Physics 105 - How Things Work

Problem 1:

The outside temperature is 0 °F (-18 °C) and there is snow on the ground. Oddly enough, the snow is gradually disappearing into the air. The snow is disappearing because

(A) while water molecules are both landing on and leaving the snow’s surface, they are landing more often than they are leaving.

(B) water molecules release potential energy as they separate into a gas, so the need to reduce total potential energy causes the snow to convert into water vapor.

(C) there is always liquid water present when both solid ice and gaseous water vapor exist. The ice is becoming water and the water is evaporating as water vapor.

(D) while water molecules are both landing on and leaving the snow’s surface, they are leaving more often than they are landing.

Answer: (D) while water molecules are both landing on and leaving the snow’s surface, they are leaving more often than they are landing.

Why: When two phases of matter are present, the action at the interface between the two is very important. If molecules depart one phase for the other more often than they return, the first phase will shrink and the other will grow.

Problem 2:

Copper is a much better conductor of heat than glass because copper

(A) contains mobile electrons while glass does not.

(B) has a reddish color while glass is transparent.

(C) is much softer than glass.

(D) is shiny and reflective while glass is not.

Answer: (A) contains mobile electrons while glass does not.

Why: The same electrons that make copper a good electrical conductor also make it a goo thermal conductor.

Problem 3:

As the wind blows past one of the entry doors to a modern office building, the wind is bending inward—toward the building’s surface. This entry door can swing either inward (into the building) or outward (out of the building). The effect of the wind is to make it

(A) easy to open the door outward and hard to open it inward.

(B) hard to open the door outward and easy to open it inward.

(C) easy to open the door both outward and inward.

(D) hard to open the door both outward and inward.

Answer: (A) easy to open the door outward and hard to open it inward.

Why: The pressure outside the door is below atmospheric because the air is bending toward it. Air accelerates toward low pressure, so the air near the door must be lower than the atmospheric pressure far from the door (atmospheric pressure). With low pressure outside the door and atmospheric pressure inside the building, the door experiences an outward push from the pressure imbalance.

Problem 4:

A stream is flowing gently under a bridge and past a cylindrical support post. The flow around this vertical post is smooth and laminar, so the water separates in front of the post, arcs around its sides, and rejoins behind the post before continuing down the stream. Compared with the average water level in the stream, the water level is

(A) high in front of the post and low on the sides and behind the post.

(B) high in front of and behind the post, and low on the sides of the post.

(C) low in front of the post and high on the sides and behind the post.

(D) low in front of and behind the post, and high on the sides of the post.

Answer: (B) high in front of and behind the post, and low on the sides of the post.

Why: With laminar flow around the post, the water's total energy will be constant along a streamline. If we look at the water in a horizontal slice in the stream, the water will be bending away from the post at its front and back and toward the post on its sides. As with laminar flow around a ball, the pressure will rise in front of and behind the post and will drop on the sides of the post. The elevated pressure in front of and behind the post will support more water, so the level there will be high. The decreases pressure on the sides of the post will support less water, so the level there will be low.

Problem 5:

Two cars are heading northward on a highway. The red car is in front and the blue car is behind it. The blue car gently bumps the red car and the two cars push on one another and exchange momentum. The force that the blue car exerts on the red car is

(A) equal in amount to the force the red car exerts on the blue car and the momentum lost by the blue car is equal to the momentum gained by the red car.

(B) larger in amount than the force the red car exerts on the blue car, but the momentum lost by the blue car is equal to the momentum gained by the red car.

(C) equal in amount to the force the red car exerts on the blue car, but the momentum lost by the blue car is greater than the momentum gained by the red car.

(D) smaller in amount than the force the red car exerts on the blue car, but the momentum lost by the blue car is equal to the momentum gained by the red car.

Answer: (A) equal in amount to the force the red car exerts on the blue car and the momentum lost by the blue car is equal to the momentum gained by the red car.

Why: The two must push on one another with equal but oppositely directed forces, in agreement with Newton's third law. They will therefore give one another equal but oppositely directed impulses and the momentum will be transferred perfectly... no lost or gained momentum.

Problem 6:

While the strings of a violin or guitar are all the same length, the strings in a piano get shorter as their pitch (frequency) increases. The highest frequency strings of a piano are only a few inches long. But suppose the strings of a piano were all the same long length. To compensate from their dramatic increase in length, those highest frequency strings would have to have

(A) more tension and less mass per inch.

(B) more tension and more mass per inch.

(C) less tension and less mass per inch.

(D) less tension and more mass per inch.

Answer: (A) more tension and less mass per inch.

Why: Lengthing the strings will effectively reduce their stiffness and slow their periods. To compensate, the strings must be stiffened by increasing their tensions and the strings' masses must be decreased to lessen their inertia.

Problem 7:

A rocket is heading upward quickly and a plume of exhaust is streaming downward from its tail. The amount of force that the rocket is exerting on the plume of exhaust is

(A) less than the amount of force the plume is exerting on the rocket, but not zero.

(B) greater than the amount of force the plume is exerting on the rocket.

(C) equal to the amount of force the plume is exerting on the rocket.

(D) zero.

Answer: (C) equal to the amount of force the plume is exerting on the rocket.

Why: In accordance with Newton's third law, the force that the plume exerts on the rocket must be equal and opposite to the force that the rocket is exerting on the plume.

Problem 8:

It’s a cold winter day and you are in an unheated cabin in the woods. Everything in the cabin is at the same cold temperature. You find that when you stand in front of a full-length mirror in the bedroom that you feel warmer. That is because the mirror

(A) emits more of its own thermal radiation at you than any other surface in the house.

(B) concentrates the thermal radiation from the rest of the bedroom and focuses it on you.

(C) reflects your own thermal radiation back at you, so you lose heat more slowly.

(D) emits less of its own thermal radiation at you than any other surface in the house.

Answer: (C) reflects your own thermal radiation back at you, so you lose heat more slowly.

Why: The whole room has a single, cold temperature so its thermal radiation is relatively cold. The mirror is particularly bad at emitting its own thermal radiation and mostly reflects the thermal radiation that hits it. When you stand in front of the mirror, your own thermal radiation bounces back at you. Since you're the warmest thing around, having your own radiation come back it you is a welcome treat and helps keep you warmer.

Problem 9:

A single violin string can vibrate in several different ways, as though it were several separate harmonic oscillators. Each of these vibrational modes has its own frequency. If the frequency of the fundamental (lowest frequency) vibrational mode is 440 Hz (Hertz or cycles-per-second), then the frequencies of the string’s overtones are

(A) 880 Hz, 1320 Hz, 1760 Hz, and so on.

(B) 880 Hz, 1760 Hz, 3520 Hz, and so on.

(C) 550 Hz, 660 Hz, 770 Hz, and so on.

(D) 660 Hz, 880 Hz, 1100 Hz, and so on.

Answer: (A) 880 Hz, 1320 Hz, 1760 Hz, and so on.

Why: A string has harmonics at all the integer multiples of the fundamental frequency. The fundamental's frequency is 440 Hz, so the second harmonic's frequency is 2 * 440 Hz or 880 Hz. The third harmonic's frequency is 3 * 440 Hz or 1320 Hz, and so on.

Problem 10:

Gliders often have very long wings so that they contact lots of air each second. These long wings enable the glider to stay aloft longer because it can then give the air it contacts

(A) large amounts of downward momentum and energy.

(B) a large amount of downward momentum but a small amount of energy.

(C) small amounts of downward momentum and energy.

(D) a small amount of downward momentum but a large amount of energy.

Answer: (B) a large amount of downward momentum but a small amount of energy.

Why: The glider needs to tranfer downward momentum into something else. The last thing it wants to do is transfer energy at the same time because energy is precious. If the glider could transfer downward momentum into a mountain peak, that would be great because the mountain peak wouldn't move and the glider would do no work on it and transfer no energy to it. Since there is no mountain peak to touch, the glider's next best choice is to transfer downward momentum to as much air as it can touch. The air will move and have work done on it and take away the glider's precious energy. But by pushing on lots of air, the glider minimizes the distance that air travels during the downward push and therefore the amount of energy the air receives.

Problem 11:

When a sport utility vehicle (SUV) makes a sudden left turn on the highway, it is in danger of flipping over. One valid explanation for this effect is:

(A) the SUV’s angular mass increases as it turns and its angular momentum decreases. A transfer of angular momentum from its wheels to its frame then causes it to begin rotating so that it flips over.

(B) the centrifugal force on the SUV as it goes around the turn pushes outward on the top of the SUV. The wheels are held in place by friction, so only the top of the SUV accelerates outward and it flips over.

(C) the leftward frictional force that causes the SUV to accelerate leftward during the turn also exerts a torque on the SUV about its center of mass and can cause the SUV to undergo angular acceleration and flip over.

(D) the force of the SUV’s momentum pushes it forward while the road pushes it to the left and these two forces twist the SUV so that it undergoes angular acceleration and flips over.

Answer: (C) the leftward frictional force that causes the SUV to accelerate leftward during the turn also exerts a torque on the SUV about its center of mass and can cause the SUV to undergo angular acceleration and flip over.

Why: The SUV's problem is that its static stability is poor. It's just too tall for its base and its center of gravity is too high above that small base. While it can recover from a modest tip, it cannot survive tips that would not put lower lying automobiles in jeopardy. So an SUV is an accident waiting to happen. During a high-speed turn, the frictional forces exert low on the car (on its wheels, in fact) can rotate the SUV pretty far from verticle during the turn and may take it beyond the limits of its static stability.

Problem 12:

As an incandescent light bulb ages, it develops a dark spot on the glass directly above the filament. This dark spot indicates that the bulb has gas inside it because the spot is produced by

(A) plasma.

(B) conduction.

(C) convection.

(D) radiation.

Answer: (C) convection.

Why: The tungsten atoms that form the dark spot were carried there in the convection fluid flow that develops inside the bulb. The presence of gas is bad for energy efficiency because convection does carry heat upward from the filament and wastes that heat on the surface of the bulb and the air above it. But the gas does help the filament's life by bouncing many of the tungsten atoms that try to leave the filament back onto the filament.

Problem 13:

You’re a passenger in a car and you have your eyes closed. The car is traveling at a constant speed on a level road that is turning toward the left. You feel a gravity-like sensation toward your right, but you remain in your car seat. The net force acting on you during this turn

(A) is zero.

(B) points toward your left.

(C) points toward your right.

(D) points forward.

Answer: (B) points toward your left.

Why: You always feel a gravity-like sensation in the direction opposite your acceleration. It's your inertia trying to make you go straight. In this case, you are accelerating to your left so you feel this gravity-like sensation toward your right.

Problem 14:

A plastic bottle, partly full of water, has been sitting on the kitchen counter for the past hour. Some of the water molecules in the bottle are gaseous and some are liquid. To increase the amount of liquid water in the bottle, you could (note: neglect any effect squeezing has on the temperatures in the bottle)

(A) cool the bottle down, but you must not squeeze it to increase the density of its gas.

(B) open the bottle to the kitchen air.

(C) cool the bottle down and/or squeeze it to increase the density of its gas.

(D) warm the bottle up and/or squeeze it to increase the density of its gas.

Answer: (C) cool the bottle down and/or squeeze it to increase the density of its gas.

Why: You can shift the balance between the liquid and gaseous phases of water by either slowing the leaving process (molecules leave the liquid less often) or speeding up the landing process (molecules land on the liquid more often). Cooling the bottle will slow the leaving process and making the gas above the water more dense will speed up the landing process.

Problem 15:

To make a large surface wave on the ocean travel faster, you could

(A) increase its amplitude (the height difference between its crests and troughs).

(B) decrease its amplitude (the height difference between its crests and troughs).

(C) decrease its wavelength (the distance between adjacent crests).

(D) increase its wavelength (the distance between adjacent crests).

Answer: (D) increase its wavelength (the distance between adjacent crests).

Why: Surface waves on water travel with a speed that depends on their wavelengths. Longer wavelength waves simply travel faster.

Problem 16:

To catch a football successfully, you should allow the ball to push your hands in the direction of its travel. Allowing your hands to move with the football is crucial because it allows you to

(A) add energy to the football.

(B) add momentum to the football.

(C) extract momentum from the football.

(D) extract energy from the football.

Answer: (D) extract energy from the football.

Why: It's easy to exchange momentum with the football, but getting it to stop and stay stopped requires that you extract all of its energy. By letting it do work on your hands, you help it lose energy so that it stays put in your hands.

Problem 17:

As you are going around the loop-the-loop on a roller coaster, you are basically traveling in a vertical circle. For simplicity, let’s imagine that you are traveling around and around that circle. When during this circular motion do you feel the heaviest?

(A) When you are at the bottom of the circle.

(B) When you are going down the descending side of the circle.

(C) When you are going up the rising side of the circle.

(D) When you are at the top of the circle.

Answer: (A) When you are at the bottom of the circle.

Why: You're traveling in something like uniform circular motion, although you're actually going fastest when you're lowest (at the bottom of the circle). Since acceleration is toward the center of the circle in uniform circular motion, you are accelerating upward most rapidly when you are at the bottom of the circle. That's when you feel the strongest gravity-like sensation due to acceleration and it points in the same direction as your real weight. You feel extremely heavy at the bottom of the loop.

Problem 18:

You drop a bouncy ball from rest and it bounces off a granite floor. The ball rebounds to its original height. During the bounce, the ball transferred

(A) both energy and momentum to the floor.

(B) neither energy nor momentum to the floor.

(C) energy but not momentum to the floor.

(D) momentum but not energy to the floor.

Answer: (D) momentum but not energy to the floor.

Why: It's easy to transfer momentum to the floor and the ball actually transfers twice as much downward momentum to the floor as it had before it hit. The result is that the ball has a negative amount of downward momentum, which is a positive amount of upward momentum. All it takes to transfer momentum is an impulse: a force exerted on the floor for a time. But transferring energy to the floor requires that the floor move so that the ball can do work on the floor. Since the granite floor won't move, the ball does not work on it and there is no transfer of energy.

Problem 19:

You are standing still in the basket of a hot air balloon and you are wearing a backpack. As the hot air balloon rises upward, you are doing

(A) (positive) work on the backpack and the hot air balloon is doing zero work on you.

(B) (positive) work on the backpack and the hot air balloon is doing (positive) work on you.

(C) zero work on the backpack and the hot air balloon is doing zero work on you.

(D) zero work on the backpack and the hot air balloon is doing (positive) work on you.

Answer: (B) (positive) work on the backpack and the hot air balloon is doing (positive) work on you.

Why: The balloon is pushing up on you, supporting your weight, and you move upward. Therefore, the balloon is doing work on you. You are pushing up on your backpack, supporting its weight, and it moves upward. Therefore, you are doing work on the backpack.

Problem 20:

You are swinging a bucket full of water around you in a big horizontal circle at a constant speed. You are at the center of its circular path. The net force on the bucket points directly

(A) toward you.

(B) downward.

(C) along the bucket’s velocity (along its direction of travel).

(D) away from you.

Answer: (A) toward you.

Why: You are providing the centripetal force that causes the bucket to travel around in a large horizontal circle. You are also keeping the bucket from falling by balancing its weight. However, the observation is the bucket is simply traveling in a circle at a constant speed. Therefore, it is experiencing a net force directly toward the center of the circle: you.

Problem 21:

A guitar string vibrating in its fundamental mode (as a single arc) behaves as a harmonic oscillator, which means that the restoring force acting on the string is

(A) proportional to how far it is from equilibrium and that its pitch (frequency) can vary but its volume (amplitude) cannot.

(B) proportional to how far it is from equilibrium and that its volume (amplitude) can vary but its pitch (frequency) cannot.

(C) independent of how far it is from equilibrium and that its volume (amplitude) can vary but its pitch (frequency) cannot.

(D) independent of how far it is from equilibrium and that its pitch (frequency) can vary but its volume (amplitude) cannot.

Answer: (B) proportional to how far it is from equilibrium and that its volume (amplitude) can vary but its pitch (frequency) cannot.

Why: why

Problem 22:

You are at a campfire in an open field. It is a cool, windless evening and you are sitting 10 feet from the fire. The fire is keeping you warm by transferring heat to you via

(A) radiation alone.

(B) conduction alone.

(C) conduction and convection.

(D) radiation and convection.

Answer: (A) radiation alone.

Why: why

Problem 23:

A kitchen mixer combines the ingredients best when its blades turn fairly rapidly through the batter you are making. If the blades turn too slowly, the various ingredients don't mix thoroughly, even if you wait a considerable amount of time. The advantage of rapid motion of the mixer blades reflects the need for

(A) inertia to dominate the flow of batter in the mixer, so that the resulting turbulent flow can fully combine the ingredients.

(B) inertia to dominate the flow of batter in the mixer, so that the resulting laminar flow can fully combine the ingredients.

(C) viscosity to dominate the flow of batter in the mixer, so that the resulting turbulent flow can fully combine the ingredients.

(D) viscosity to dominate the flow of batter in the mixer, so that the resulting laminar flow can fully combine the ingredients.

Answer: (A) inertia to dominate the flow of batter in the mixer, so that the resulting turbulent flow can fully combine the ingredients.

Why: why

Problem 24:

If you use a microwave oven to heat clean water in a smooth glass container, you can sometimes superheat the water—that is, raise its temperature well above water’s boiling temperature without it boiling. In that case, the very hot water fails to boil because

(A) even though bubbles of pure steam are stable at such high temperatures, they cannot form without help.

(B) bubbles of pure steam must stick to the surface of a container in order to grow in size, but slick glass surfaces won’t hold these bubbles in place long enough for boiling to occur.

(C) bubbles of pure steam are not stable in clean water that is uniformly heated, even at such high temperatures.

(D) bubbles of pure steam shrink quickly when they are exposed to atmospheric pressure, even at such high temperatures.

Answer: (A) even though bubbles of pure steam are stable at such high temperatures, they cannot form without help.

Why: why

Problem 25:

You and your best friend are trying to pull one another toward your respective dorm rooms. You’re the stronger of the two and, with a mighty tug, you drag your friend into your room. As you are pulling your friend toward your room, the force you exert on your friend is

(A) equal in amount to the force your friend exerts on you.

(B) definitely equal to three times the weight of Spongebob Squarepants.

(C) less in amount than the force your friend exerts on you.

(D) greater in amount than the force your friend exerts on you.

Answer: (A) equal in amount to the force your friend exerts on you.

Why: why

Problem 26:

A steam engine manages to do real work using only hot steam and cold air. It needs both the steam and the air because

(A) there is order present in their difference in temperature.

(B) total entropy decreases when heat flows spontaneously from hot steam to cold air.

(C) work is released when heat flows from cold air to hot steam.

(D) there is order present in the hot steam and energy present in the cold air.

Answer: (A) there is order present in their difference in temperature.

Why: why

Problem 27:

At the heart of a quartz watch is a tiny

(A) quartz sphere that bounces about like a ball in a box.

(B) container of quartz powder that emits electrical pulses because of thermal energy.

(C) quartz sphere that spins with constant angular acceleration.

(D) quartz tuning fork that vibrates like a pair of masses on a spring.

Answer: (D) quartz tuning fork that vibrates like a pair of masses on a spring.

Why: why

Problem 28:

Because of bad planning during the design and construction of a high-rise apartment building, all 50 floors of the building receive their water from a single pipe. That pipe is fed from a water tank located on the building’s roof. On opening day, residents on various floors begin taking showers and have different experiences. They quickly discover that the total energy per liter in the spraying water is (neglecting any effects of viscosity and friction)

(A) less on higher floors

(B) greater on higher floors.

(C) the same on all floors, but the speed of the spraying water is slower on higher floors.

(D) the same on all floors, but the speed of the spraying water is higher on higher floors.

Answer: (C) the same on all floors, but the speed of the spraying water is slower on higher floors.

Why: why

Problem 29:

Tides are caused by the moon’s gravity acting on the earth and its oceans. The moon reshapes the earth’s oceans so that they bulge slightly outward

(A) only on the side of the earth closest to the moon.

(B) all the way around the earth, from north pole to south pole.

(C) on the side of the earth closest to the moon and the side of the earth farthest from the moon.

(D) all the way around the earth’s equator.

Answer: (C) on the side of the earth closest to the moon and the side of the earth farthest from the moon.

Why: why

Problem 30:

Before it burns the fuel and air mixture, the piston and cylinder of an automobile engine compress that mixture to high density. During this compression, the temperature of the mixture rises because

(A) the piston does work on the mixture during the compression process.

(B) friction between the piston and cylinder heats the mixture.

(C) high density gases are hotter than low density gases.

(D) high pressure gases are hotter than low pressure gases.

Answer: (A) the piston does work on the mixture during the compression process.

Why: why

Problem 31:

You cannot cool room air simply by converting its thermal energy into electrical energy because

(A) that would be a statistically unlikely event.

(B) that would violate Newton’s third law of motion.

(C) that would violate Newton’s second law of motion.

(D) thermal energy and electrical energy are different conserved quantities and one cannot be converted into the other.

Answer: (A) that would be a statistically unlikely event.

Why: why

Problem 32:

You are bouncing gently up and down at the end of a springy diving board, with your feet never leaving its surface. If you double the overall height of the bounce, the time it takes to complete each bounce (the period of the bounce) will

(A) increase by a factor of 4.

(B) decrease by a factor of 2.

(C) stay the same.

(D) increase by a factor of 2.

Answer: (C) stay the same.

Why: why

Problem 33:

The pendulum of a grandfather clock gradually loses energy because of air drag and friction. To keep its pendulum swinging, the clock has to add a tiny amount of energy to it every swing. In essence, the clock gently pushes the pendulum in the

(A) same direction as its acceleration.

(B) same direction as its velocity.

(C) direction opposite its acceleration.

(D) direction opposite its velocity.

Answer: (B) same direction as its velocity.

Why: why

Problem 34:

The air emerging from the nozzle of a leaf blower is traveling fast but its pressure is atmospheric pressure. Before that same air passed through the nozzle it was traveling

(A) faster but its pressure was above atmospheric pressure.

(B) slower but its pressure was below atmospheric pressure.

(C) slower but its pressure was above atmospheric pressure.

(D) faster but its pressure was below atmospheric pressure.

Answer: (C) slower but its pressure was above atmospheric pressure.

Why: why

Problem 35:

A huge abstract iron sculpture has been installed in front of the local art museum. It sits there motionless, “guarding” the main doorway. The sculpture is experiencing its weight downward and an equally strong force upward from the brick courtyard beneath it. We know that these two forces on the sculpture are equal in amount but oppositely directed because

(A) for every action, there is an equal but oppositely directed reaction.

(B) the sculpture is not accelerating so the two forces must sum to zero.

(C) the sculpture has zero velocity.

(D) Newton’s third law requires that forces always appear in equal but oppositely directed pairs.

Answer: (B) the sculpture is not accelerating so the two forces must sum to zero.

Why: why

Problem 36:

You drop a bouncy ball straight down from your friend’s 30th floor balcony. It is spinning rapidly so that its northern side is turning upward (toward you). As a result of this spin, the ball does not continue falling straight downward, but instead deflects horizontally as it falls. The ball deflects the passing airstream

(A) northward and the ball accelerates northward.

(B) southward and the ball accelerates southward.

(C) southward and the ball accelerates northward.

(D) northward and the ball accelerates southward.

Answer: (C) southward and the ball accelerates northward.

Why: why

Problem 37:

When a large wave travels across the ocean, water at the ocean’s surface moves

(A) steadily forward at the wave’s velocity.

(B) steadily forward at half the wave’s velocity.

(C) directly up and down and returns to its starting point.

(D) in a circular path and returns to its starting point.

Answer: (D) in a circular path and returns to its starting point.

Why: why

Problem 38:

You toss a quarter straight up into the air. It rises to a peak and then descends. After it leaves your hand and neglecting air resistance, the quarter experiences

(A) a downward net force that is proportional to the quarter’s speed.

(B) a constant downward net force.

(C) an upward net force that gradually diminishes to zero at its peak height and then becomes a downward net force.

(D) a constant upward net force on the way up and a constant downward net force on the way down.

Answer: (B) a constant downward net force.

Why: why

Problem 39:

The light from an incandescent light bulb is redder than sunlight and photographs taken in incandescent lighting often have a reddish cast to them. There is a practical reason why an incandescent light bulb cannot produce a larger fraction of blue light. To shift its light spectrum toward the blue, the bulb would have to

(A) shorten the distance between coils in its spiral filament and no one knows how to wind these tiny spirals any tighter.

(B) use a blue metal filament and all known metals are either shiny, yellow, orange, or red.

(C) increase the length of its filament and the filament would consume too much electricity to be cost effective.

(D) operate its filament at a higher temperature and the filament would die quickly.

Answer: (D) operate its filament at a higher temperature and the filament would die quickly.

Why: why

Problem 40:

You roll a marble down the side of a round bowl—a bowl with a spherical bottom. The marble rolls back and forth between the front and back of the bowl, passing right through the bottom of the bowl on each trip. At this exact moment, the marble has just rolled down your side of the bowl and is exactly at the bottom of the bowl. It is about to begin rolling up the far side of the bowl. It is accelerating

(A) away from you.

(B) upward.

(C) downward.

(D) toward you.

Answer: (B) upward.

Why: why

Problem 41:

Two marbles are in a large bowl and the system is completely isolated from its surroundings. Each of the four arrangements listed below as possible answers contains exactly the same amount of total energy, although that energy may be in different forms in the different arrangements. Which arrangement has the most entropy:

(A) the marbles high up on the same side of the bowl, but not touching.

(B) the marbles both rolling quickly through the bottom of the bowl, but not touching.

(C) the marbles resting motionless in the bottom of the bowl.

(D) the marbles high up on opposite sides of the bowl.

Answer: (C) the marbles resting motionless in the bottom of the bowl.

Why: why

Problem 42:

If you throw a ball straight forward at high speed with no spin at all, the air pressure at its surface will not be uniform. The location of the highest air pressure on the ball’s surface will be at its

(A) front (the side farthest from you).

(B) bottom.

(C) top.

(D) back (the side nearest to you).

Answer: (A) front (the side farthest from you).

Why: why

Problem 43:

When you blow gently across the top of a soda bottle, the air inside it vibrates as a harmonic oscillator and it emits a clear tone. If you replace the air in the bottle with helium, the bottle’s new tone will definitely occur at a

(A) lower pitch (a lower frequency).

(B) lower volume (a smaller amplitude).

(C) higher pitch (a higher frequency).

(D) higher volume (a larger amplitude).

Answer: (C) higher pitch (a higher frequency).

Why: why

Problem 44:

A liquid pours smoothly down a channel and into a basin, experiencing only laminar flow. To make this flow become turbulent, you should

(A) decrease the speed of the flow and/or decrease the viscosity of the liquid.

(B) increase the speed of the flow and/or increase the viscosity of the liquid.

(C) increase the speed of the flow and/or decrease the viscosity of the liquid.

(D) decrease the speed of the flow and/or increase the viscosity of the liquid.

Answer: (C) increase the speed of the flow and/or decrease the viscosity of the liquid.

Why: why

Problem 45:

Two children are trying to lift toys using identical-looking helium balloons. However, one child is located in Norfolk, where the altitude is roughly sea level, and the other child is located in Denver, where the altitude is about 5,000 feet above sea level. Which balloon, if any, can lift more weight?

(A) I am just randomly picking answers and do not deserve credit for this problem.

(B) The Denver balloon can lift more weight.

(C) They can both lift equal weights.

(D) The Norfolk balloon can lift more weight.

Answer: (D) The Norfolk balloon can lift more weight.

Why: why

Problem 46:

A wooden sculpture weighs exactly 1 pound. You very gently float this sculpture in a pot that was perfectly full of water, so some of that water overflows. The pot is again perfectly full of water, but now the sculpture is floating in it. If you weigh the water that overflowed, you’ll find that it weighs

(A) slightly less than 1 pound.

(B) much less than 1 pound.

(C) exactly 1 pound.

(D) more than 1 pound.

Answer: (C) exactly 1 pound.

Why: why

Problem 47:

You’re standing at the top of a tall lighthouse. You are holding two balls, one twice as heavy as the other, and you throw them together horizontally with one hand. They leave your hand at the same time and the same speed. Both balls soon hit the water. Neglecting any air resistance, the heavier ball reaches the water

(A) at the same time but half as far from the lighthouse as the lighter ball.

(B) in half the time and half as far from the lighthouse as the lighter ball.

(C) in half the time but at the same place as the lighter ball.

(D) at the same time and at the same place as the lighter ball.

Answer: (D) at the same time and at the same place as the lighter ball.

Why: why

Problem 48:

An air conditioner is most effective at cooling a room on a warm summer day when the work done while operating its compressor ends up as

(A) thermal energy split evenly between the outside air and the indoor air.

(B) thermal energy in the indoor air.

(C) work back at the power company.

(D) thermal energy in the outside air.

Answer: (D) thermal energy in the outside air.

Why: why

Problem 49:

A high-speed chase ends when the robbers drive their car into the concrete base of a bridge. The car is badly crumpled and the robbers are dazed, but the bridge is unscathed. When the car collided with the bridge, it transferred

(A) both momentum and energy to the bridge.

(B) momentum to the bridge but did not transfer energy to the bridge.

(C) neither momentum nor energy to the bridge.

(D) energy to the bridge but did not transfer momentum to the bridge.

Answer: (B) momentum to the bridge but did not transfer energy to the bridge.

Why: why

Problem 50:

A kiln is used to fire pottery—to heat it red or even yellow hot so that it fuses into a hard, finished ceramic. You have four pots to fire, each with its own surface color. One is black, one is white, one is shiny aluminum-like, and one is essentially transparent. You place the four pots in a kiln and heat them to the same yellow-hot temperature. When you look inside the kiln, the one that is glowing most brightly is the one that is

(A) white.

(B) shiny aluminum-like.

(C) black.

(D) essentially transparent.

Answer: (C) black.

Why: why