ch 3,4,5,7

HW6: People fighting forest fires carry emergency tents that have shiny aluminum outer surfaces. If there is trouble, a fire fighter can lie under the tent to block the heat from burning trees overhead. The tent helps because a) conduction carries heat downward toward the fire fighter and the aluminum tent conducts that heat harmlessly into the ground. b) radiation carries heat downward toward the fire fighter and the aluminum tent reflects most of that radiation. c) convection carries heat downward toward the fire fighter and the aluminum tent blocks most of the heat carried by convection. d) both conduction and radiation carry heat downward toward the fire fighter and the aluminum tent blocks most of that heat.

(B) only radiation carries heat downward toward the fire fighter and the aluminum tent reflects most of that radiation.

HW4 A ball hits a wall going 10 m/s, & bounces off going 7 m/s. The coefficient of restitution is

0.70

3EOCQ If you pull down on the basket of a hanging grocery store scale so that it reads 15 N, how much downward force are you exerting on the basket?

15N

HW4 you put apples in a basket of a hanging grocery scale. It is pulled downward 1cm & reports a weight of 4N. If you pull downward on the basket 4cm., what weight will it report?

16N

HW5 Escape velocity at the surface of the Earth is about

25000 mph

3EOCQ An elastic ball that wastes 30% of the collision energy as heat when it bounces on a hard floor will rebound to 70% of the height from which it was dropped. Explain the 30% loss in height

A 30% loss of rebound height is a 30% loss of gravitational potential energy—equal to the energy that became thermal energy.

3EOCQ The best running tracks have firm but elastic rubber surfaces. How does a lively surface assist a runner?

An elastic track stores energy as it dents when the runner's foot presses against it and then returns that energy to the runner as it undents.

3EOCQ There have been baseball seasons in which so many home runs were hit that people began to suspect that something was wrong with the baseballs. What change in the baseballs would account for them traveling farther than normal?

An increase in the balls' coefficients of restitution = faster it rebounds from a surface for a given collision speed

3ST An object @ equilibrium is always motionless. T/F?

An object @ equilibrium is not accelerating, but its velocity (speed) may not be zero. If it was moving when it reached equilibrium, it will coast through that equilibrium @ constant velocity (speed).

3EOCQ As you wind the mainspring of a mechanical watch or clock, why does the knob get harder and harder to turn?

As the mainspring winds farther and farther from its equilibrium shape, the restoring force it exerts on its end gets stronger. You must overcome this increasing force as you twist the knob.

3EOCQ . Some athletic shoes have inflatable air pockets inside them. These air pockets act like springs that become stiffer as you pump up the air pressure. High pressure also makes you bounce back up off the floor sooner. Why does high pressure shorten the bounce time?

At high pressure, the shoes are stiffer and exert larger forces when distorted. They accelerate more rapidly and bounce faster.

3EOCQ When two trains are traveling side by side at breakneck speed, it's still possible for people to jump from one train to the other. Explain why this can be done safely

Because the trains' relative velocity is zero, a person jumping between them views them both as essentially stationary.

3EOCQ A RIF (reduced injury factor) baseball has the same coefficient of restitution as a normal baseball except that it deforms more severely during a collision. Why does this increased deformability lessen the forces exerted by the ball during a bounce & reduce the chances of its causing injury?

During a bounce, the work done on a RIF ball—to store energy in it—involves a smaller force exerted for a longer distance.

3EOCQ Bumper cars are an amusement park ride in which people drive small electric vehicles around a rink and intentionally bump them into one another. All of the cars travel at about the same speed. Why are head-on collisions more jarring than other types of collisions?

During a head-on collision, the relative velocity is enormous--the sum of the two individual velocities--and the forces, accelerations, and momentum transfers are enormous as well.

3EOCQ During rehabilitation after hand surgery, patients are often asked to squeeze and knead putty to strengthen their muscles. How does the energy transfer in squeezing putty differ from that in squeezing a rubber ball?

Energy transferred while deforming putty is converted into thermal energy and never returns to the person's hand. However, energy transferred while deforming a rubber ball becomes elastic potential energy in the ball and returns to the person's hand when the rubber ball returns to its spherical shape.

HW5 At various times in the news it has been reported that certain vehicles are susceptible to rolling over. The physical mechanism that is responsible for rollovers is

Frictional torques from the road on the veichle about its center of mass.

Inertia is a force which keeps stationary objects at rest and moving objects in motion at constant velocity.

Inertia is NOT a force. (Crap)

Inertia is the tendency of all objects to resist motion and ultimately stop.

Inertia is NOT the tendency to resist motion, but rather to resist changes in the state of motion. For instance, its the tendency of a moving object to keep moving at a constant velocity (or a stationary object to resist changes from its state of rest). (CRAP)

3EOCQ If you put your bathroom scale on a ramp and stand on it, will the weight it reports be high, low, or correct?

Less. The scale only reports how hard it pushes perpendicular to its surface. When you weigh yourself on a ramp, your weight doesn't push directly toward the scale's surface and so the scale doesn't push back directly perpendicular to its surface. The perpendicular component of its force on you is less than you full weight.

HW5 Lead sinkers actually float on the surface of liquid mercury. Which takes up less space: 1 kg of mercury or 1 kg of lead?

Mercury

In a gravity-free environment (should there be one), a person with a lot of inertia would have the same ability to make a turn as a person with a small amount of inertia.

Once more (refer to g), inertia is unaffected by alterations in the gravitational environment. An alteration in the g value effects the weight of an object but not the mass or inertia of the object. (CRAP)

3EOCQ Steep mountain roads often have emergency ramps for trucks with failed brakes. Why are these ramps most effective when they are covered with deep, soft sand?

Sand dents easily as the truck plows through it and extracts energy from the truck. The truck does work in pushing the sand out of the way. The sand converts that work into safe thermal energy.

3 Hooke's Law

Stretching a spring: the restoring force exerting by an elastic object is proportional to how far it has been distortd 4m its equilibrium shape.

3EOCQ When you lie on a spring mattress, it pushes most strongly on the parts of you that stick into it the farthest. Why doesn't it push up evenly on your entire body?

The deeper you dent the mattress's surface at any given point, the stronger the local restoring force becomes.

3EOCQ If you drop a steel marble on a wooden floor, why does the floor receive most of the collision energy and contribute most of the rebound energy?

The marble is stiffer than the floor, so the floor dents more than the marble. Since the forces are equal in magnitude, the floor also has the most work done on it during the collision & is thus responsible for most of the rebound energy.

Fast-moving objects have more inertia than slow-moving objects.

The speed of an object has no impact upon the amount of inertia that it has. Inertia has to do with mass alone. (CRAP)

HW5 For an object that is floating on a fluid

The weight of the fluid displaced equals the object's weight.

3EOCQ Padded soles in running shoes soften the blow of hitting the pavement. Why does padding reduce the forces involved in bringing your foot to rest?

With padding in your shoes, you feet stop over a longer period of time when they encounter the pavement. The momentum transfer or impulse is the same, but the force is smaller while the time is longer.

3EOCQ Why is it so exhausting to run on soft sand?

You do work on the sand as you step on it, but the sand doesn't return this energy to you as you lift your foot back up again.

3EOCQ Your car is on a crowded highway with everyone heading south at about 100 km/h (62 mph). The car ahead of you slows down slightly and your car bumps into it gently. Why is the impact so gentle?

Your relative velocity is small—in your frame of reference the car in front of you is barely moving, so the impact is very gentle

HW6: If you double the radiating surface area of a star, its power output increases by a factor of a) 2 b) 4 c) 8 d) 16

a) 2

HW6: A shiny, white transparent surface prevents heat transfer because a) It has emissivity of zero & reflects all light that strikes it. b) It has emissivity of 1 & reflects all light that strikes it. c) It has emissivity of zero & absorbs all light that strikes it. d) It has emissivity of 1 & absorbs all light that strikes it.

a) It has emissivity of zero & reflects all light that strikes it.

HW6: Which part of a welding flame is the coldest? a) red b) blue c) orange d) yellow

a) red

HW4 Suppose you are standing on a bathroom scale when you are flying in a jet airplane. For a moment the scale reads less than your actual weight. During that moment, it's exerting an upward force on you that is less than your weight & you are

accelerating downward (Ex: feeling light when going ^ on an elevator = accelerating ^; feeling heavy when going down elevator = accelerating down)

HW4 You go through a loop in a roller coaster at a constant speed. Where is your apparent weight a minimum?

at the top

HW6: Which temperature scale has 100 as the boiling point of water? a) Fahrenheit b) Celsius c) Kelvin

b) Celsius

HW6: Housing and natural animal insulation works because a) the insulating materials are poorer heat conductors than air. b) the insulating materials trap air and prevent convection. c) the insulating materials prevent radiation d) the insulating materials reflect radiation

b) The insulation materials trap air and prevent convection.

HW6: Why are good thermal conductors generally good electric conductors also? a) Heat and electricity are the same things. b) The molecules in both don't move around very much. c) Electron motion is responsible for determining both properties. d) The answer depends on the specific metal.

c) Electron motion is responsible for determining both properties.

HW6: Why does an ice cube feel cold? a) Heat from your hand enters the cube by convection. b) Cold from the cube enters your hand by convection. c) Heat from your hand enters the cube by conduction. d) Cold from the cube enters your hand by conduction.

c) Heat from your hand enters the cube by conduction.

as temp increases....

coeff. of expansion increases

3EOCQ There's a bathroom scale on your kitchen table and your friend climbs up to weigh himself on it. One of the table's legs is weak and you're afraid that he'll break it, so you hold up that corner of the table. The table remains level as you push upward on the corner with a force of 100 N. Is the weight reported by the scale high, low, or correct?

correct

HW6: If you double the absolute temperature of a star, its power output increases by a factor of a) 2 b) 4 c) 8 d) 16

d) 16

HW6: A pedestrian bridge crosses a street. This bridge is entirely supported by columns from below. A gap at each end of the bridge separates the bridge's surface from the sidewalks leading to the bridge. The width of each gap changes with time. This width is largest a) on hot days. b) at noon. c) at midnight. d) on cold days.

d) on cold days.

HW4 You're playing w/ your younger cousins & find yourself on all fours, w/ your hands & feet on four bathroom scales on the level ground. (a hand & foot is on ea. scale) You can make a reasonable guess that the reading on ea. scale is obtained by

dividing your weight by 4

work =

force * distance

pressure = air pressure is proportional to =

force/surface area density (m/v) & absolute temp (K)

Eq1.3.2 gravitational potential energy =

height above the ground

Unlike velocity, acceleration, force, & momentum, the kinetic energy of an object is completely described by _________ alone. Like work and potential energy, the standard unit for kinetic energy is ______.

magnitude, Joules

momentum =

mass * velocity

HW4 When a baseball player hits a ball the bat can respond by

rebounding, vibrating, and spinning

HW5 When sitting still on the ground, a tricycle is in a state of

stable static equilibrium

types of electromagnetic waves in an object's thermal radiation depend on its

temp

HW5 You are riding in a hot air balloon. You have not used the burner for some time and the balloon begins to sink. Assuming that no air is allowed to leave or enter the balloon,

the average density of the balloon became greater than the surrounding air and the buoyant force on the balloon became less than its weight.

Explain mechanism of hot air balloon

the hotter the air in the envelope, the lower the density & the less the balloon weighs. Balloon's weight is v. nearly = buoyant force on the balloon. Hence balloon floats! Note: pressure is not a factor in this scenario

HW4 Like a baseball bat, a tennis racket has a sweet spot at its center of percussion. If a tennis ball hits this center of percussion, the racket's handle does not accelerate. This is bc

the racket's center of mass accelerate backwards while its handle rotates forward about its center of mass & the two motions cancel one another at the handle (the ball puts a torque on it that causes it to rotate)

HW5 Suppose you have an aquarium and you notice that you have a solid plastic decoration and a rock of about the same volume but twice as dense, and both are totally submerged in the water. The buoyant force on the plastic object is about ___________as that on the rock.

the same. (The buoyant force only depend on the submerged volume (the amount of displaced water)

HW5 It is important for cyclists to lean their bicycles while negotiating sharp turns because

the torque created by their leaning balances out the frictional torque from the road which would tend to flip them.

HW5 Astronauts in the space shuttle feel weightless because

they are in a constant state of free fall.

HW4 Why do elastic balls bounce so well?

they store energy through compression like a spring

HW4 You are riding an amusement park ride where you are strapped to the inside of a giant metal wheel that is rotating quite rapidly. Your acceleration is

toward the center

Approaching speed:

traveling opposite direction, add #'s

Separating speed:

traveling same direction, subtract #'s

HW4 You are flying on a airplane & you notice that a ball of aluminum foil from lunch starts to roll to the left. From the motion of the foil ball you can conclude that the plane is

turning toward the right (the inertia from the foil keeps it sitting there-it wants to stay there- which means that the plane is turning to the right.)