PART I: MULTIPLE CHOICE QUESTIONS
Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on this examination.
Problem 1:
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
(A) equal to atmospheric pressure.
(B) lower than atmospheric pressure, but more than zero.
(C) exactly zero.
(D) higher than atmospheric pressure.
Answer: (D) higher than atmospheric pressure.
Problem 2:
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 doesn't accelerate. That's because
(A) the racket's center of mass accelerates backward while its handle rotates forward and the two motions cancel one another at the handle.
(B) an impact at the center of percussion transfers no momentum to the racket and doesn't cause the racket to accelerate.
(C) the racket's velocity doesn't change when the ball hits its center of percussion.
(D) an impact at the center of percussion exerts no torque about the racket's center of mass and doesn't cause the racket to undergo angular acceleration.
Answer: (A) the racket's center of mass accelerates backward while its handle rotates forward and the two motions cancel one another at the handle.
Problem 3:
As a wave passes by it, a stick that was floating motionless in the water will
(A) begin to move backward, opposite the wave's direction of travel.
(B) begin to move forward with the wave, at the speed of the moving wave crest.
(C) travel in a circular motion and then end up almost motionless again.
(D) travel directly upward and downward, returning eventually to its original position.
Answer: (C) travel in a circular motion and then end up almost motionless again.
Problem 4:
A boy throws a steel ball straight up. Disregarding any effects of air resistance, the force(s) acting on the ball until it returns to the ground is (are)
(A) a steadily decreasing upward force from the moment it leaves the hand until it reaches its highest point beyond which there is a steadily increasing downward force of gravity as the object gets closer to the earth.
(B) a constant downward force of gravity along with an upward force that steadily decreases until the ball reaches its highest point, after which there is only the constant downward force of gravity only.
(C) a constant downward force of gravity only.
(D) its weight vertically downward along with a steadily decreasing upward force.
Answer: (C) a constant downward force of gravity only.
Problem 5:
A bicycle is remarkably stable while it's moving forward because, when you start to fall over,
(A) the bicycle's base of support becomes much wider so that a torque from the ground returns it to an upright orientation.
(B) your center of mass naturally descends.
(C) your center of mass naturally rises.
(D) the bicycle naturally steers so that its wheels move under your center of mass.
Answer: (D) the bicycle naturally steers so that its wheels move under your center of mass.
Problem 6:
When a rubber ball dropped from rest bounces off the floor, its direction of motion is reversed because
(A) energy of the ball is conserved.
(B) the floor is in the way and the ball has to keep moving.
(C) momentum of the ball is conserved.
(D) the floor exerts a force on the ball that stops its fall and then drives it upward.
Answer: (D) the floor exerts a force on the ball that stops its fall and then drives it upward.
Problem 7:
Pat and Minnie are sitting at opposite ends of a seesaw. Pat is heavier than Minnie, so Minnie is suspend motionless high above the ground. Out of frustration, Minnie puts her feet on the seat, bends her knees, and then pushes downward with all her might. The seesaw rotates and Pat rises off the ground. As she pushes downward, Minnie
(A) accelerates away from the seesaw's pivot.
(B) accelerates upward.
(C) remains motionless.
(D) accelerates downward.
Answer: (B) accelerates upward.
Problem 8:
When the tire of your bicycle or car has too little air in it, it flattens out and a larger portion of its surface touches the ground. Where the flattened tire touches the ground, it exerts a downward pressure (force per area) that is
(A) the same as would be exerted by a properly inflated tire.
(B) less than would be exerted by a properly inflated tire, but not zero.
(C) zero.
(D) more than would be exerted by a properly inflated tire.
Answer: (B) less than would be exerted by a properly inflated tire, but not zero.
Problem 9:
You are swinging gently on a playground swing that is hanging by chains from a beam about 4 meters overhead. The swing moves through one complete cycle (over and back) every 4 seconds. If you want to shorten that time, you should
(A) swing less vigorously back and forth, so that the swing's amplitude of motion decreases and its period of motion decreases.
(B) shorten the swing's chains.
(C) swing more vigorously back and forth, so that the swing's amplitude of motion increases and its period of motion decreases.
(D) lengthen the swing's chains.
Answer: (B) shorten the swing's chains.
Problem 10:
Two steel balls, one of which weighs twice as much as the other, roll off of a horizontal table with the same speeds. In this situation,
(A) the heavier ball hits considerably closer to the base of the table than the lighter, but not necessarily half the horizontal distance.
(B) the lighter ball impacts the floor at about half the horizontal distance from the base of the table than does the heavier.
(C) both balls impact the floor at approximately the same horizontal distance from the base of the table.
(D) the heavier ball impacts the floor at about half the horizontal distance from the base of the table than does the lighter.
Answer: (C) both balls impact the floor at approximately the same horizontal distance from the base of the table.
Problem 11:
You fill a metal pot completely with cold water, so that the water's surface is right at the lip of the pot. When you begin heating the pot on the stove, it soon overflows because
(A) metal contracts as its temperature increases while water maintains the same volume.
(B) water expands more as its temperature increases than metal does.
(C) metal contracts as its temperature increases while water expands.
(D) water expands as its temperature increases while metal does not.
Answer: (B) water expands more as its temperature increases than metal does.
Problem 12:
On a humid summer day, perspiration doesn't cool you off much because
(A) the air density is extremely low and its pressure is too high to permit water to evaporate.
(B) the water vapor in the air is moving too fast to condense on your skin as perspiration.
(C) the air density is extremely low and its pressure is too low to permit water to evaporate.
(D) the air is almost saturated with water vapor and there is almost no evaporation.
Answer: (D) the air is almost saturated with water vapor and there is almost no evaporation.
Problem 13:
Mike and Johnny are in trouble for scuffling in the school cafeteria. While Mike admits that he pushed Johnny, who immediately fell over backward, Mike claims that Johnny pushed back and is thus just as guilty. From the perspective of physics,
(A) Johnny did push back on Mike, with exactly the same amount of force.
(B) Johnny didn't push back on Mike.
(C) Johnny pushed back on Mike, but with more force than Mike exerted on him.
(D) Johnny pushed back on Mike, but with less force than Mike exerted on him.
Answer: (A) Johnny did push back on Mike, with exactly the same amount of force.
Problem 14:
Copper conducts heat better than plastic because
(A) Copper atoms can move about and carry heat via convection. Plastic molecules are too long to move about and do not undergo convection.
(B) Copper atoms vibrate more rapidly than the atoms in plastic, so the copper atoms transfer thermal energy better in a bucket brigade.
(C) plastic is transparent and does not emit or absorb radiation well. Copper is not transparent and radiation carries heat through it quickly.
(D) copper has mobile electrons that can carry heat through it quickly, while plastic has none.
Answer: (D) copper has mobile electrons that can carry heat through it quickly, while plastic has none.
Problem 15:
Your shopping cart is rolling frictionlessly down the aisle of a grocery store. You pick up a large can of candied brussels sprouts (your favorite treat, particularly on ice cream) and drop them straight down into the cart. As a result, the cart
(A) retains exactly the same forward velocity because velocity is conserved.
(B) retains exactly the same forward velocity because only its weight has increased.
(C) slows down because its momentum is distributed over more mass.
(D) speeds up because its mass has increased.
Answer: (C) slows down because its momentum is distributed over more mass.
Problem 16:
Which has the higher temperature: a glass of water and ice that's outside on a hot summer day or a glass of water and ice that's outside on a cold winter night?
(A) They both have essentially the same temperature.
(B) The winter glass has a higher temperature.
(C) The summer glass has a higher temperature.
(D) It depends on the amount of moisture in the air.
Answer: (A) They both have essentially the same temperature.
Problem 17:
You are paddling yourself forward in a Kayak, across a calm lake in the mountains. 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
(A) sideways, toward you and the boat.
(B) in the forward direction, toward the front of boat.
(C) in the backward direction, toward the back of the boat.
(D) sideways, away from you and the boat.
Answer: (B) in the forward direction, toward the front of boat.
Problem 18:
A commercial jetliner must compress the outside air before circulating it in the cabin. But the compressed air must first be sent through an air conditioner because compressing the air
(A) turns it into a liquid and the air conditioner's evaporator allows it to return to a gas.
(B) increases its humidity.
(C) decreases its humidity.
(D) makes its temperature rise.
Answer: (D) makes its temperature rise.
Problem 19:
You are bouncing up and down gently on a trampoline, without actually leaving its surface. Each bounce takes 0.5 seconds. If your friend joins you on the trampoline, each bounce will take
(A) more than 0.5 seconds.
(B) less than 0.5 seconds.
(C) 0.5 seconds because the restoring force remains the same.
(D) 0.5 seconds because the mass that is moving remains the same.
Answer: (A) more than 0.5 seconds.
Problem 20:
There is energy in a water wave and this energy is found in
(A) the water's kinetic energy.
(B) the water's elastic potential energy.
(C) the water's gravitational potential energy.
(D) both the water's gravitational potential energy and its kinetic energy.
Answer: (D) both the water's gravitational potential energy and its kinetic energy.
Problem 21:
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 into the turn keeps you from flipping over because the overall force that the ground exerts on the wheel
(A) points directly upward so that you don't fall downward.
(B) points exactly at right angles to your center of mass and exerts a torque on you that keeps you from falling over.
(C) points directly at your center of mass and exerts no torque on you about that center.
(D) is exactly zero and causes no acceleration of your center of mass.
Answer: (C) points directly at your center of mass and exerts no torque on you about that center.
Problem 22:
The tides are caused by
(A) Centrifugal "force" due to the rotation of the earth.
(B) the gravitational forces of the sun and the moon on the earth's oceans.
(C) Sunlight striking the earth's surface.
(D) the angular momentum of the earth orbiting the sun and the moon orbiting the earth.
Answer: (B) the gravitational forces of the sun and the moon on the earth's oceans.
Problem 23:
Heat can flow spontaneously from a warm hot tub to a cool swimming pool because this heat transfer causes
(A) the total entropy of the two objects to decrease.
(B) the total entropy of the two objects to increase.
(C) the entropy of the cooler object (the pool) to decrease.
(D) the entropy of the warmer object (the hot tub) to increase.
Answer: (B) the total entropy of the two objects to increase.
Problem 24:
If the sun suddenly stopped exerting gravitational forces on the planets, each planet would
(A) move in a straight line.
(B) would fall into the sun.
(C) stop moving since there is no gravity acting on it.
(D) would continue to orbit the sun because of its inertia.
Answer: (A) move in a straight line.
Problem 25:
During the winter, a heat pump transfers heat from the outdoor air to the indoor air. If you put your hands near the evaporator coils of the heat pump's outdoor portion, you'll find that the air there is
(A) hotter than the surrounding air.
(B) missing, because it's been sucked into the heat pump.
(C) at the same temperature as the surrounding air.
(D) colder than the surrounding air.
Answer: (D) colder than the surrounding air.
Problem 26:
If you place a window air conditioning unit on a table in the middle of your sealed room, plug it in, and turn it on, you will find that the average temperature of your room will
(A) rise slightly for about one minute, then fall dramatically as the unit begins to function.
(B) Remain the same.
(C) rise.
(D) fall.
Answer: (C) rise.
Problem 27:
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?
(A) The hammer is pushing harder on the nail than the nail is pushing on the hammer.
(B) They are both exerting the same amount of force on one another.
(C) The nail is pushing harder on the hammer than the hammer is pushing on the nail.
(D) The nail isn't pushing on the hammer at all, which is why the hammer pushes the nail into the wood.
Answer: (B) They are both exerting the same amount of force on one another.
Problem 28:
Goose down is the soft, fluffy feathers near the animal's skin. The reason that a down jacket keeps you so warm in the winter is that
(A) the fine structure of the down traps air and prevents it from undergoing convection.
(B) the fibrous material in down is a poorer conductor of heat than air is.
(C) down's white coloring assists the radiative transfer of heat from the outer surface of the coat to your skin.
(D) down's white coloring impedes the radiative transfer of heat from the outer surface of the coat to your skin.
Answer: (A) the fine structure of the down traps air and prevents it from undergoing convection.
Problem 29:
During a brown out, the electric power company drops the voltage of the power line and the incandescent light bulbs in your house become dimmer. When this happens, the filaments in the bulbs operate at a lower than normal temperature, so
(A) their color is the same as normal but is simply less bright.
(B) their color is somewhat redder than normal.
(C) their color is somewhat bluer than normal.
(D) that they emit ultraviolet light.
Answer: (B) their color is somewhat redder than normal.
Problem 30:
If you try to cook vegetables with 100° C air, it takes a very long time. But if you cook those same vegetables with 100° C steam, they cook quickly. This is because the steam
(A) condenses on the colder vegetables and absorbs a large amount of heat from the vegetables.
(B) causes moisture inside the vegetables to boil and transfer heat to the vegetables.
(C) condenses on the colder vegetables and releases a large amount of heat to the vegetables.
(D) causes moisture inside the vegetables to boil and absorb heat from the vegetables.
Answer: (C) condenses on the colder vegetables and releases a large amount of heat to the vegetables.
Problem 31:
Just mixing gasoline and air together won't make them burst into flames because
(A) their densities must be increased considerably before they will ignite.
(B) something must provide the activation energy needed to initiate the chemical reaction.
(C) they must be compressed into liquid before they can burn.
(D) their velocities must be increased considerably before they will ignite.
Answer: (B) something must provide the activation energy needed to initiate the chemical reaction.
Problem 32:
The strings of a guitar vibrate at different frequencies. They have different thicknesses because
(A) a thicker string has more tension than a thinner string and vibrates more quickly.
(B) a thinner string has more tension than a thicker string and vibrates more quickly.
(C) a thicker, more massive string vibrates more slowly than a thinner string of the same length and tension.
(D) a thinner, less massive string vibrates more slowly than a thicker string of the same length and tension.
Answer: (C) a thicker, more massive string vibrates more slowly than a thinner string of the same length and tension.
Problem 33:
When a violinist first bows a violin string, the sound begins softly and rises in volume. That's because
(A) the string's frequency or pitch starts too low to hear and then gradually shifts into the audible range.
(B) bowing transfers energy into the string slowly, a little bit each cycle of the string.
(C) the string's frequency or pitch starts too high to hear and then gradually shifts into the audible range.
(D) it takes time for the ends of the string to begin moving at the same velocity as the bow.
Answer: (B) bowing transfers energy into the string slowly, a little bit each cycle of the string.
Problem 34:
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
(A) wider.
(B) shorter
(C) thinner.
(D) taller.
Answer: (D) taller.
Problem 35:
You must move 100 baseballs from the lower shelf of a display to the upper shelf. Will you do more total work on the 100 baseball by raising them one at a time or by raising them all at once?
(A) You will do the same total work on the baseballs either way.
(B) The answer depends on how far you move the baseballs horizontally before putting them on the upper shelf.
(C) You will do more total work on them by raising them all at once.
(D) You will do more total work on them by raising them one at a time.
Answer: (A) You will do the same total work on the baseballs either way.
Problem 36:
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
(A) the propane burner located below the opening keeps pushing the hot air back into the balloon.
(B) hot air has more inertia than cold air and doesn't accelerate easily.
(C) hot air has a lower pressure than cold air, so hot air is drawn into the balloon by the partial vacuum inside it.
(D) the air pressure inside the balloon's opening is the same as the air pressure outside that opening.
Answer: (D) the air pressure inside the balloon's opening is the same as the air pressure outside that opening.
Problem 37:
The giant waves or tsunamis created by earthquakes have extremely long wavelengths. They travel
(A) extremely slowly because they involve only the water near the ocean's surface.
(B) extremely quickly because they involve only the water near the ocean's surface.
(C) extremely slowly and involve water to great depth in the ocean.
(D) extremely quickly and involve water to great depth in the ocean.
Answer: (D) extremely quickly and involve water to great depth in the ocean.
Problem 38:
You wish to lift a very heavy object with a hydraulic ram by pushing down on a second hydraulic ram. The two cylinders of the two rams are connected by a pipe. You only need to lift that heavy object a small distance and must push down on the second ram by hand. The piston lifting the heavy object should be
(A) much shorter than the piston you push down on.
(B) much smaller in diameter than the piston you push down on.
(C) much larger in diameter than the piston you push down on.
(D) much longer than the piston you push down on.
Answer: (C) much larger in diameter than the piston you push down on.
Problem 39:
If you were in a spaceship and fired a cannonball into frictionless, empty space, how much force would have to be exerted on the ball to keep it going after it left your ship?
(A) A force equal to the cannonball's mass.
(B) A force equal to the cannonball's momentum.
(C) A force equal to the cannonball's velocity.
(D) Zero force.
Answer: (D) Zero force.
Problem 40:
If the earth had no atmosphere, its temperature would be
(A) essentially the same as it is now.
(B) almost that of empty space-a few degrees above absolute zero.
(C) somewhat cooler than it is now.
(D) somewhat hotter than it is now.
Answer: (C) somewhat cooler than it is now.
Problem 41:
After clearing the bar in the high jump, you land softly on a giant mattress. Landing on the mattress is much more comfortable than landing on a sand heap of equal size because
(A) you transfer more momentum to the mattress in coming to a stop than you would have transferred to the sand heap in coming to a stop.
(B) the force that the mattress exerts on you to stop your descent is much less than the force that the sand heap would have exerted on you.
(C) your velocity is less as you land on the mattress than it would have been if you'd landed on the sand heap.
(D) you transfer less momentum to the mattress in coming to a stop than you would have transferred to the sand heap in coming to a stop.
Answer: (B) the force that the mattress exerts on you to stop your descent is much less than the force that the sand heap would have exerted on you.
Problem 42:
Once water is heated to 100 °C (212 °F) at sea level,
(A) it begins to boil, but it requires additional energy to convert from a liquid to a gas without changing temperature.
(B) Water molecules first begin to leave the water's surface as a gas.
(C) it begins to boil, but its requires additional energy to convert from a liquid to a gas because it must change temperature to undergo the change of phase.
(D) it immediately turns to steam without the further introduction of energy.
Answer: (A) it begins to boil, but it requires additional energy to convert from a liquid to a gas without changing temperature.
Problem 43:
You're the first person to visit Mars. While discussing your experiences with a group of school children by radio link, one of them asks you how your weight and mass have changed. You take a moment to measure both and reply correctly that
(A) your weight is still essentially unchanged but your mass is less than on earth.
(B) your mass is still essentially unchanged but your weight is less than on earth.
(C) your weight and mass have both changed significantly.
(D) neither your weight nor your mass have changed much.
Answer: (B) your mass is still essentially unchanged but your weight is less than on earth.
Problem 44:
You are riding in a train. While the train is stopped at a station, you begin throwing a ball into the air and catching it. You find that throwing it straight up causes it to return directly to your hand. A few minutes later, the train is moving at a constant speed along straight tracks between stations. You again toss the ball up and down and find that the ball will return directly to your hand if you throw it
(A) a bit toward the front of the train, as viewed from your new moving reference frame.
(B) a bit toward the back of the train, as viewed from your new moving reference frame.
(C) a bit toward the left side of the train, as viewed from your new moving reference frame.
(D) straight up, as viewed from your new moving reference frame.
Answer: (D) straight up, as viewed from your new moving reference frame.
Problem 45:
The second harmonic vibration of a violin string, in which the string vibrates as two half-strings, has
(A) the same frequency or pitch of the string's fundamental vibration, but twice the amplitude of vibration.
(B) a frequency or pitch half that of the string's fundamental vibration.
(C) the same frequency or pitch of the string's fundamental vibration, but half the amplitude of vibration.
(D) a frequency or pitch twice that of the string's fundamental vibration.
Answer: (D) a frequency or pitch twice that of the string's fundamental vibration.
Problem 46:
A toy top is spinning steadily on a table, with only its sharp point touching the table's surface. What is keeping it spinning?
(A) A torque about the top's point, produced by the top's weight.
(B) No torque at all, just its own angular momentum.
(C) A torque about its axis of rotation, produced by the force of friction on the top's point.
(D) A torque about its center of mass, produced by the support force that the table exerts on the top's point.
Answer: (B) No torque at all, just its own angular momentum.
Problem 47:
Tightening the strings of a guitar
(A) raises their frequency or pitch because it softens the restoring force.
(B) lowers their frequency or pitch because it stiffens the restoring force.
(C) lowers their frequency or pitch because it softens the restoring force.
(D) raises their frequency or pitch because it stiffens the restoring force.
Answer: (D) raises their frequency or pitch because it stiffens the restoring force.
Problem 48:
When you jump while standing on a bathroom scale, it briefly reads more than your actual weight. During that moment, it's exerting an upward force on you that is greater than your weight and
(A) your velocity is constant but downward.
(B) you are accelerating upward.
(C) your velocity is constant but upward.
(D) you are accelerating downward.
Answer: (B) you are accelerating upward.
Problem 49:
When you sit facing a campfire outdoors, your face feels quite warm because
(A) convection from the fire is carrying hot air sideways to your face.
(B) microwaves from the moving charged particles in the flames heat your skin.
(C) the campfire is radiating heat toward your skin.
(D) conduction in the air is carrying heat from the flames to your skin.
Answer: (C) the campfire is radiating heat toward your skin.
Problem 50:
A large box is being pushed across the floor at a constant speed of 4.0 m/s. What can you conclude about the forces acting on the box?
(A) The amount of force applied to move the box at a constant speed must be more than its weight.
(B) If the force applied to the box is doubled, the constant speed of the box will increase to 8.0 m/s.
(C) The amount of force applied to move the box at a constant speed must be equal to the amount of the frictional forces that resist its motion.
(D) The amount of force applied to move the box at a constant speed must be more than the amount of the frictional forces that resist its motion.
Answer: (C) The amount of force applied to move the box at a constant speed must be equal to the amount of the frictional forces that resist its motion.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Part II is worth 33% of the grade on this examination.
Problem 1:
Since firefighter use water to put out fires, handling that water is half their job. That job becomes particularly difficult when they are fighting a fire in a high-rise building.
(A) The water pressure in a fire hydrant is about 500,000 pascals (about 5 times atmospheric pressure). When the firefighters attached a hose directly to a fire hydrant, they find that water won't flow out of the hose above the 10th floor of the building, located about 50 meters above the sidewalk. Why won't the water flow?
Answer: The water pressure at ground level can't support a column of water taller than 50 meters.
(B) A spectator suggests that the firefighters return to the 8th floor, where water does flow from the hose, and attempt to spray water up the stairwell to the 11th floor. Explain why this strategy will or will not work.
Answer: The water's total energy isn't large enough to lift it more than 50 meters above the ground, regardless of how you try to get it up higher.
(C) The firefighters find that water leaving a hose with a narrow nozzle on it travels faster than water leaving that same hose with no nozzle. Why does narrowing the opening of the hose speed up the water emerging from it?
Answer: Using a narrow nozzle allows the water to move slowly through the hose and speed up only as it sprays out of the nozzle. The slower moving water wastes less energy to friction in the hose.
(D) When water hits hot material in a fire, it turns to steam. This steam takes up much more space than the water did and displaces air from the room. Without oxygen to sustain it, the fire quickly goes out. But the hot material must be much, much hotter than 100° C, water's boiling temperature, for the water that hits it to turn to completely into steam. Why?
Answer: It takes considerable thermal energy to convert water at 100° C into steam at 100° C.
Problem 2:
You're taking a step aerobics class. In front of you is a small platform that you step onto and off of during the course of the exercises. Most of the time, one foot remains stationary on the platform and you use it to lift your body up and down.
(A) As you step up onto the platform, your leg does work on your body. What characteristics of you and the platform determine how much work your leg must do?
Answer: Your weight and the height of the step.
(B) How much work (whether positive, negative, or zero) is your leg doing on your body as it lowers you gently back down to the ground?
Answer: A negative amount of work equal to your weight times the step height.
(C) If you let yourself drop back down to the ground, rather than lowering yourself gently, you may injure the leg you land on. Why does wearing padded shoes reduce your risk of injury?
Answer: The padded shoes allow your momentum to drop to zero more slowly, with smaller forces.
(D) The platforms are all identical and are made of a sturdy plastic that acts like a stiff spring. When you step up onto yours, it distorts downward by about 4 millimeters. You're curious about the weight of the person to your right so you watch the platform as that person steps onto it. It distorts downward by 8 millimeters. How much does that person weigh?
Answer: Twice your weight.
Problem 3:
It's your first day on the staff of the U.S. Patents Office and you're excited about having some new ideas come across your desk.
(A) An inventor comes to you with a small box that is supposed to make batteries obsolete. The inventor claims that the box can produce electricity forever without having to be recharged. You can be sure that this claim is nonsense because
Answer: it can't conserve energy and still provide limitless energy.
(B) Another inventor comes in with a small motor-like device that is powered by a burning candle. The inventor claims that this device takes the heat from the candle and turns it entirely into work. The device thus doesn't warm the room at all. Your knowledge of the laws of thermodynamics assures you that this claim, too, is nonsense because
Answer: Some thermal energy can only be converted to work as heat flows from a hot object to a cold object. Some heat must be delivered to the cold object.
(C) As though the entire population of loser inventors was released on you in one day, another clown comes in claiming to have a heat pump that can transfer heat out of a box of corn flakes for as long as you like. The cereal just gets colder and colder. Once again, you know that this is impossible because
Answer: As the temperature of the cereal approaches absolute zero, this cooling process must slow and eventually stop.
(D) Just when you thought you were out of the woods, someone comes in with a coffee mug that automatically reassembles itself if you accidentally break it. You're supposed to put the broken mug in a box and shake. In a minute or two, the mug will be as good as new. You agree with the inventor that this reconstruction trick doesn't violate any of the laws of motion. However, you can still be sure that it won't work because
Answer: it violates the second law of thermodynamics (i.e. it's incredibly unlikely).
Problem 4:
You have recently arrived on the galactic megacruiser "Hollywood." This enormous spaceship was designed in part by unemployed screenwriters, so it has a few quirks that naturally occur when you ignore the laws of physics and build things according to what makes the stars look their best.
(A) The ship has a huge flight deck from which small sightseeing spacecraft can depart. You watch from the flight office as tourists walk across the flight deck and board one of the little ships. The flight attendant shuts the passengers in and the flight controller immediately opens a huge door between the flight deck and empty space beyond. Suddenly there is a horrendous roar as air rushes out of the flight deck. Why does the air begin moving toward the flight deck doors?
Answer: Air accelerates toward lower pressure.
(B) Think of the air in the flight deck as being in two halves: one nearest to the doors and one farthest from the doors. Use the definition of work to show that the half farthest from the doors does work on the half nearest to the doors.
Answer: The air farthest from the doors pushes the air nearest the doors toward the doors and that air moves toward the doors.
(C) What happens to the temperature of the air farthest from the doors as the doors open?
Answer: That air becomes colder.
(D) Even though it's turned off, the small ship is unceremoniously tossed into space moments after the doors open. What force causes the ship's spaceward acceleration?
Answer: Pressure drag.
Problem 5:
If you extend a ruler off the edge of the table and pluck it with one hand while holding it against the table with the other, the ruler will vibrate up and down.
(A) Why does the ruler's frequency of oscillation decrease when you extend more of it off the table?
Answer: The mass of the oscillating portion becomes larger.
(B) Why doesn't the ruler's frequency of oscillation depend on how hard you pluck it?
Answer: The ruler acts as a harmonic oscillator.
(C) What forms does its energy take as the ruler vibrates up and down?
Answer: Kinetic energy and elastic potential energy (in the bent ruler).
(D) What eventually happens to the ruler's energy?
Answer: It eventually becomes thermal energy.
Problem 6:
You're trying to lift construction materials up to your new tree house, so that you can finish the marble fireplace. You begin lifting a marble slab by pulling it upward with a rope.
(A) Since the slab weighs 1000 newtons and the tension you exert upward is 1000 newtons, the tension in the rope is
Answer: 1000 newtons.
(B) As you lift the slab upward at constant velocity, what forces are acting on the slab?
Answer: Its weight downward and the rope's tension force upward.
(C) The slab is just too heavy for you to lift with a rope. So you attach a multiple pulley to the slab and begin pulling the rope down through the top pulley. There are 5 segments of rope extending between the pulleys. The force you exert on the rope as the slab rises at constant velocity is
Answer: 200 newtons.
(D) The tree house is 10 meters up in a 250 year-old oak tree. To lift the slab up that high, you will have to pull how many meters of rope through the pulleys?
Answer: 50 meters.