzle near the ground?
The water emerging from the nozzle near the top of the ladder travels more slowly
You are kayaking in Alaska. You reach forward with your paddle, lower the paddle into the water, and then pull the paddle backward. You accelerate forward. What force acts on the paddle to cause you to accelerate forward?
The water exerts a drag force on the paddle, pushing the paddle in the forward direction.
You live in a two-story house with identical bathrooms on the ground floor and second floor. You turn on the showers in both bathrooms. How do the water pressures before each shower nozzle and the water speeds after each nozzle compare?
The water pressure and water speed are both larger in the ground floor shower
A pressure washer is used to clean stone surfaces. Its pump delivers high-pressure water to a hose and that water is sprayed through a nozzle so this travels through the air and hits the stone surface. Which of the following correctly describes the water's pressure?
The water's pressure is high before the nozzle, it is atmospheric pressure as it travels through the air, and it is high when it is slowed down by the stone surface
At takeoff, an airplane tilts its wings so that the leading edge (front portion of the wing) is higher than the trailing edge (back portion of the wing) and thereby obtains enough upward lift force to rise into the sky. The more aggressively it tilts its wings (the greater its angle of attack), the more lift it obtains and the faster it rises. There is a limit, however, to the rate at which the airplane can rise. If its angle of attack is too great, what happens?
The wings become blunt objects and they stall
You are riding a bus that is moving straight ahead at high speed. The bus comes to a curve in the road and begins to turn toward the left. You find yourself pressed against the right side of your seat. What force, if any, is pushing you toward the right?
There is no force pushing you toward the right.
When a hammer exerts a force on a nail, how does the amount of that force compare to that of the nail on the hammer?
They are both exerting the same amount of force on one another
A music shop has numerous stringed instruments on display, including violins, guitars, and harps. You pluck the string of one instrument and it emits a tone with a frequency of 440 cycles per second (a pitch known as Concert A). When you stop that string from vibrating, you notice that other strings in the room are now vibrating at 440 cycles per second. Why did this happen?
Those other strings were also tuned to vibrate at 440 cycles per second and energy was passed gradually from the string you plucked to those other strings via resonant energy transfer.
You are carrying a child on your back as you walk down a hill. The child is traveling straight at a steady speed. In which direction is the force you are exerting on the child?
Upward (vertical)
When a ray of sunlight passes through a darkened room, you see dust particles seemingly suspended in the air. What upward forces are keeping those dust particles from falling to the ground?
Viscous drag forces—the drag forces associated with air's viscosity.
You wake up one very cold winter morning to find the grass outside covered with frost. How did this ice form on the grass?
Water molecules from the moist air landed on the cold grass more frequently than they left the grass, and there was a net accumulation of water molecules as ice
As part of your new circus routine, you jump off a high platform onto a trampoline. You bounce up and down spectacularly before eventually settling at equilibrium on the trampoline. When during your descent from the high platform do you first stop accelerating downward?
When you pass through equilibrium on the trampoline for the first time
A large hot air balloon drifts horizontally with the wind. What causes the balloon's velocity to change when the wind's velocity changes? [For simplicity, neglect any vertical motions of the balloon or wind.]
Whenever the two velocities are different, the wind exerts a pressure drag force on the balloon and the balloon accelerates until its velocity is that of the wind
To avoid tipping over while turning right on a bicycle, why must you lean the bicycle toward your right?
You accelerate rightward as you turn right and your inertia tends to make you tip over toward your left. By leaning toward your right, you cancel that tendency and avoid tipping over
You work at a camp and play the bugle, a simple musical horn with a single brass pipe. Despite having no valves to change the length of that pipe, you can produce several different notes (or pitches) from the bugle to wake everyone up at dawn. What are you doing to change the notes (or pitches) of the bugle?
You are causing the column of air inside the pipe to vibrate in its different vibrational modes, each of which has a different pitch
You are at a tropical resort and your friends have talked you into diving off the top of a cliff into the ocean below. It's a 4-second fall to the ocean. When you have been falling for exactly 2 seconds, you change your mind and instantly grab onto the tree projecting outward from the cliff. Miraculously, you stop safely and hold onto the tree. Your decision to climb up or down now depends on your height on the cliff. Where are you and the tree located on that cliff?
You are much closer to the top of the cliff than to the ocean below
You are on a 3rd floor hotel balcony, 10 meters above the bar. You lower a long straw into a pitcher of diet soda (essentially pure water) and find that you can just barely suck the soda up to your mouth. You move the straw over to pitcher filled with full-calorie soda (essentially sugar water, with a density that is 10% more than pure water). Now what happens when you begin sucking?
You are unable to suck the full-calorie soda up to your mouth
You are on a 10th floor hotel balcony, overlooking the pool. You lower a 100-foot-long straw into the pitcher of lemonade at the bar and try to suck the lemonade up to your mouth. What happens?
You are unable to suck the lemonade up to your mouth
You and your friend are riding a carousel. Your horses are side-by-side and your friend is closer to the center of the carousel than you are. As the carousel turns steadily, which of the following is true?
You experience a larger inward acceleration than your friend does
When you drop a beanbag on cement, the bag doesn't bounce. Suppose you are playing tennis with a beanbag instead of a tennis ball. How should you adjust your tennis racket to make the beanbag travel as fast as possible after you hit it with the racket?
You should loosen the strings so that the racket surface is like a soft trampoline
You and several friends take turns bungee jumping from Beta Bridge, using the same spring-like bungee cord. Near the end of your turn, you bounce gently up and down on the cord with a period (the time it takes for each bounce) of 1 second. One friend has your same height, but is heavier than you. Another friend has your same weight, but is shorter than you. During your friends' turns, what are the periods of their bounces?
Your heavier friend has a period longer than 1 second, your shorter friend has a period of 1 second
You and several friends take turns bungee jumping from Beta Bridge, using the same spring-like bungee cord. Near the end of your turn, you bounce gently up and down on the cord with a period (the time it takes for each bounce) of 1 second. One friend has your same height, but is heavier than you. Another friend has your same weight, but is shorter than you. During your friends' turns, what are the periods of their bounces?
Your heavier friend has a period longer than 1 second; your shorter friend has a period of 1 second
you are walking a dog on a leash when a squirrel runs by. Suddenly, you are all accelerating forward. If the dog is exerting a 50 newton forward force on the leash, the force that the leash is exerting on the dog is
a 50 newton backward force.
A cross-country skier is struggling to get up a hill, so you offer to help. As the skier passes you, you reach out with your hand and exert an uphill force of 80 N (18 pounds) on the skier. When you do this, the skier exerts
a downhill force of 80 N on you
Two children are balanced on a seesaw, but one child weighs twice as much as the other child. The heavier child is sitting half as far from the pivot as is the lighter child. Since the seesaw is balanced, the heavier child is exerting on the seesaw
a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting
Wrapping a hot potato in aluminum foil helps keep the potato warm because the shiny aluminum foil is
a very poor emitter of thermal radiation
You drive too fast over a small hill and your car's wheels lose contact with the road. While your car is not touching the road, you are
accelerating downward and you feel lighter than you do while at rest
You are riding on a playground swing. As you swing forward, there is a moment when you are directly below the pivot that supports the swing. At that moment, you are
accelerating upward
You are riding on a playground swing. As you swing forward, there is a moment when you are directly below the pivot that supports the swing. At that moment, your speed is momentarily constant and you are
accelerating upward
To transform 1 kilogram of boiling-hot water at 100 °C into 1 kilogram of steam at 100 °C, you must
add a large amount of heat to the water
A hot air balloon is passing over your head when the pilot turns on the flame and heats up the air inside the balloon's envelope (the balloon's fabric skin). That envelope is open at the bottom, where the flame heats it. As the temperature of the air inside the unsealed envelope increases, the
air molecules flow out of the envelope's opening and the weight of the air inside the envelope decreases
After preventing a score, the goalie kicks the soccer ball toward midfield. The soccer ball gradually slows down, even though it is travelling almost horizontally. The primary reason for this slowing is that the
air pressure at the front of the ball is greater than atmospheric pressure, while the air pressure behind the ball is approximately atmospheric
A skateboarder rides swiftly up the edge of a bowl-shaped surface and leaps into the air. While in the air, the skateboarder flips upside and tosses the skateboard from hand to hand. The skateboarder then rides safely back down the bowl. During the time that the skateboarder and skateboard are not touching anything, one aspect of their motion that is constant is their total (or combined) [note: neglect any effects due to the air]
angular momentum
You're filming a movie and you arrange for two unoccupied cars to drive horizontally off a tall cliff. The Mercedes sedan weighs twice as much as the Mini Cooper, but the Mini Cooper is traveling at twice the velocity of the Mercedes when the two cars careen off the cliff side-by-side. In this situation, the two cars hit the level ground below the cliff
at approximately the same time, but the Mini Cooper hits considerably farther from the cliff than the Mercedes sedan
After a volleyball is served and is heading through the air and over the net, it gradually slows down. The primary reason for this slowing is that the air pressure
at the front of the ball is greater than atmospheric pressure, while the pressure behind the ball is approximately atmospheric
Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because it
barely moves downward as you push downward on it, so you do almost zero work on it
A dog and a cat jump horizontally off a wall at the same moment and soon land on the level horizontal field that extends outward from the base of the wall. The dog weighs twice as much as the cat, but the cat was moving forward horizontally twice as fast as the dog when the two animals left the wall. In this situation,
both animals land at approximately the same time, but the cat lands considerably farther from the wall than the dog does
Two bars of soap slide off of a horizontal counter at the same time, side by side. The green bar of soap weighs twice as much as the orange bar of soap, but the orange bar has twice the velocity of the green bar when the two first lose contact with the counter. In this situation
both bars hit the floor at approximately the same time, but the orange bar lands considerably farther from the counter than the green bar doe
A curve ball's path bends to the right as it flies toward home plate because it's experiencing an aerodynamic force to its right. One reason why the ball is experiencing this aerodynamic lift force is that the ball is spinning and that spin
causes the air to flow faster and farther around the right side of the ball than around the left side of the ball
If you try to cook vegetables with 100 °C air, it takes a long time. But if you cook those same vegetables with 100 °C steam, they cook quickly. This is because the steam
condenses on the colder vegetables and releases a large amount of heat to the vegetables.
On a hot day, you drink the last drop of water in your plastic water bottle and then seal it closed. As the bottle cools off in your air conditioned car, the bottle's sides bend slightly inward so that the bottle now has slightly less volume than when you sealed it. Since you seal it, the air pressure inside the bottle has
decreased and the density inside the bottle has increased
When water rushing through a fire hose flows around a bend in that hose, its pressure changes. The water pressure
decreases near the inside of the bend and increases near the outside of the bend
When water flows around a bend in a garden hose, the water pressure
decreases near the inside of the curve and increases near the outside of the curve
As heat flows from hot soup to a cold kitchen, the entropy (disorder) of the soup
decreases, the entropy of the kitchen increases, and their combined entropy increases
As heat flows from hot soup to a cold kitchen, the entropy (disorder) of the soup
decreases, the entropy of the kitchen increases, and their combined entropy increases.
The hotter it is outside, the more electric energy an air conditioner must use to remove each joule of heat from the 72 °F indoor air in your home. It must consume more electric energy on a hotter day because
delivering that joule of heat to the outdoor air creates less entropy as the outdoor air gets hotter
The plumbing in a new high-rise hotel was carefully designed so that the water pressure is exactly the same on all floors and the bathroom showers spray exactly the same on all floors. Neglecting any effects due to viscosity or friction, the total ordered energy (the sum of gravitational potential energy, pressure potential energy, and kinetic energy) per liter of water is
different on different floors, but the sum of pressure potential energy and kinetic energy per liter is the same on different floors
A racecar is driving around a circular track at a steady speed of 100 mph (160 km/h). The net force on the racecar is
directed toward the center of the track
The curator (artistic director) of a museum's gallery has laid a wool rug on a stone block and placed a copper vase on the rug. That gallery has a uniform temperature throughout, including all of the objects it contains. As a result of contact between those three objects, heat
does not flow at all
You are riding a roller coaster and your speed is increasing as you plunge downhill on the first hill. Your acceleration is directed
downhill, your feeling of acceleration is directed uphill, and your apparent weight is smaller in amount than your actual weight
The roller coaster you are riding is going through a loop-the-loop at high-speed. At the moment when you are at the top of the loop, you and your car are inverted yet you feel an apparent weight directed toward the sky. At this moment, your acceleration is
downward and greater than the acceleration due to gravity
A spacecraft is moving eastward in a circular orbit 200 miles above the Earth's equator. It is time for the spacecraft to return to the ground. The most effective way to cause the spacecraft to begin descending toward the Earth's surface is to fire its rocket engine. In which direction should the engine direct its rocket exhaust
eastward
A gust of wind shakes your home and causes a hanging chandelier to begin swinging gently back and forth on its support chain. When its overall motion is small, the time it takes the chandelier to complete one full oscillation of that motion (its period of oscillation) does not depend on
either the chandelier's weight or the chandelier's amplitude of motion
To win a prize at the fair, you must toss a basketball into a stiff fruit basket and have the ball remain in that basket. The basket is tipped toward you and nailed to the wall. Its lower lip has just enough uphill slope to keep a stationary ball from rolling out. Each time you throw the ball into the basket, the ball bounces and rolls onto the floor. The conserved quantity that the basketball is unable to get rid of in order to stay in the basket is
energy
An astronaut is orbiting the Earth inside the International Space Station and she is hovering in the middle of the cabin. The net force on that astronaut is
equal to her weight and she is accelerating downward (toward the center of the Earth).
A satellite is orbiting the Earth at an altitude of 100 miles. The net force on that satellite is
equal to its weight and it is accelerating toward the center of the Earth
You are competing in an "egg toss" event in which pairs of people toss raw eggs back and forth at gradually increasing distances. The winning pair is the one that tosses the egg the greatest distance without breaking it. To catch your egg without breaking it, you must allow that egg to push your hands a long distance in the direction of its velocity, a pushing process that takes a long period of time. Allowing your hands to move with the egg in this manner is crucial because it allows your hands to
extract both the egg's kinetic energy and the egg's forward momentum while exerting only small forces on the egg
To catch a football successfully, you should allow the ball to push your hands in the direction of its travel. Allowing your hands to move with the football is crucial because it allows you to
extract energy from the football
To put a satellite in orbit around the earth, a rocket has two important tasks. First, the rocket must lift the satellite above the Earth's atmosphere. Second, the rocket must
give the satellite such a large horizontal speed that the falling satellite arcs endlessly around the Earth rather than descending to the ground.
A rocket designed to put a satellite in orbit around the earth has two main tasks. First, it must lift the satellite above earth's atmosphere to eliminate air resistance. Second, it must
give the satellite such a large horizontal speed that the falling satellite arcs endlessly around the earth rather than descending to the ground
If you blow carefully across the top of a drinking straw, it will emit a tone. If you now close off the bottom of the straw with your finger, the tone that it emits will
go down by an octave (its frequency will decrease by a factor of 2)
As water in steady state flows straight down in a vertical pipe with a uniform diameter, that water transforms its
gravitational potential energy into pressure potential energy
When you jump while standing on a bathroom scale, it briefly reads more than your actual weight. During that moment, it is exerting an upward force on you that is
greater than your weight and you are accelerating upward
A rigid two-blade wind turbine that is experiencing zero net torque
has a constant angular velocity, which may be zero
Some fire extinguishers use spray carbon dioxide. The mass of a carbon dioxide molecule is about 50% greater than the mass of an average air molecule. At room temperature, a liter of carbon dioxide
has the same number of particles as a liter of air, but the carbon dioxide weighs more than the air it displaces and it sinks to the floor.
If you strike a stiff, spring-like surface with a mallet and listen to the sound it emits, you'll notice that this sound is more complicated than that emitted by a string or a thin bar. That's because surfaces
have overtones that are not integer multiples of their fundamental frequencie
Water is flowing gently out of the end of a garden hose. You block off most of the hose's opening with your thumb and now the water sprays out at high speed because you
have reduced the water's speed in the hose so that it wastes less of its ordered energy
Water is flowing gently out of the end of a garden hose. You block off most of the hose's opening with your thumb and now the water sprays out at high speed because you
have slowed the water flow through hose and plumbing so that the water wastes less ordered energy doing work against viscous forces
All current rockets eject stages or booster rockets as they climb from the earth's surface up to orbit. A rocket that didn't eject any stages or booster rockets while climbing from the ground to orbit would
have to have a launch weight that is about 90% rocket fuel
The curves on bicycle racetracks are steeply banked, so that the outer edge of each curve is much higher than its inner edge. This banking tips the support force that the track exerts on the bicycle wheel toward the center of each turn. That center-directed or centripetal force on the bicycle is important because it
helps the bicycle accelerate inward to complete each turn without skidding
You are cleaning a wall by spraying water at it from a hose. At the center of the stream of water, right where it hits the wall, the water is coming to a complete stop. If you were to measure the water pressure at that point, you would find that it is
higher than atmospheric pressure
You are cleaning the siding on your house by spraying water at it from a hose and nozzle. At the center of the stream of water, right where it hits the siding, the water is coming to a complete stop. The pressure in the water at the center of the stream as it touches the siding is
higher than atmospheric pressure
You are playing with a squirt gun and the narrow stream of water from your toy hits your friend's cheek. At the center of the stream of water, right where it hits his cheek, the water is coming to a complete stop. The pressure in the water at the center of the stream as it touches his cheek is
higher than atmospheric pressure
Flowing honey is less likely to become turbulent than flowing water because
honey's larger viscosity favors laminar flow
You are paddling yourself forward in a Kayak, across a calm lake. Each time you pull the paddle backward through the water, from the front of the boat to its rear, the water exerts a force on the paddle that is
in the forward direction, toward the front of boat
On a cold morning, you seal your empty plastic water bottle and thus trap the air inside it. Later in the day, the bottle and its contents are much warmer and the sides of the bottle are now bowed outward. As a result of these changes, the air pressure inside the bottle has
increased and the density inside the bottle has decreased
A satellite is traveling around the earth in a circular orbit. It briefly fires its rocket engine to increase its speed in the forward direction; it is suddenly going faster but its direction of travel didn't change. As a result of this speed increase, the orbiting satellite's average distance from the center of the earth
increases
As air flows around a fast-moving ball, that air has difficulty maintaining laminar flow from the middle of the ball to the back of the ball. During the air's passage from the ball's middle to its back, the pressure along each streamline
increases and the airflow slows down
At high altitude, a commercial jetliner must compress the outside air before circulating it in the pressurized cabin. The compressed air must first be sent through an air conditioner, however, because compressing air
increases its temperature
Increasing the amount of carbon dioxide (a greenhouse gas) in the earth's atmosphere
increases the altitude of the earth's effective radiating surface
An automobile engine becomes more energy efficient (better gas mileage) as the temperature reached when it burns fuel in air increases (when its flame becomes hotter) because increasing the temperature difference between the hot flame and the cold outdoor air
increases the entropy produced by transferring 1 joule of thermal energy from the flame to the air.
In a famous ice skating trick, a skater begins spinning with her arms out and then pulls her arms tightly against her body. She is then spinning extremely fast on the tip of one skate. Her angular velocity increases because her angular momentum
is constant but her rotational mass decreases
Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because
it barely moves downward as you push downward on it, so you do almost zero work on it
You throw a curve ball that is bending toward your right as it travels away from you. It is able to bend in this manner because
it is deflecting the airflow around it toward your left
As the water stream from a fountain rises upward in the open air, its gravitational potential energy increases,
its kinetic energy decreases, but its pressure potential energy remains constant
As the water stream from a fountain rises upward in the open air, its gravitational potential energy increases
its kinetic energy decreases, but its pressure potential energy remains constant.
You are standing in a slow-moving stream, facing upstream so that the water is coming toward the front of your body. You are wearing shorts and the water level is slightly above your bare knees. The water experiences perfect laminar flow around each of your nearly cylindrical legs. You measure the water pressure at a specific altitude below your right knee and find that the water pressure is
largest on the front and back of your leg and smallest on the sides of your leg
A pendulum consists of a metal ball hanging on a cable from a fixed point on the ceiling. To increase the period of that pendulum (the time it takes to complete one full cycle of motion), you should
lengthen the cable
When you light a candle, its flame turns wax molecules and oxygen into water and carbon dioxide molecules. The molecules that form during this combustion have
less chemical potential energy than the original wax and oxygen molecules
When wood burns in air, the water and carbon dioxide molecules that form as the result of combustion reactions have
less chemical potential energy than the original wood and air molecules
You stop for a cappuccino at Alderman Café and notice that the tiny white bubbles of steamed milk remain on the surface of the coffee. These air-filled bubbles stay where they are, rather than descending into the coffee or rising into the air, because they are
less dense than the coffee but more dense than the air above the coffee
The energy needed to operate an air conditioner during the summer decreases as the outdoor temperature becomes less hot and as you adjust the thermostat to let the indoor air become less cold. The air conditioner needs less electric energy to move each joule of heat from indoor air to outdoor air because, as the temperature difference between the cool indoor air and the warm outdoor air decreases,
less ordered energy must become thermal energy to make up for the entropy lost when heat moves against its natural direction of flow
Your bottle of oil and vinegar salad dressing has separated, so that the oil is floating on the vinegar. You notice that a large herb is floating motionless at the interface between the oil and the vinegar. That herb's average density is
less than that of vinegar but more than that of oil
The maximum speed of a rocket-propelled spaceship with an exhaust speed of 5000 mph is
limited only by the ratio of fuel to spaceship
You hold a new glass bottle of root beer upright in one hand and pound its bottle cap downward hard with a rubber mallet. As a consequence of this action,
liquid colliding with the bottom of the bottle knocks the bottom out of the bottle
You hold an unopened glass bottle of root beer upright in one hand and pound its bottle cap downward hard with a rubber mallet. As a consequence of this action
liquid colliding with the bottom of the bottle knocks the bottom out of the bottle
A friend suggests that you invest in a new invention that supposedly absorbs heat from the room and converts it into electricity. According to a brochure, the device sits in the middle of the room, gradually cooling the room air while providing a steady and endless supply of electric power. You wisely choose not to invest in the device because in order to work as described, it would have to do something impossible-it would have to
lower the world's total entropy
Your table at a family-style restaurant has a "lazy Susan" in the middle. This large circular platform rotates frictionlessly so that you can "pass" an entree to your friends by placing it on the platform and then rotating the platform. When the server places a new entree on the platform, it
makes it harder to change the platform's angular velocity
Your luxury apartment has a dining room table that is so enormous, it has a "lazy Susan" in the middle. This large circular platform rotates frictionlessly so that you can "pass" the marmalade jar to your guests by placing that jar on the platform and then rotating the platform. When you replace the empty jar with a full jar from the pantry, the full jar definitely
makes it harder to change the platform's angular velocity.
Water hammer occurs when you suddenly stop the rapid flow of water in a pipe and results in the pipe-end experience a huge force in the direction of the water's original motion. It happens because flowing water has
momentum
A block of wood is floating easily on the surface of a lake. As a storm approaches, the density and pressure of the air above the lake decrease. As a result of this decrease in air pressure and density, the block of wood
moves downward slightly and floats a little deeper in the water
A block of wood is floating easily on the surface of a lake. The sun comes out and warms the air above the lake. The warming air's density decreases and the block of wood
moves downward slightly and floats a little deeper in the water
Near sea level, water normally boils at about 100 °C (212 °F). However, a microwave oven can heat water in a glass container above that temperature if
no initial steam bubbles form within the liquid water
A pitcher throws a curveball. While the spinning baseball is in the air between the pitcher and home plate, the forces acting on the ball are its weight, a buoyant force, drag forces, a lift force, and
no other force
You are out for a bicycle ride on a calm, windless day. You are heading northward on a level road and are experiencing a pressure drag force that pushes you toward the south. This air resistance explains why you have to keep pedaling to maintain your constant speed. If someone were to examine the air that you have left behind you after you have passed through it, they would find that the air's average velocity is
northward
If a golf ball were smooth rather than dimpled, it would
not travel as far after being hit by a golf club
In the game of shuffleboard, you push plastic disks forward and then release them so that they slide across a level playing surface. Once you release a disk, you are no longer pushing and it gradually skids to a stop. Its final position determines your score. As each disk skids to a stop, what becomes of the kinetic energy it had when you released it? That energy is
now thermal energy in the disk and playing surface
Two puppies are fighting over a toy. Each puppy grips one end of that toy in its mouth and pulls. Suddenly, the puppy on the right pulls especially hard on the toy and moves its end of the toy to the right. The other puppy manages to keep its end of the toy from moving. Alas, the toy breaks and the game ends. Breaking the toy required energy and that energy was provided
only by the puppy on the right
Flowing pancake syrup is less likely to become turbulent than flowing water because
pancake syrup's larger viscosity favors laminar flow
You're standing on the middle of the Ruffner Hall/Bavaro Hall pedestrian bridge when a group of prospective students jumps up and down and sets the bridge bouncing. As you bounce up and down about equilibrium, your speed is greatest when you are
passing through equilibrium on your way upward or downward
The characteristic of a harmonic oscillator that makes it especially suitable for use as a timekeeper in a clock is that its
period of oscillation does not depend on its amplitude (extent of motion)
When you ride a bicycle, the ground exerts an upward support force on the wheel. When you turn the bicycle, the ground also exerts a horizontal frictional force on the wheel. Leaning the bicycle toward the inside of a turn keeps you from tipping over because the overall force that the ground exerts on the wheel (the sum of the support force and the frictional force)
points directly at your combined center of mass and produces zero torque on you and the bicycle
When you ride a bicycle, the ground exerts an upward support force on the wheel. When you turn the bicycle, the ground also exerts a horizontal frictional force on the wheel that causes you to accelerate sideways. Leaning the bicycle toward the inside of a turn keeps you from flipping over because the overall force that the ground exerts on the wheel
points directly toward your combined center of mass and produces zero torque
An toddler's sipping cup has a round base (like the bottom half of a ball) that is weighed so that the cup always returns to upright after being tipped. The cup is in a stable equilibrium when it is upright and tipping it causes its total
potential energy to increase
When the air moving passed an airplane flows over the top surface of the airplane's wing, that air speeds up significantly. One important consequence of the air's increase in speed is that the air's
pressure decreases
You are drying your hair and you have put a diffuser (an anti-nozzle) on the end of the dryer. As it goes through the diffuser, the air slows down and its
pressure increases
You are watering the garden with a hose and nozzle. As the water flows through the nozzle, it converts
pressure potential energy into kinetic energy
You are watering the garden with a hose and nozzle. You are holding the nozzle horizontally in order to spray a distant flower. As the water flows through that nozzle, it converts
pressure potential energy into kinetic energy
You are eating at an outdoor restaurant on a cold day and the restaurant has a red hot heating element positioned about 10 feet above you that keeps you surprisingly warm. Heat from that element reaches you
primarily via radiation, with almost no heat flowing to you via conduction or convection
The waiting room in a bus station gets cold in winter, so the company has installed an electric heater high on one wall where no one can accidently touch it. When that heater is turned on, it turns electric power directly into thermal power, like an electric toaster. Heat then flows from the heater to people seated on benches near the floor
primarily via radiation, with almost no heat flowing via conduction or convection
Tightening a guitar string
raises the string's frequency or pitch by stiffening the string's restoring force
Modern double-pane windows provide excellent thermal insulation. The inner surface of one of those panes is coated with a transparent electrical conductor because that coating
reflects infrared light and blocks radiative heat transfer from one pane to the other
You are standing in the middle of a level subway car that is moving forward at constant velocity when another passenger accidentally spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Because nothing else is touching you, you
remain in the middle of the subway car
You are standing in the middle of a subway car that is moving forward at constant velocity when another passenger accidently spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Nothing else is touching you, so you
remain in the middle of the subway car
You are practicing tennis alone by hitting a tennis ball forward toward a cement wall. Each time the ball hits the wall, it bounces backward at high speed so that you can hit it again. During its bounce, the ball
retains approximately all of its energy but transfers more forward momentum than it had to the wall
You purchase an antique wall clock that uses a pendulum as its timekeeper. The seller, who lives in Northern Canada truthfully said that the clock keeps perfect time. When you mount it on the wall of your dorm room, where the earth's gravity is slightly weaker, the clock
runs a little slow (causing you to be late for classes)
Just mixing gasoline and air together won't make them burst into flames because
something must provide the activation energy needed to initiate the chemical reaction
When a liquid flows through a collection of stationary obstacles, its flow can be smooth and laminar, or it can be swirling and turbulent. The liquid's flow is more likely to become turbulent if you increase its
speed
When a liquid flows through a collection of stationary obstacles, its flow can be smooth and laminar, or it can be swirling and turbulent. The liquid's flow is more likely to become turbulent if you increase its
speed or its density
A helium-filled balloon weighs only about 1/7th as much as the air it displaces. If you replace the helium gas in the balloon with hydrogen gas (which is half as dense as helium gas), the upward buoyant force on the balloon would
stay the same and the net force on the balloon would increase slightly in the upward direction
If you're trying to increase the pressure in the water distribution system by modifying the local water tower, you should make the water tower
taller
An air conditioner's compressor takes low density gas and compresses that gas to high density. During this compression process, the gas's
temperature rises and so does its pressure
Moments before it's ignited by the sparkplug, the mixture of fuel and air inside an automobile engine cylinder is compressed to very high density. During the compression process, the mixture's
temperature rises dramatically and so does its pressure
A bicycle touches the ground at only two points, forming a base of support that is a line. As the bicycle (including the rider) moves forward, it steers itself automatically so that it
tends to put its base of support below its center of gravity, thereby returning the bicycle to its unstable equilibrium after any minor tip
You are floating along in a hot air balloon. You look up and notice that the bottom of the balloon is open. Apart from a few molecules that diffuse out, hot air remains inside the balloon despite this opening because
the air pressure inside the balloon's opening is the same as the air pressure outside that opening.
An apple sits motionless on a table. The amount of momentum that apple is transferring to the table each second is
the apple's weight times 1 second
You are filling a plastic party balloon with helium. At first, the under-filled balloon doesn't float. But as you keep filling it with helium, it eventually begins floating because
the average density of the helium-filled bag decreases as it fills and its density eventually becomes less than that of the air it displaces
A bicycle is remarkably stable while it's moving forward because, when you start to tip over,
the bicycle spontaneously steers so that its wheels drive under your center of gravity
You are at a barbeque and the bottle of hot sauce is almost empty. You put the cap on the bottle and swing it rapidly in a circle with its cap end pointing outward. The remaining sauce collects just inside the cap and you are able to extract enough to spice up your vegiburger. This technique works because
the bottle's rapid inward acceleration leaves the hot sauce behind so that it drifts toward the cap end of the bottle
Your French fries are getting cold as you wait for your server to bring you a new bottle of ketchup. In frustration, you decide to get the last bits of ketchup out of the old bottle by swinging it rapidly in a circle with the cap-end of the bottle pointing away from you. This technique works because
the bottle's rapid inward acceleration leaves the ketchup behind so that it drifts toward the cap-end of the bottle.
You have forgotten to buy more shampoo and there is only a tiny bit left in your old bottle. To move that shampoo toward the opening of the bottle, you swinging the bottle rapidly in a circle with its cap-end pointing away from the center of the circle. This technique works because
the bottle's rapid inward acceleration leaves the shampoo behind so that it moves toward the cap-end of the bottle
You make a sharp left turn in your car and your cellphone slips off the dashboard and out the right passenger window. The cellphone leaves the car because
the car did not exert enough leftward force on the cellphone to make the cellphone accelerate with the car
You're at the lake and watch two children jump off a dock at the same time. They both kick equally hard during their jumps, but one child jumps mostly upward while the other child jumps mostly forward. After they leave the dock,
the child who jumps forward reaches the water before the child who jumps upward
You are studying for final exams in a perfectly insulated and isolated room. Absolutely nothing passes into or out of this room, not even sound, light, electricity, or heat. As you study in this special room,
the entropy (disorder) of the room and its contents is increasing
Goose down is the soft, fluffy feathers near a goose's skin. The reason that a goose down jacket keeps you so warm in the winter is that
the fine structure of the goose down traps air and prevents it from undergoing convection
You are visiting a playground and you notice that a girl on one swing completes each full cycle of motion in much less time than a boy on another swing. The girl has a much shorter period of motion (time between cycles) than the boy because
the girl is riding on a much shorter swing than the boy is riding
You're at the lake and watch two children jump off a dock. They jump at the same time and at the same speed, but the boy jumps mostly upward while the girl jumps mostly forward. After they leave the dock,
the girl reaches the water before the boy
A seemingly "haunted" house emits creaking sounds as it cools off at night. Since sound carries energy, the house must obtain that energy from somewhere (other than ghosts and goblins). While this energy begins as thermal energy, the sound doesn't appear until the outside temperature drops well below the temperature of the house because
the house's thermal energy can only become work and sound as heat flows from the hotter house to the colder outside air
Two glass marbles, one of which weighs twice as much as the other, roll off of a horizontal table. As they leave the table, the lighter marble is traveling twice as fast as the heavier marble. In this situation,
the lighter marble hits the floor about twice as far from the table as the heavier marble
Two squirrels are fighting over a tasty acorn. Each squirrel is pulling on one side of the acorn and they are not making any progress. At last, the female squirrel holds her side of the acorn motionless as the male squirrel tears his half of the acorn away. Tearing the acorn in half required energy and that energy was provided by
the male squirrel.
When you pull a tablecloth out from under a set of dishes, it's important to pull the cloth as fast as possible because
the momentum transferred to the dishes is proportional to the time during which the cloth pulls on them.
You're filling a jar of honey from the spigot at the bottom of a large, nearly empty barrel at the grocery store. The honey flows extremely slowly, so the store manager has the barrel refilled. Now the honey flows much more rapidly from the spigot because
the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases.
In Charlottesville, water boils at approximately 100 °C (212 °F). In the Blue Ridge Mountains, however, water boils at about 97 °C (207 °F) because
the reduced atmospheric pressure in the mountains allows steam bubbles in the water to grow by evaporation at that lower temperature
When a modern car crashes into a tree and comes to an abrupt stop, the driver's face and chest collide with an air bag rather than with the steering wheel. The driver's chances of serious injury are reduced by hitting the air bag rather than the steering wheel because the driver transfers
the same amount of momentum to the air bag as he would to the steering wheel if there were no air bag, but he does so with a smaller force because of the air bag
A dozen houses have just been built on a steep hillside overlooking a beautiful lake. A simple water distribution system provides water to all of those houses; they share a single delivery pipe with a branch for each house. Residents at various altitudes above the lake begin taking showers and have different experiences. They observe that the total ordered energy per liter in the spraying water is (neglecting any effects of viscosity and friction)
the same at all altitudes, but the speed of the spraying water is smaller at higher altitudes
You take off your shoes to sneak quietly into your home late at night. Unfortunately, it's too dark to see the concrete block your friend left on the floor and your big toe collides with it. The block doesn't move and your foot comes to a complete stop due to the impact. Luckily, you are wearing soft woolen socks because when your foot stops during the impact, your toe transfers
the same momentum, whether or not you are wearing socks, but that transfer takes more time when you are wearing socks
A helium balloon floats in air because the helium balloon contains
the same number of particles as the air it displaces, but each helium atom weighs less than the average air molecule
A hydrogen-filled dirigible (a huge blimp or elongated balloon) floats in air because the hydrogen-filled dirigible contains
the same number of particles as the air it displaces, but each hydrogen molecule weighs less than the average air molecule
A builder outsourced the design and construction of a high-rise apartment building, as a result, all 50 floors receive their water from a single pipe. That pipe is fed from a water tank located on the building's roof. On opening day, residents on various floors begin taking showers and have different experiences. They quickly discover that the total energy per liter in the spraying water is (neglecting any effects of viscosity and friction)
the same on all floors, but the speed of the spraying water is smaller on higher floors.
Your friend puts an open container of vegetable soup in a frost-free freezer and lets it freeze solid. He then forgets about it for more than year. We he rediscovers the open container, he finds it almost empty except for the vegetables because
the soup's water molecules have gone directly from solid ice to gaseous water vapor
You are watching children play a game of tug-o-war with a plastic clothesline. The two teams are pulling at opposite ends of the cord and each team is trying to drag the other team into a mud puddle that lies between them. After a few minutes without progress, the team on the right suddenly pulls hard toward the right. The team on the left has anticipated this threat and is able to keep their end of the rope from moving. The right end of the rope stretches toward the right and the rope breaks. Breaking the rope required energy and that energy was provided by
the team on the right
You are inflating a shiny plastic balloon with helium. The balloon starts as a thin, flat bag but it gradually gets thicker as it fills. At first, the thin plastic balloon doesn't float. But as you keep adding helium to the balloon and it gets thicker, it eventually begins floating because
the weight of the helium-filled balloon increases as it fills, but the buoyant force it experiences increases much faster and eventually the buoyant force exceeds the balloon's weight.
When the A-string of a guitar vibrates in its fundamental mode without anyone touching it, it has a frequency (or pitch) of 110 cycles per second. When that same string is vibrating at 330 cycles per second without anyone touching it, it is vibrating in it
third harmonic mode, as three 1/3rd-length strings
You are swinging a bucket full of water around you in a big horizontal circle at a constant speed. You are at the center of its circular path. The net force on the bucket points directly
toward you
You are swinging a can of paint around you in a big horizontal circle at a constant speed. You are at the center of its circular path. The net force on the paint can points directly
toward you
As water in steady state flows straight up in a vertical pipe with a uniform diameter, that water
transforms its pressure potential energy into gravitational potential energy
A spaceship is moving at constant velocity toward a distant star. The ship experiences no gravitational forces. To maintain its velocity, the ship must have its rocket engines
turned off.
The first kind of perpetual motion machine provides an inexhaustible supply of mechanical work (or the equivalent of work), but consumes nothing at all from its environment, not even heat. An example would be a black box that consumes nothing yet provides electricity forever. This form of perpetual motion machine is impossible because it
violates the law of conservation of energy
The second kind of perpetual motion machine provides an inexhaustible supply of mechanical work (or the equivalent of work), but consumes only heat from its constant-temperature environment. An example would be a black box that absorbs heat from the 70 °F room air and converts that heat into electricity forever. This form of perpetual motion machine is impossible because it
violates the law of entropy
The flow of air around a ball can be perfectly laminar only when the ball is traveling extremely slowly through the air. During such laminar flow, the ball experiences
viscous drag but not pressure drag
You are jogging on a hot, humid summer day and are covered with perspiration. Unfortunately, that perspiration doesn't cool you off much because
water molecules are landing from the air onto your perspiration almost as often as they are leaving your perspiration for the air.
Ice cubes gradually shrink in a frost-free freezer because
water molecules go directly from solid ice to gaseous water vapor
When you launch a toy water rocket, compressed air inside the rocket pushes water out of the rocket's tail as "exhaust" and the rocket blasts forward at high speed. Normally, you fill the rocket's hollow shell half-full with water and half-full with compressed air. Eliminating the water would allow you to put twice as much compressed air in the rocket and would double the rocket's stored energy. However, the air-filled rocket will barely move forward when you launch it because its air exhaust
will have almost no mass and therefore carry almost no backward momentum
When you stand in front of an open refrigerator, trying to decide which flavor of Ben and Jerry's to eat, you feel cold even though no chilled air actually touches your skin. The reason you feel cold is that
you are radiating heat toward the refrigerator but receiving relatively little radiated heat in return
Air mattresses are hollow, air-filled bags that are used as beds while camping or to accommodate occasional visitors at home. When you lie on a fully inflated air mattress, it supports your weight easily and barely dents at all. But if you stand on it, its surface dents severely and you may actually pop the air mattress. Standing on the air mattress causes the air pressure inside it to rise much more than lying on it does because
you're supported by the mattress's air pressure and as your contact surface with the mattress decreases, the air pressure needed to support you increases
You are pulling your niece uphill on a sled and the sled experiences a small downhill frictional force as it slides uphill on the snow. Your niece is traveling in a straight line at a constant speed. The net force your niece is experiencing is
zero
You are tossing popcorn straight up and catching it in your mouth. At the moment each piece of popcorn reaches its peak height, its velocity is
zero and its acceleration is downward
You throw a snowball straight up into the air. It rises to a peak height and then descends. At the moment it reaches its peak height, the snowball's velocity is
zero and its acceleration is downward
A skateboarder is being pulled up a gradual hill by a bicyclist and they are both moving at constant velocity. The net force on the skateboarder is
zero and the net force on the bicyclist is zero
It's a snowy day and you are pulling your sled up a hill. Both you and the sled are moving at constant velocity. The net force on you is
zero and the net force on the sled is zero
You are bicycling at constant velocity along a horizontal road and you are wearing a backpack. As you bicycle, you are doing
zero work on the backpack and it is doing zero work on you
You are an astronaut floating in empty space far from Earth or anything else. You throw a wrench forward as hard as you can. While that wrench is moving forward at great speed, the net force on that wrench is
zero.
