intro to physical science final
energy
A combination of energy and matter make up the universe. Energy •Mover of substances •Both a thing and a process •Observed when it is being transferred or being transformed •A conserved quantity •Property of a system that enables it to do work •Anything that can be turned into heat Example: Electromagnetic waves from the Sun Matter •Substance we can see, smell, and feel •Occupies space
Consider a cart pushed along a track with a certain force. If the force remains the same while the mass of the cart decreases to half, the acceleration of the cart A. remains relatively the same. B. halves. C. doubles. D. changes unpredictably.
Acceleration = net force / mass Because, mass is in the denominator, acceleration increases as mass decreases. So, if mass is halved, acceleration doubles. Answer is C
Push a cart along a track so twice as much net force acts on it. If the acceleration remains the same, what is a reasonable explanation? A.The mass of the cart doubled when the force doubled. B.The cart experiences a force that it didn't before. C.The track is not level. D.Friction reversed direction.
Acceleration = net force / mass If force doubles, acceleration will also double, But it does not, so mass must also be doubling to cancel out effects of force doubling. answer is A
When a 20-N falling object encounters 5 N of air resistance, its acceleration of fall is A.less than g. B.more than g. C. g. D.terminated.
Acceleration of a non-free fall is always less than g. Acceleration will actually be (20 N - 5 N)/2 kg = 7.5 m/s2. answer is A
Must a car with momentum have kinetic energy? A. Yes, due to motion alone B.Yes, when motion is nonaccelerated C.Yes, because speed is a scalar and velocity is a vector quantity D.No
Acceleration, speed being a scalar, and velocity being a vector quantity are irrelevant. Any moving object has both momentum and kinetic energy. answer is A
A fast-moving car hitting a haystack or a cement wall produces vastly different results. 1. Do both experience the same change in momentum? 2. Do both experience the same impulse? 3. Do both experience the same force? A. Yes for all three B.Yes for 1 and 2 C.No for all three D.No for 1 and 2
Although stopping the momentum is the same whether done slowly or quickly, the force is vastly different. Be sure to distinguish among momentum, impulse, and force. answer is B
When Sanjay pushes a refrigerator across a kitchen floor at a constant speed, the force of friction between the refrigerator and the floor is A.less than Sanjay's push. B.equal to Sanjay's push. C.equal and opposite to Sanjay's push. D.more than Sanjay's push.
Answer is C
Suppose you take a sharper turn than before and halve the radius, by what factor will the centripetal force need to change to prevent skidding? A. Double B. Four times C. Half D. One-quarter
Because the term for "radius" is in the denominator, if you halve the radius, the centripetal force will double. answer is A
Suppose you double the speed at which you round a bend in the curve, by what factor must the centripetal force change to prevent you from skidding? A. Double B. Four times C. Half D. One-quarter
Because the term for "tangential speed" is squared, if you double the tangential speed, the centripetal force will be double squared, which is four times. answer is B
free fall vs non free fall
Coin and feather fall with air present •Feather reaches terminal velocity very quickly and falls slowly at constant speed, reaching the bottom after the coin does. •Coin falls very quickly and air resistancedoesn't build up to its weight over short-falling distances, which is why the coin hits the bottom much sooner than the falling feather. Coin and feather fall in vacuum •There is no air, because it is vacuum. •So, no air resistance. •Coin and feather fall together.
the moving earth
Copernicus proposed that Earth was moving. •This idea was refuted by people. •Example: If Earth moved, how can a bird swoop from a branch to catch a worm? •Solution: As it swoops, due to inertia, it continues to go sideways at the speed of Earth along with the tree, worm, etc.
What is the distance covered of a freely falling object starting from rest after 4 s? A.4 m B.16 m C.40 m D.80 m
Distance = (1/2) x acceleration x time x time So: Distance = (1/2) x 10 m/s2 x 4 s x 4 s So: Distance = 80m. Answer is D
efficiency
Efficiency •Percentage of work put into a machine that is converted into useful work output •In equation form: • efficiency= useful energy output/total energy input
A bowling ball is in equilibrium when it A.is at rest. B.moves steadily in a straight-line path. C.Both of the above. D.None of the above.
Equilibrium means no change in motion, so there are two options: •If at rest, it continues at rest. •If in motion, it continues at a steady rate in a straight line.
equilibrium of moving things
Equilibrium: a state of no change with no net force acting -Static equilibrium Example: hockey puck at rest on slippery ice -Dynamic equilibrium Example: hockey puck sliding at constant speed on slippery ice Equilibrium test: whether something undergoes changes in motion Example: A crate at rest is in static equilibrium. Example: When pushed at a steady speed, it is in dynamic equilibrium.
free fall
Falling under the influence of gravity only - with no air resistance •Freely falling objects on Earth accelerate at the rate of 10 m/s/s, i.e., 10 m/s2 (more precisely, 9.8 m/s2). The velocity acquired by an object starting from rest is So, under free fall, when acceleration is 10 m/s2, the distance is 5 m/s after 1 s. 20 m/s after 2 s. 45 m/s after 3 s. And so on.
collisions
For all collisions in the absence of external forces, •net momentum before collision equals net momentum after collision. •in equation form: (net mv)before = (net mv)after Elastic collision -occurs when colliding objects rebound without lasting deformation or any generation of heat. Inelastic collision -occurs when colliding objects result in deformation and/or the generation of heat. Example of elastic collision: single car moving at 10 m/s collides with another car of the same mass, m, at rest From the conservation of momentum, (net mv)before = (net mv)after (m x 10)before = (2m x V)after V = 5 m/s
acceleration
Formulated by Galileo based on his experiments with inclined planes. Rate at which velocity changes over time Involves a •change in speed, or •change in direction, or •both. Example: Car making a turn Unit of acceleration is unit of velocity / unit of time. Example: You car's speed right now is 40 km/h. Your car's speed 5 s later is 45 km/h. Your car's change in speed is 45 - 40 = 5 km/h. Your car's acceleration is 5 km/h/5 s = 1 km/h/s.
The force of friction can occur A. with sliding objects. B.in water. C.in air. D.All of the above.
Friction can also occur for objects at rest. If you push horizontally on your book and it doesn't move, then friction between the book and the table is equal and opposite to your push. Answer is D
Acceleration part 2
Galileo increased the inclination of inclined planes. •Steeper inclines gave greater accelerations. •When the incline was vertical, acceleration was max, same as that of the falling object. •When air resistance was negligible, all objects fell with the same unchanging acceleration.
A hoop and a disk are released from the top of an incline at the same time. Which one will reach the bottom first? A. Hoop B. Disk C. Both together D. Not enough information
Hoop has larger rotational inertia, so it will be slower in gaining speed. answer is B
Two people of equal mass on slippery ice push off from each other. Will both move at the same speed in opposite directions? a.Yes B.Yes, but only if both push equally C.No D.No, unless acceleration occurs
However they push, the result is equal-magnitude forces on equal masses, which produces equal accelerations; therefore, there are equal changes in speed. answer is A
bouncing
Impulses are generally greater when objects bounce. Example: Catching a falling flower pot from a shelf with your hands. You provide the impulse to reduce its momentum to zero. If you throw the flower pot up again, you provide an additional impulse. This "double impulse" occurs when something bounces. Pelton wheel designed to "bounce" water when it makes a U-turn on impact with the curved paddle.
instantaneous speed
Instantaneous speed is the speed at any instant. Example: -When you ride in your car, you may speed up and slow down. -Your instantaneous speed is given by your speedometer.
forces and interactions
Interaction •is between one thing and another. •requires a pair of forces acting on two objects. Example: interaction of hand and wall pushing on each other Force pair—you push on wall; wall pushes on you.
Newton's second law of motion
Isaac Newton was the first to connect the concepts of force and mass to produce acceleration. Newton's second law (the law of acceleration) relates acceleration to force. The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object. In equation form: acceleration = net force/mass • Example: If net force acting on object is doubled Þ object's acceleration will be doubled. If mass of object is doubled Þ object's acceleration will be halved.
When the string is pulled down quickly, the bottom string breaks, which best illustrates the A.weight of the ball. B.mass of the ball. C.volume of the ball. D.density of the ball.
It is the "laziness" of the ball that tends to keep it at rest, resulting in the breaking of the bottom string. answer is B
conservation of energy
Law of conservation of energy •Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. Example: Energy transforms without net loss or net gain in the operation of a pile driver.
conservation of momentum
Law of conservation of momentum: In the absence of an external force, the momentum of a system remains unchanged. Examples: •When a cannon is fired, the force on the cannonball inside the cannon barrel is equal and opposite to the force of the cannonball on the cannon. •The cannonball gains momentum, while the cannon gains an equal amount of momentum in the opposite direction—the cannon recoils. When no external force is present, no external impulse is present, and no change in momentum is possible. Examples (continued): •Internal molecular forces within a baseball come in pairs, cancel one another out, and have no effect on the momentum of the ball. •Molecular forces within a baseball have no effect on its momentum. •Pushing against a car's dashboard has no effect on its momentum.
machines
Machine •Device for multiplying forces or changing the direction of forces •Cannot create energy but can transform energy from one form to another, or transfer energy from one location to another •Cannot multiply work or energy Principle of a machine • •Conservation of energy concept: Work input = work output • •Input force x input distance = Output force x output distance • •(Force x distance)input = (force x distance)output Simplest machine •Lever -rotates on a point of support called the fulcrum -allows small force over a large distance and large force over a short distance Pulley -operates like a lever with equal arms— changes the direction of the input force Example: This pulley arrangement can allow a load to be lifted with half the input force. •Operates as a system of pulleys (block and tackle) •Multiplies force
mechanical energy
Mechanical energy is due to position or to motion, or both. There are two forms of mechanical energy: •Potential energy •Kinetic energy
motion is relative
Motion of objects is always described as relative to something else. For example: You walk on the road relative to Earth, but Earth is moving relative to the Sun. So your motion relative to the Sun is different from your motion relative to Earth.
The average speed of driving 30 km in 1 hour is the same as the average speed of driving A.30 km in 1/2 hour. B.30 km in 2 hours. C.60 km in 1/2 hour. D.60 km in 2 hours.
Now, if we drive 60 km in 2 hours: Average speed = 60 km / 2 h = 30 km/h. Answer is D
A free-falling object has a speed of 30 m/s at one instant. Exactly 1 s later its speed will be A.the same. B.35 m/s. C.more than 35 m/s. D.60 m/s.
One second later its speed will be 40 m/s, which is more than 35 m/s. Answer is C
potential energy- gravitational
Potential energy due to elevated position Example: •water in an elevated reservoir •raised ram of a pile driver •Equal to the work done (force required to move it upward ´ the vertical distance moved against gravity) in lifting it • •In equation form: Potential energy = mass x acceleration due to gravity ´height = mgh
power
Power: •Measure of how fast work is done Example: •A worker uses more power running up the stairs than climbing the same stairs slowly. •Twice the power of an engine can do twice the work of one engine in the same amount of time, or twice the work of one engine in half the time or at a rate at which energy is changed from one form to another. Unit of power •joule per second, called the watt after James Watt, developer of the steam engine •1 joule/second = 1 watt •1 kilowatt = 1000 watts
Suppose the girl on the left suddenly is handed a bag of apples weighing 50 N. Where should she sit order to balance, assuming the boy does not move? A. 1 m from pivot B. 1.5 m from pivot C. 2 m from pivot D. 2.5 m from pivot
She should exert same torque as before. Torque = lever arm ´ force = 3 m ´ 250 N = 750 Nm Torque = new lever arm ´ force 750 Nm = new lever arm ´ 250N Þ New lever arm = 750 Nm / 250 N = 2.5 m. answer is D
kinetic energy and momentum compared
Similarities between momentum and kinetic energy: •Both are properties of moving things. Difference between momentum and kinetic energy: •Momentum is a vector quantity and therefore is directional and can be canceled. •Kinetic energy is a scalar quantity and can never be canceled. •Velocity dependence -Momentum depends on velocity. -Kinetic energy depends on the square of velocity. Example:An object moving with twice the velocity ofanother with the same mass, has twice themomentum but 4 times the kinetic energy
sources of energy
Sources of energy Sun Example: •Sunlight evaporates water; water falls as rain; rain flows into rivers and into generator turbines; then back to the sea to repeat the cycle. •Sunlight can be transformed into electricity by photovoltaic cells. •Wind power turns generator turbines. Sources of energy Sun Example: •Photovoltaic cells on rooftops catch the solar energy and convert it to electricity. Fuel cell •Runs opposite to the battery shown (where electricity separates water into hydrogen and oxygen). •In a fuel cell, hydrogen and oxygen are compressed at electrodes and electric current is produced at electrodes. Concentrated energy •Nuclear power -stored in uranium and plutonium -by-product is geothermal energy •held in underground reservoirs of hot water to provide steam that can drive turbogenerators •Dry-rock geothermal power is a producer of electricity. -Water is put into cavities in deep, dry, hot rock. Water turns to steam and reaches a turbine, at the surface. After exiting the turbine, it is returned to the cavity for reuse.
potential energy
Stored energy held in readiness with a potential for doing work Example: •A stretched bow has stored energy that can do work on an arrow. •A stretched rubber band of a slingshot has stored energy and is capable of doing work. Example: Potential energy of 10-N ball is the same in all 3 cases because work done in elevating it is the same.
support force
Support force (normal force) is an upward force on an object that is opposite to the force of gravity. Example: A book on a table compresses Atoms in the table, and the compressed atoms produce the support force. When you push down on a spring, the spring pushes back up on you. Similarly, when a book pushes down on a table, the table pushes back up on the book.
A ladybug sits halfway between the rotational axis and the outer edge of the turntable . When the turntable has a rotational speed of 20 RPM and the bug has a tangential speed of 2 cm/s, what will be the rotational and tangential speeds of her friend who sits at the outer edge? A. 1 cm/s B. 2 cm/s C. 4 cm/s D. 8 cm/s
Tangential speed = Rotational speed of both bugs is the same, so if radial distance doubles, tangential speed also doubles. So, tangential speed is 2 cm/s ´ 2 = 4 cm/s. answer is C
When the string is pulled down slowly, the top string breaks, which best illustrates the A.weight of the ball. B.mass of the ball. C.volume of the ball. D.density of the ball.
Tension in the top string is the pulling tension plus the weight of the ball, both of which break the top string. answer is A
A cannonball shot from a cannon with a long barrel will emerge with greater speed because the cannonball receives a greater a.average force. B.impulse. C.Both of the above. D.None of the above.
The average force on the cannonball will be the same for a short- or long-barreled cannon. The longer barrel provides for a longer time for the force to act, and therefore, a greater impulse. (The long barrel also provides a longer distance for the force to act, providing greater work and greater kinetic energy to the cannonball.) answer is B
You are riding in a van at a steady speed and toss a coin up. Where will the coin land? A.behind you B.ahead of you C.back in your hand D.There is not enough information.
The coin has inertia. It continues sideways with the van and your hand and lands back in your hand. Answer is C
You are pushing a crate at a steady speed in a straight line. If the friction force is 75 N, how much force must you apply? A.more than 75 N B.less than 75 N C.equal to 75 N D.not enough information
The crate is in dynamic equilibrium, so, SF = 0. Your applied force balances the force of friction. Answer is C
circular motion- tangential speed
The distance traveled by a point on the rotating object divided by the time taken to travel that distance is called its tangential speed (symbol v). •Points closer to the circumference have a higher tangential speed that points closer to the center.
Compared with a lightweight glider, a heavier glider would have to push air a.downward with greater force. B.downward with the same force. C.downward with less force. D.None of the above.
The force on the air deflected downward must equal the weight of the glider. answer is A
impulse changes momentum
The greater the impulse exerted on something, the greater the change in momentum. •In equation form: Ft = D(mv) •Case 1: increasing momentum -Apply the greatest force for as long as possible and you extend the time of contact. -Force can vary throughout the duration of contact. Examples: •Golfer swings a club and follows through. •Baseball player hits a ball and follows through. •Case 2: decreasing momentum over a long time -extend the time during which momentum is reduced Examples: When a car is out of control, it is better to hit a haystack than a concrete wall. Physics reason: Same impulse either way, but extension of hitting time reduces the force. Example (continued): In jumping, bend your knees when your feet make contact with the ground because the extension of time during your momentum decrease reduces the force on you. In boxing, ride with the punch. •Case 3: decreasing momentum over a short time -short time interval produces large force. - Example: Karate expert splits a stack of bricks by bringing her arm and hand swiftly against the bricks with considerable momentum. Time of contact is brief and force of impact is huge.
free fall
The greater the mass of the object... •the greater its force of attraction toward the Earth. •the smaller its tendency to move i.e., the greater its inertia. So, the acceleration is the same. It is equal to the acceleration due to gravity: 10 m/s2 (precisely 9.8 m/s2). When acceleration is g—free fall •Newton's second law provides an explanation for the equal accelerations of freely falling objects of various masses. •Acceleration is equal when air resistance is negligible. •Acceleration depends on force (weight) and inertia.
You run horizontally at 4 m/s in a vertically falling rain that falls at 4 m/s. Relative to you, the raindrops are falling at an angle of a. 0°. B.45°. C.53°. D.90°.
The horizontal 4 m/s and vertical 4 m/s combine by the parallelogram rule to produce a resultant of 5.6 m/s at 45°. answer is B
When Sanjay pushes a refrigerator across a kitchen floor at an increasing speed, the amount of friction between the refrigerator and the floor is A. less than Sanjay's push. B.equal to Sanjay's push. C.equal and opposite to Sanjay's push. D.more than Sanjay's push.
The increasing speed indicates a net force greater than zero. The refrigerator is not in equilibrium. Answer is A
center of gravity - stability
The location of the center of gravity is important for stability. •If we draw a line straight down from the center of gravity and it falls inside the base of the object, it is in stable equilibrium; it will balance. •If it falls outside the base, it is unstable.
When a dish falls, will the change in momentum be less if it lands on a carpet than if it lands on a hard floor? (Careful!) a. No, both are the same. B.Yes, less if it lands on the carpet. C.No, less if it lands on a hard floor. D.No, more if it lands on a hard floor.
The momentum becomes zero in both cases, so both change by the same amount. Although the momentum change and impulse are the same, the force is less when the time of momentum change is extended. Be careful to distinguish among force, impulse, and momentum. answer is A
mass resists acceleration
The same force applied to •Twice the mass produces half the acceleration. •3 times the mass, produces 1/3 the acceleration. •Acceleration is inversely proportional to mass. Acceleration ~ 1/mass
Suppose the potential energy of a drawn bow is 50 joules and the kinetic energy of the shot arrow is 40 joules. Then A. energy is not conserved. B.10 joules go to warming the bow. C.10 joules go to warming the target. D.10 joules are mysteriously missing.
The total energy of the drawn bow, which includes the poised arrow, is 50 joules. The arrow gets 40 joules and the remaining 10 joules warms the bow—still in the initial system. answer is B
Consider a problem that asks for the distance of a fast-moving crate sliding across a factory floor and then coming to a stop. The most useful equation for solving this problem is A. F = ma. B. Ft = Dmv. C. KE = 1/2mv2. D. Fd = D1/2mv2.
The work-energy theorem is the physicist's favorite starting point for solving many motion-related problems. answer is D
If a 50-N person is to fall at terminal speed, the air resistance needed is A.less than 50 N. B.50 N. C.more than 50 N. D.None of the above.
Then, SF = 0 and acceleration = 0. answer is B
Work is done in lifting a barbell. How much work is done in lifting a barbell that is twice as heavy the same distance? A. Twice as much B.Half as much C.The same D.Depends on the speed of the lift
This is in accord with work = force ´ distance. Twice the force for the same distance means twice the work done on the barbell. answer is A
The work done in bringing a moving car to a stop is the force of tire friction ´ stopping distance. If the initial speed of the car is doubled, the stopping distance is A. actually less. B.about the same. C.twice. D.None of the above.
Twice the speed means four times the kinetic energy and four times the stopping distance. answer is D
vectors
Vector quantity •has magnitude and direction. •is represented by an arrow. Example: velocity, force, acceleration Scalar quantity •has magnitude. Example: mass, volume, speed Vector components •Vertical and horizontal components of a vector are perpendicular to each othe.r •Determined by resolution.
At one instant, an object in free fall has a speed of 40 m/s. Its speed 1 second later is A. also 40 m/s. B.45 m/s. C.50 m/s. D.None of the above.
We assume the object is falling downward. answer is C
slightly tilted wings of airplanes deflect a. oncoming air downward to produce lift. B.oncoming air upward to produce lift. C.Both A and B. D.Neither A nor B.
When a wing diverts air downward, it exerts a downward force on the air. The air simultaneously exerts an upward force on the wing. The vertical component of this upward force is lift. (The horizontal component is drag.) answer is A
non-free fall
When an object falls downward through the air it experiences •force of gravity pulling it downward. air drag force acting upward When acceleration of fall is less than g, non-free fall •occurs when air resistance is non-negligible. •depends on two things: •speed and •frontal surface area. •When the object is moving fast enough that force of gravity equals its air resistance •Then no net force Þ No acceleration Þ Velocity does not change Terminal speed •occurs when acceleration terminates (when air resistance equals weight and net force is zero). • Terminal velocity •same as terminal speed, with direction implied or specified. •A skydiver jumps from plane. •Weight is the only force until air resistance acts. •As falling speed increases, air resistance on diver builds up, net force is reduced, and acceleration becomes less. •When air resistance equals the diver's weight, net force is zero and acceleration terminates. •Diver reaches terminal velocity, then continues the fall at constant speed.
newtons third law of motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Action and reaction forces •one force is called the action force; the other force is called the reaction force. •are co-pairs of a single interaction. •neither force exists without the other. •are equal in strength and opposite in direction. •always act on different objects. •Re-expression of Newton's third law: To every action there is always an opposed equal reaction. Example: Tires of car push back against the road while the road pushes the tires forward. Simple rule to identify action and reaction •Identify the interaction—one thing interacts with another -Action: Object A exerts a force on object B. -Reaction: Object B exerts a force on object A. Example: Action—rocket (object A) exerts force on gas (object B). Reaction—gas (object B) exerts force on rocket (object A). Action and Reaction on Different Masses Cannonball: F/m=a Cannon: F/m=a •The same force exerted on a small mass produces a large acceleration. •The same force exerted on a large mass produces a small acceleration. Defining Your System •Consider a single enclosed orange. -Applied external force causes the orange to accelerate in accord with Newton's second law. -Action and reaction pair of forces is not shown. •Consider the orange and the apple pulling on it. -Action and reaction do not cancel (because they act on different things). -External force by apple accelerates the orange. •Consider a system comprised of both the orange and the apple -The apple is no longer external to the system. -Force pair is internal to system, which doesn't cause acceleration. -Action and reaction within the system cancel. -With no external forces, there is no acceleration of system. •Consider the same system, but with external force of friction on it. -Same internal action and reaction forces (between the orange and apple) cancel. -A second pair of action-reaction forces (between the apple's feet and the floor) exists. -One of these acts by the system (apple on the floor) and the other acts on the system (floor on the apple). -External frictional force of floor pushes on the system, which accelerates. -Second pair of action and reaction forces do not cancel.
work
Work •involves force and distance. •is force ´ distance. •in equation form: W = Fd. • Two things occur whenever work is done: •application of force •movement of something by that force Examples: •Twice as much work is done in lifting 2 loads 1 story high versus lifting 1 load the same vertical distance. Reason: force needed to lift twice the load is twice as much. •Twice as much work is done in lifting a load 2 stories instead of 1 story. Reason: distance is twice as great Example: •a weightlifter raising a barbell from the floor does work on the barbell. Unit of work: newton-meter (Nm) or joule (J)
In an ideal pulley system, a woman lifts a 100-N crate by pulling a rope downward with a force of 25 N. For every 1-meter length of rope she pulls downward, the crate rises A. 50 centimeters. B.45 centimeters. C.25 centimeters. D.None of the above.
Work in = work out; Fd in = Fd out. One-fourth of 1 m = 25 cm. answer is c
work-energy theorem
Work-energy theorem •Gain or reduction of energy is the result of work. •In equation form: work = change in kinetic energy (W = DKE). •Doubling speed of an object requires 4 times the work. •Applies to decreasing speed: -reducing the speed of an object or bringing it to a halt Example: Applying the brakes to slow a moving car, work is done on it (the friction force supplied by the brakes x distance).
When you step off a curb, Earth pulls you downward and you pull the force upward. Why do you not sense Earth moving upward toward you? a.Earth is fixed, so it cannot move. B.Earth can move, but other objects on it prevent it from moving. C.It moves, but a very small amount that you cannot see. D.None of the above.
You exert a force on Earth that is equal to the force it exerts on you. But you move more than the Earth does, because its mass is so great compared to your mass that it moves very little and you do not notice it. answer is C
A certain machine is 30% efficient. This means the machine will convert A. 30% of the energy input to useful work—70% of the energy input will be wasted. B.70% of the energy input to useful work—30% of the energy input will be wasted. C.Both of the above. None of the above
answer is A
As the skydiver falls faster and faster through the air, air resistance A.increases. B.decreases. C.remains the same. D.Not enough information.
answer is A
Does a car hoisted for repairs in a service station have increased potential energy relative to the floor? A. Yes B.No C.Sometimes D.Not enough information
answer is A
If the mass of an object is halved, the weight of the object is A.halved. B.twice. C.depends on location. D.None of the above.
answer is A
A 5-kg iron ball and a 10-kg iron ball are dropped from rest. For negligible air resistance, the acceleration of the heavier ball will be A. less. B.the same. C.more. D.undetermined.
answer is B
A 5-kg iron ball and a 10-kg iron ball are dropped from rest. When the free-falling 5-kg ball reaches a speed of 10 m/s, the speed of the free-falling 10-kg ball is A. less than 10 m/s. B.10 m/s. C.more than 10 m/s. D.undetermined.
answer is B
A soccer player kicks a ball with 1500 N of force. The ball exerts a reaction force against the player's foot of a.somewhat less than 1500 N. B.1500 N. C.somewhat more than 1500 N. D.None of the above.
answer is B
As the skydiver continues to fall faster and faster through the air, her acceleration A.increases. B.decreases. C.remains the same. D.Not enough information.
answer is B
As the skydiver continues to fall faster and faster through the air, net force A.increases. B.decreases. C.remains the same. D.Not enough information.
answer is B
Freight car A is moving toward identical freight car B that is at rest. When they collide, both freight cars couple together. Compared with the initial speed of freight car A, the speed of the coupled freight cars is A. the same. B.half. C.twice. D.None of the above.
answer is B
When lift is greater, the helicopter a.climbs down. B.climbs up. C.hovers in midair. D.None of the above.
answer is B
When the force that produces an impulse acts for twice as much time, the impulse is not changed. B.doubled. C.quadrupled. D.halved.
answer is B
When the speed of an object is doubled, its momentum a.remains unchanged in accord with the conservation of momentum. B.doubles. C.quadruples. D.decreases.
answer is B
You do work when pushing a cart with a constant force. If you push the cart twice as far, then the work you do is A. less than twice as much. B.twice as much. C.more than twice as much. D.zero.
answer is B
a bird flies by a.flapping its wings. B.pushing air down so that the air pushes it upward. C.hovering in midair. D.inhaling and exhaling air.
answer is B
Consider a high-speed bus colliding head-on with an innocent bug. The force of impact splatters the unfortunate bug over the windshield.Which is greater, the force on the bug or the force on the bus? a.Bug B.Bus C.Both are the same. D.Cannot say
answer is C
When a cannon is fired, the accelerations of the cannon and cannonball are different because the a.forces don't occur at the same time. B.forces, although theoretically the same, in practice are not. C.masses are different. D.ratios of force to mass are the same.
answer is C
When lift equals the weight of a helicopter, the helicopter a.climbs down. B.climbs up. C.hovers in midair. D.None of the above.
answer is C
When the air is removed by a vacuum pump and the coin and feather activity is repeated, a.the feather hits the bottom first, before the coin hits. B.the coin hits the bottom first, before the feather hits. C.both the coin and feather drop together side-by-side. D.Not enough information.
answer is C
When you step off a curb, Earth pulls you downward. The reaction to this force is a. slight air resistance. B.nonexistent in this case. C.you pulling Earth upward. D.None of the above.
answer is C
a moving object has A. momentum. B.energy. C.speed. All of the above
answer is D
If you push against a stationary brick wall for several minutes, you do no work a. on the wall. B.at all. C.Both of the above. D.None of the above.
answer is a
A job can be done slowly or quickly. Both may require the same amount of work, but different amounts of A. energy. B.momentum. C.power. D.impulse.
power is the rate at which work is done. answer is c
velocity
•A description of -the instantaneous speed of the object -what direction the object is moving •Velocity is a vector quantity. It has -magnitude: instantaneous speed -direction: direction of object's motion
force causes acceleration
•Acceleration depends on the net force. •Acceleration is directly proportional to net force. •To increase the acceleration of an object, you must increase the net force acting on it. Acceleration ~ net force
centrifugal force
•Although centripetal force is center directed, an occupant inside a rotating system seems to experience an outward force. This apparent outward force is called centrifugal force. • •Centrifugal means "center-fleeing" or "away from the center." •It is a common misconception that a centrifugal force pulls outward on an object. •Example: -If the string breaks, the object doesn't move radially outward. -It continues along its tangent straight-line path—because no force acts on it. (Newton's first law)
rotational inertia
•An object rotating about an axis tends to remain rotating about the same axis at the same rotational speed unless interfered with by some external influence. • •The property of an object to resist changes in its rotational state of motion is called rotational inertia (symbol I). Depends upon •mass of object. •distribution of mass around axis of rotation. -The greater the distance between an object's mass concentration and the axis, the greater the rotational inertia. •The greater the rotational inertia, the harder it is to change its rotational state. -A tightrope walker carries a long pole that has a high rotational inertia, so it does not easily rotate. -Keeps the tightrope walker stable. Depends upon the axis around which it rotates •Easier to rotate pencil around an axis passing through it. •Harder to rotate it around vertical axis passing through center. •Hardest to rotate it around vertical axis passing through the end. The rotational inertia depends upon the shape of the object and its rotational axis.
centripetal force
•Any force directed toward a fixed center is called a centripetal force. •Centripetal means "center-seeking" or "toward the center." Example: To whirl a tin can at the end of a string, you pull the string toward the center and exert a centripetal force to keep the can moving in a circle. •Depends upon -mass of object. -tangential speed of the object. -radius of the circle. - •In equation form: (mass x tangential speed^2)/ radius •When a car rounds a curve, the centripetal force prevents it from skidding off the road. •If the road is wet, or if the car is going too fast, the centripetal force is insufficient to prevent skidding off the road.
energy for life
•Body is a machine, so it needs energy. •Our cells feed on hydrocarbons that release energy when they react with oxygen (like gasoline burned in an automobile). •There is more energy stored in the food than in the products after metabolism.
center of mass and center of gravity
•Center of mass is the average position of all the mass that makes up the object. • •Center of gravity (CG) is the average position of weight distribution. -Since weight and mass are proportional, center of gravity and center of mass usually refer to the same point of an object. To determine the center of gravity, -suspend the object from a point and draw a vertical line from suspension point. -repeat after suspending from another point. •The center of gravity lies where the two lines intersect.
An automobile is accelerating when it is A.slowing down to a stop. B.rounding a curve at a steady speed. C.Both of the above. D.Neither of the above.
•Change in speed (increase or decrease) is acceleration, so slowing is acceleration. •Change in direction is acceleration (even if speed stays the same), so rounding a curve is acceleration. Answer is C
speed and velocity
•Constant speed is steady speed, neither speeding up nor slowing down. •Constant velocity is -constant speed and -constant direction (straight-line path with no acceleration). - Motion is relative to Earth, unless otherwise stated.
speed
•Defined as the distance covered per amount of travel time. •Units are meters per second. •In equation form: Example: A girl runs 4 meters in 2 sec. Her speed is 2 m/s.
kinetic energy
•Energy of motion •Depends on the mass of the object and square of its speed •Include the proportional constant 1/2 and kinetic energy = 1/2 ´ mass ´ speed ´ speed •If object speed is doubled Þ kinetic energy is quadrupled. Kinetic energy and work of a moving object •Equal to the work required to bring it from rest to that speed, or the work the object can do while being brought to rest •In equation form: net force x distance = kinetic energy, or Fd = 1/2 mv2
mass and weight
•Mass: The quantity of matter in an object. It is also the measure of the inertia or sluggishness that an object exhibits in response to any effort made to start it, stop it, or change its state of motion in any way. • •Weight: The force upon an object due to gravity. Mass •A measure of the inertia of a material object •Independent of gravity Greater inertia Þ greater mass •Unit of measurement is the kilogram (kg) Weight •The force on an object due to gravity •Scientific unit of force is the newton (N) •Unit is also the pound (lb) Mass and weight in everyday conversation are interchangeable. Mass, however, is different and more fundamental than weight. Mass versus weight •on the Moon and Earth: -Weight of an object on the Moon is less than on Earth. -Mass of an object is the same in both locations. 1 kilogram weighs 10 newtons (9.8 newtons to be precise). Relationship between kilograms and pounds: •1 kg = 2.2 lb = 10 N at Earth's surface •1 lb = 4.45 N
impulse
•Product of force and time (force ´ time) •In equation form: Impulse = Ft Example: •A brief force applied over a short time interval produces a smaller change in momentum than the same force applied over a longer time interval. or •If you push with the same force for twice the time, you impart twice the impulse and produce twice the change in momentum.
recycled energy
•Re-employment of energy that otherwise would be wasted. • •Edison used heat from his power plant in New York City to heat buildings. • •Typical power plants waste about 30% of their energy to heat because they are built away from buildings and other places that use heat.
circular motion - rotational speed
•Rotational (angular) speed is the number of rotations or revolutions per unit of time(symbol w). •All parts of a rigid merry-go-round or turntable turn about the axis of rotation in the same amount of time. So, all parts have the same rotational speed tangential speed = radial distance x rotational speed v=rw
more complicated collisions
•Sometimes the colliding objects are not moving in the same straight line. •In this case you create a parallelogram of the vectors describing each initial momentum to find the combined momentum. •Example: collision of two cars at a corner Another example: A firecracker exploding; the total momentum of the pieces after the explosion can be added vectorially to get the initial momentum of the firecracker before it exploded.
average speed
•The entire distance covered divided by the total travel time -Doesn't indicate various instantaneous speeds along the way. Example: Drive a distance of 200 km in 2 h and your average speed is 100 km/h.
resultant
•The sum of two or more vectors -For vectors in the same direction, add arithmetically. -For vectors in opposite directions, subtract arithmetically. -Two vectors that don't act in the same or opposite direction: •use parallelogram rule. -Two vectors at right angles to each other •use Pythagorean Theorem: R2 = V2 + H2.
torque
•The tendency of a force to cause rotation is called torque. • •Torque depends upon three factors: -Magnitude of the force -The direction in which it acts -The point at which it is applied on the object •The equation for Torque is •The lever arm depends upon -where the force is applied. -the direction in which it acts.
Consider a heavy and light person jumping together with same-size parachutes from the same altitude. Who will reach the ground first? A.The light person. B.The heavy person. C.Both will reach at the same time. D.Not enough information.
•They both have the same drag force (for the same speed). •The man (heavier) has a greater downward force than the woman (lighter). •The man has to drop farther to receive drag force equal to his downward force, so a higher terminal velocity. answer is B
Acceleration and velocity are actually A.the same. B.rates but for different quantities. C.the same when direction is not a factor. D.the same when an object is freely falling.
•Velocity is the rate at which distance changes over time, •Acceleration is the rate at which velocity changes over time. Answer is B
circular motion
•When an object turns about an internal axis, it is undergoing circular motion or rotation. • •Circular Motion is characterized by two kinds of speeds: -tangential (or linear) speed. -rotational (or circular) speed.
When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read A.your weight. B.half your weight. C.zero. D.more than your weight.
•You are at rest, so SF=0. •Forces from both scales add to cancel your weight. •Force from each scale is one-half your weight
momentum
•a property of moving things •means inertia in motion •more specifically, mass of an object multiplied by its velocity •in equation form: Momentum = mass ´ velocity Example: •A moving boulder has more momentum than a stone rolling at the same speed. •A fast boulder has more momentum than a slow boulder. •A boulder at rest has no momentum.
the force of friction
•depends on the kinds of material and how much they are pressed together. •is due to tiny surface bumps and to "stickiness" of the atoms on a material's surface. Example: Friction between a crate on a smooth wooden floor is less than that on a rough floor.
