Problem 1:
A narrow boat, such as a canoe, kayak, or racing shell, is like a long cylinder that can rotate about a horizontal axis (see figure). When the boat's occupants are seated, its overall center of gravity is below the rotation axis and the boat is upright and stable. When someone stand up, however, the boat's center of gravity shifts above the rotation axis and the boat tips over. Why does this center gravity shift cause the boat to become unstable?
(A) Once the center of gravity is above the rotation axis, tipping causes the boat's buoyant force to decrease. [18.3% picked]
(B) Once the center of gravity is above the rotation axis, tipping causes the boat's total potential energy to decrease. [79.4% picked]
(C) Once the center of gravity is above the rotation axis, tipping causes the boat's total mass to decrease. [0.0% picked]
(D) Once the center of gravity is above the rotation axis, tipping causes the boat's total weight to decrease. [2.3% picked]
Answer: (B) Once the center of gravity is above the rotation axis, tipping causes the boat's total potential energy to decrease. [79.4% picked]
Why: When everyone is seated in the boat, the boat's center of gravity -- the effective location of its weight -- is low and therefore below the rotational axis of the boat. If the boat were to tip away from equilibrium, its center of gravity would rise and so would its total potential energy (which is pretty much only its gravitational potential energy). Since an object accelerates in the direction that reduces their total potential as quickly as possible, the boat tends to accelerate back toward equilibrium. So the equilibrium is stable. But when someone stands up, the boat's center of gravity rises. If it rises above the rotational axis of the boat, the boat's equilibrium becomes unstable. If everything is just right, the boat's center of gravity is directly above the rotational axis and the boat is in equilibrium (zero net force and zero net torque). But the slightest disturbance moves the boat's center of gravity to one side or another and then disaster occurs. The boat accelerates so as to reduce its total potential energy as quicly as possible -- it tips farther and farther to one side. So standing up in a canoe or similar boat is asking for trouble!
Problem 2:
You inflate a rubber balloon with oxygen and then release it. The balloon flies around the room until it is empty and deflated. What pushed the balloon forward?
(A) The air on all sides of the balloon pushed the balloon forward. [1.8% picked]
(B) As the balloon pushed oxygen backward out of the balloon's opening that oxygen pushed the balloon forward. [90.4% picked]
(C) The air behind the balloon pushed the balloon forward. [1.8% picked]
(D) The buoyant force due to the atmosphere's pressure gradient pushed the balloon forward. [6.0% picked]
Answer: (B) As the balloon pushed oxygen backward out of the balloon's opening that oxygen pushed the balloon forward. [90.4% picked]
Why: The balloon's elastic skin pressurizes the oxygen inside it. That means that the pressure of the oxygen is slightly greater than the pressure of the surrounding air. When you release the balloon, oxygen accelerates toward lower pressure and flows out of the balloon's opening. That acceleration of the oxygen is caused by a push from the balloon and its taut skin. The oxygen pushes back on the balloon and its skin, causing the balloon to accelerate in the other direction -- action and reaction.
Problem 3:
You live in a two-story house with identical bathrooms on the ground floor and second floor. You turn on the showers in both bathrooms. How do the water pressures before each shower nozzle and the water speeds after each nozzle compare?
(A) The water pressure is larger in the ground floor shower and the water speed is larger in the second floor shower. [13.8% picked]
(B) The water pressure and water speed are both larger in the ground floor shower. [77.1% picked]
(C) The water pressure and water speed are both larger in the second floor shower. [1.8% picked]
(D) The water pressure is larger in the second floor shower and the water speed is larger in the ground floor shower. [7.3% picked]
Answer: (B) The water pressure and water speed are both larger in the ground floor shower. [77.1% picked]
Why: The higher you go in the house, the more of the water's total ordered energy (the energy that is equivalent to work) is tied up as gravitational potential energy. On the ground floor, the water has relatively little gravitational potential energy, so it can have more pressure potential energy before the shower nozzle or more kinetic energy after the shower nozzle.
Problem 4:
You drop an unopened metal can of soup and it lands bottom-first on the floor. When you inspect the can, you see that it has dented outward in places. Where did it dent outward and why?
(A) The impact between the floor and the bottom of the can buckled the can outward near its middle (halfway between its top and bottom). [2.3% picked]
(B) The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom. [78.9% picked]
(C) The floor pushed the bottom of the can upward extremely hard on impact and the momentum of that upward force dented the can outward near its top. [10.1% picked]
(D) The soup bounced upward after the can hit the floor and when the soup hit the top of the can, it dented the top of the can outward. [8.7% picked]
Answer: (B) The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom. [78.9% picked]
Why: The falling soup acquires downward momentum. After all, gravity is pulling it downward for a period time, thereby doing an impulse on the soup. When the can hits the floor, the soup must stop and it must therefore give up all its downward momentum. It does this by exerting pressure forces on the bottom of the can for a period of time. That effect requires a surge in pressure at the bottom of the can. While the floor supports the can's bottom and prevents damage there, the sides of the can near the bottom are in jeopardy. The pressure surge in the soup at the bottom of the can pushes the sides of the can outward and dents them outward near the bottom of the can. This effect is an example of water hammer, though in this case it's soup hammer!
Problem 5:
You are riding a roller coaster and your speed is increasing as you plunge downhill on the first hill. Your acceleration is directed
(A) vertically downward, your feeling of acceleration is directed vertically upward, and your apparent weight is smaller in amount than your actual weight. [38.2% picked]
(B) downhill, your feeling of acceleration is directed downhill, and your apparent weight is directed vertically downward. [5.1% picked]
(C) downhill, your feeling of acceleration is directed uphill, and your apparent weight is smaller in amount than your actual weight. [49.3% picked]
(D) vertically downward, your feeling of acceleration is directed vertically upward, and your apparent weight is vertically upward. [7.4% picked]
Answer: (C) downhill, your feeling of acceleration is directed uphill, and your apparent weight is smaller in amount than your actual weight. [49.3% picked]
Why: When you are going faster and faster down a steep ramp, you are accelerating downhill on that ramp. Your feeling of acceleration is directed opposite your acceleration. In this case, that feeling is uphill on the ramp. Combining your feeling of weight itself with this uphill feeling of acceleration, your apparent weight is weak -- the uphill feeling of acceleration cancels much of your feeling of weight itself.
Problem 6:
A birdfeeder hangs motionless from a tree branch on a long string. After a bird disturbs it, the birdfeeder swings back and forth below the branch before settling back to its original stable equilibrium position. Why is the birdfeeder's equilibrium position stable?
(A) Since the birdfeeder's total potential energy always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium. [85.8% picked]
(B) Since the birdfeeder's total energy always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium. [6.0% picked]
(C) Since the birdfeeder's momentum always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium. [2.3% picked]
(D) Since the birdfeeder's total kinetic energy always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium. [6.0% picked]
Answer: (A) Since the birdfeeder's total potential energy always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium. [85.8% picked]
Why: The motionless birdfeeder is in equilibrium and that equilibrium is stable. That's because any disturbance of the birdfeeder away from equilibrium causes its center of gravity to rise and therefore its total potential energy to increase. Since objects accelerate in the direction that reduces their total potential energy as quickly as possible, the birdfeeder accelerates back toward equilibrium. The equilibrium is stable.
Problem 7:
A spaceship is moving at constant velocity toward a distant star. The ship experiences no gravitational forces. To maintain its velocity, the ship must have its rocket engines
(A) turned on, with their exhaust direct toward the distant star. [0.5% picked]
(B) turned on, with their exhaust direct away from the distant star. [29.4% picked]
(C) turned off. [68.8% picked]
(D) turned on, with their exhaust direct perpendicular to the ship's velocity. [1.4% picked]
Answer: (C) turned off. [68.8% picked]
Why: To maintain its current velocity, the spaceship needs to be inertial -- it needs to experience zero net force. With no gravitational force acting on the ship, it just needs to ensure that nothing else pushes on it in order to move at constant velocity. By turning off its rocket engines, the spaceship eliminates those engines as a source of unwanted forces. It is then coasting and will move at constant velocity.
Problem 8:
A spacecraft is moving eastward in a circular orbit 200 miles above the Earth's equator. It is time for the spacecraft to return to the ground. The most effective way to cause the spacecraft to begin descending toward the Earth's surface is to fire its rocket engine. In which direction should the engine direct its rocket exhaust?
(A) Northward [35.8% picked]
(B) Westward [30.3% picked]
(C) Eastward [24.3% picked]
(D) Southward [9.6% picked]
Answer: (C) Eastward [24.3% picked]
Why: If the spacecraft maintains its current eastward velocity, it will move in a circular orbit. There will then be a perfect balance between the spacecraft's inertia, which tends to make it move in a straight path and thus experience in increase in altitude above the Earth's curved surface, and the spacecraft's weight, which makes it accelerate toward the center of the Earth and thus experience a decrease in altitude. To descend toward the Earth's surface and return to the ground, the spacecraft merely needs to let weight and gravity win, so that the ship's altitude will decrease. The spacecraft does this by slowing down -- it directs its rocket exhaust eastward so that the spacecraft accelerates westward. As the spacecraft's eastward velocity decreases, it begins to descend and soon reenters the Earth's atmosphere.
Problem 9:
The exposed air ducts in warehouses and gymnasiums are often several feet in diameter. If a wide duct and a narrow duct are delivering the same amount of air each second, the fan pushing air through the wide duct consumes less power than the fan pushing air through the narrow duct. Why?
(A) Air in the wide duct has lower density, so it weighs less per liter and can flow forward due to the buoyant force alone. [7.3% picked]
(B) Air in the wide duct moves slower, requiring less kinetic energy per liter, and it experiences weaker viscous interactions with the duct. [68.8% picked]
(C) Air from the wide duct emerges into the space outside the duct at a lower pressure than air emerging from the narrow duct. [21.1% picked]
(D) Air in the wide duct has a higher density and coasts more easily forward. [2.8% picked]
Answer: (B) Air in the wide duct moves slower, requiring less kinetic energy per liter, and it experiences weaker viscous interactions with the duct. [68.8% picked]
Why: For a given air speed, a wider duct moves more air each second than a narrower duct. Correspondingly, that wider duct can move the same amount of air each second as the narrower duct while moving that air more slowly. Since viscous drag forces increase with speed through a duct and the air is closer to the duct's surface in a narrower duct, air in the narrower duct experiences much greater drag forces that air in the wider duct. It takes less fan power to move the air through the wider duct.
Problem 10:
Doctors use a centrifuge to separate red blood cells from blood plasma. The centrifuge swings a container of blood at high speed in a circular path. The red bloods drift away from the center of the circle, leaving the blood plasma nearer the center. Describe the pressures and densities in the blood as the centrifuge swings it in a circle.
(A) The pressure is highest closest to the center of the circle and the plasma is denser than the red blood cells. [21.6% picked]
(B) The pressure is highest farthest from the center of the circle and the plasma is denser than the red blood cells. [33.0% picked]
(C) The pressure is highest farthest from the center of the circle and the red blood cells are denser than the blood plasma. [38.5% picked]
(D) The pressure is highest closest to the center of the circle and the red blood cells are denser than the blood plasma. [6.9% picked]
Answer: (C) The pressure is highest farthest from the center of the circle and the red blood cells are denser than the blood plasma. [38.5% picked]
Why: Inertia alone would cause the blood to travel in a straight line, so the blood needs an inward force to bend its path in a circle. That inward force is caused by a pressure imbalance that develops between higher pressure on the outside of the centrifuge and lower pressure on the inside of the centrifuge. That pressure imbalance pushes the blood toward the center of the centrifuge, so the blood accelerates toward the center of the centrifuge and travels in a circle rather than a straight line.
Problem 11:
A log is floating motionless on a calm lake, with about half the log located beneath the water level. It's a sunny day and the temperature of the air increases, although the air pressure remains constant. How does the air's increasing temperature affect the log?
(A) The log rises slightly, so that less of it is located beneath the water level. [13.3% picked]
(B) The log sinks to the bottom of the lake. [0.0% picked]
(C) The log descends slightly, so that more of it is located beneath the water level. [56.9% picked]
(D) The log remains at the same height, so that the same fraction is located beneath the water level. [29.8% picked]
Answer: (C) The log descends slightly, so that more of it is located beneath the water level. [56.9% picked]
Why: The log initially displaces exactly its weight in two fluids: water and air. When the air's temperature increases without any change in pressure, the air becomes less dense and the log is then displacing less than its weight in water and air. The buoyant force on the log decreases, so the log has to descend deeper into the water to obtain the buoyant force it needs to balance its weight.
Problem 12:
The oil and vinegar in a salad dressing tend to separate, with the oil floating on the vinegar. Suppose an herb in the dressing sits motionless at the interface where the oil and vinegar meet. Describe the average density of that herb.
(A) Its average density is more than the density of vinegar and greater than the density of oil. [0.9% picked]
(B) Its average density is less than the density of vinegar and less than the density of oil. [0.5% picked]
(C) Its average density is more than the density of vinegar and less than the density of oil. [7.8% picked]
(D) Its average density is less than the density of vinegar and greater than the density of oil. [90.8% picked]
Answer: (D) Its average density is less than the density of vinegar and greater than the density of oil. [90.8% picked]
Why: For the herb to float to the top of the vinegar, the herb's average density must be less than that of vinegar. When immersed entirely in vinegar, the upward buoyant force on the herb is greater in amount than the herb's downward weight and the herb experiences an upward net force. For the herb to float to the bottom of the oil, the herb's average density must be more than that of oil. When immersed entirely in oil, the upward buoyant force on the herb is less in amount than the herb's downward weight and the herb experiences a downward net force. The herb therefore naturally moves to the interface between those two liquids. It seeks the height at which it displaces just the right mixture of vinegar and oil so that the weight of the mixture it displaces is equal to the weight of the herb. Equivalently, ...so that the average density of the mixture the herb displaces is equal to its average density.
Problem 13:
You hold an unopened glass bottle of root beer upright in one hand and pound its bottle cap downward hard with a rubber mallet. As a consequence of this action,
(A) the sudden expansion of gas inside the bottle causes the top layer of root beer to freeze. [0.0% picked]
(B) the sudden acceleration causes the liquid in the bottle to float above the gas in the bottle for several seconds. [4.6% picked]
(C) liquid colliding with the bottom of the bottle knocks the bottom out of the bottle. [90.8% picked]
(D) the surge in pressure near the top of the bottle causes the bottle cap to vibrate loudly. [4.6% picked]
Answer: (C) liquid colliding with the bottom of the bottle knocks the bottom out of the bottle. [90.8% picked]
Why: When the mallet strikes the top of the bottle, it causes the glass bottle itself to accelerate downward rapidly. The root beer, however, remains approximately in place because of inertia and the bottle leaves that root beer behind. During this period, the root beer shifts up into the neck of the bottle, compressing the trapped gas in that neck. When the bottle stops accelerating downward, the root beer begins returning toward the bottom of the bottle. The root beer is pushed in that direction by the pressure imbalance between high pressure in the neck and zero pressure near the bottom of the bottle. When the root beer finally reaches the bottom of the bottle, it hits that bottom hard and hammers the bottom out of the bottle. This final effect, where the liquid root beer collides with the solid glass bottom of the bottle, is an example of water hammer, although in this case it's root beer hammer.
Problem 14:
Suppose a soccer ball is approaching your leg at 30 km/h and your leg is moving toward the soccer ball at 30 km/h. What is the fastest the soccer ball can possibly travel after it hits your leg? [You can assume perfect (ideal) bouncing behaviors for the ball and your leg.]
(A) 120 km/h [8.3% picked]
(B) 30 km/h [8.7% picked]
(C) 90 km/h [34.4% picked]
(D) 60 km/h [48.6% picked]
Answer: (C) 90 km/h [34.4% picked]
Why: The two objects, the ball and your leg are approaching one another at a total speed of 60 km/h. From you leg's perspective (or frame of reference), the ball is approaching it at 60 km/h. If the ball bounces perfectly from your leg, then the ball will separate from your leg at 60 km/h. But the spectator's perspective, your leg is moving at 30 km/h (we'll assume the bouncing ball doesn't slow it down). Since your leg is moving forward at 30 km/h and the ball is "outmoving" your leg by 60 km/h, the ball's speed, as view by the spectators, is 90 km/h.
Problem 15:
You are inflating a limp plastic balloon with helium. The pressure inside the balloon remains atmospheric pressure, even as more helium accumulates inside the balloon. How do the balloon's total weight and the buoyant force on the balloon change during this filling process?
(A) The balloon's weight increases, but the buoyant force decreases. [2.3% picked]
(B) The balloon's weight decreases, but the buoyant force increases. [14.7% picked]
(C) Both forces increase, but the buoyant force increases more rapidly than the balloon's weight. [74.8% picked]
(D) Both forces increase, but the balloon's weight increases more rapidly than the buoyant force. [8.3% picked]
Answer: (C) Both forces increase, but the buoyant force increases more rapidly than the balloon's weight. [74.8% picked]
Why: As helium enters the balloon, the balloon's total weight increases. As the balloon's volume increases, the buoyant force on the balloon increases. Since the balloon's weight increases less rapidly than the buoyant force, the buoyant force eventually becomes stronger than the balloon's weight and the net force on the balloon is then upward.
Problem 16:
The roller coaster you are riding is going through a loop-the-loop at high-speed. At the moment when you are at the top of the loop, you and your car are inverted yet you feel an apparent weight directed toward the sky. At this moment, your acceleration is
(A) downward and less than the acceleration due to gravity. [1.8% picked]
(B) downward and equal to the acceleration due to gravity. [7.8% picked]
(C) downward and greater than the acceleration due to gravity. [89.4% picked]
(D) zero. [0.9% picked]
Answer: (C) downward and greater than the acceleration due to gravity. [89.4% picked]
Why: You accelerate down the ramp, propelled in that direction by the ramp force (a combination of your weight and the ramp's support force). You experience a feeling of acceleration opposite your downhill acceleration, namely in the uphill direction. Overall, your apparent weight (the sum of your downward experience of true weight and your uphill feeling of acceleration) is weak because those two experiences large cancel one another..
Problem 17:
A bicycle touches the ground at only two points, forming a base of support that is a line. As the bicycle (including the rider) moves forward, it steers itself automatically so that it
(A) turns steadily either to the left or to the right. [1.8% picked]
(B) maintains constant velocity. [0.0% picked]
(C) does not accelerate horizontally. [1.8% picked]
(D) tends to put its base of support below its center of gravity, thereby returning the bicycle to its unstable equilibrium after any minor tip. [96.3% picked]
Answer: (D) tends to put its base of support below its center of gravity, thereby returning the bicycle to its unstable equilibrium after any minor tip. [96.3% picked]
Why: TBA
Problem 18:
A horizontal pipe bends toward the right. As water flows steadily through this pipe, what happens as the water starts to travel around the bend?
(A) Water slows down. [0.5% picked]
(B) Water speeds up. [2.3% picked]
(C) Water near the inside of the bend speeds up and water near the outside of the bend slows down. [83.5% picked]
(D) Water near the inside of the bend slows down and water near the outside of the bend speeds up. [13.8% picked]
Answer: (C) Water near the inside of the bend speeds up and water near the outside of the bend slows down. [83.5% picked]
Why: TBA
Problem 19:
Water is flowing at constant velocity through a straight, horizontal pipe. The pressure at the entry end of the pipe is greater than the pressure at the exit end of the pipe. What would happen if those two pressures suddenly became equal?
(A) The water would stop flowing because of viscous interactions between the water and the pipe. [74.2% picked]
(B) The water would continue to flow at the same constant velocity. [16.6% picked]
(C) The water would flow faster than before because of viscous interactions between the water and the pipe. [8.3% picked]
(D) The water would flow through the pipe in the opposite direction. [0.9% picked]
Answer: (A) The water would stop flowing because of viscous interactions between the water and the pipe. [74.2% picked]
Why: TBA
Problem 20:
Why does a sprinter lean forward as she picks up speed at the start of a race?
(A) By leaning forward, she can use her weight to make her accelerate forward at the acceleration due to gravity. [2.8% picked]
(B) If she stood upright as the ground pushed her feet forward, the inertia of her upper body would cause her to tip over backward. [94.0% picked]
(C) Her forward lean moves her closer to the finish line so that she arrives sooner. [0.0% picked]
(D) She leans in the direction of her velocity in order to increase that forward velocity. [3.2% picked]
Answer: (B) If she stood upright as the ground pushed her feet forward, the inertia of her upper body would cause her to tip over backward. [94.0% picked]
Why: TBA
Problem 21:
A pressure washer is used to clean stone surfaces. Its pump delivers high-pressure water to a hose and that water is sprayed through a nozzle so this travels through the air and hits the stone surface. Which of the following correctly describes the water's pressure?
(A) The water's pressure is high before the nozzle, it is atmospheric pressure as it travels through the air, and it is high when it is slowed down by the stone surface. [74.8% picked]
(B) The water's pressure is high before the nozzle and it is atmospheric pressure as it travels through the air and hits the stone surface. [9.6% picked]
(C) The water's pressure is high throughout its trip: from before the nozzle to when it hits the stone surface. [8.7% picked]
(D) The water's pressure is high before the nozzle, it is medium-high as it travels through the air, and it is atmospheric pressure when it is slowed down by the stone surface. [6.9% picked]
Answer: (A) The water's pressure is high before the nozzle, it is atmospheric pressure as it travels through the air, and it is high when it is slowed down by the stone surface. [74.8% picked]
Why: TBA
Problem 22:
Suppose a rocket is launched vertically, so that its velocity always points vertically upward. By the time the rocket's fuel runs out, its altitude is 200 miles above the Earth's surface and its speed is approximately zero. What happens to the rocket after its fuel runs out?
(A) The rocket begins to orbit the Earth because it is above the Earth's atmosphere. [18.8% picked]
(B) The rocket falls, accelerating vertically downward because of its weight. [65.1% picked]
(C) The rocket begins to orbit the Earth because its weight is zero. [2.8% picked]
(D) The rocket begins to orbit the Earth because it is not going fast enough to travel into the solar system or beyond. [13.3% picked]
Answer: (B) The rocket falls, accelerating vertically downward because of its weight. [65.1% picked]
Why: TBA
Problem 23:
Firefighters are battling a fire on the 10th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the water steadily up to the 8th floor. They carry the nozzle end of that fire hose to the top of a 4-story-tall ladder and again point the nozzle upward. Their fire hose can now shoot the water steadily up to which floor?
(A) The 6th floor. [0.5% picked]
(B) The 8th floor. [93.1% picked]
(C) The 12th floor. [1.8% picked]
(D) The 10th floor. [4.6% picked]
Answer: (B) The 8th floor. [93.1% picked]
Why: TBA
Problem 24:
A scuba tank is a metal bottle that can be filled with air and used to supply that air to a person who is underwater. Suppose you fill a scuba tank with air until the particle density in the tank is 100 times the particle density of the surrounding air. At room temperature, what are the density and pressure of the air inside that tank?
(A) The density is the same as atmospheric density and the pressure is the same as atmospheric pressure. [6.0% picked]
(B) The density is about 100 times atmospheric density and the pressure is about 100 times atmospheric pressure. [57.3% picked]
(C) The density is about 100 times atmospheric density and the pressure is the same as atmospheric pressure. [16.1% picked]
(D) The density is the same as atmospheric density and the pressure is about 100 times atmospheric pressure. [20.6% picked]
Answer: (B) The density is about 100 times atmospheric density and the pressure is about 100 times atmospheric pressure. [57.3% picked]
Why: TBA
Problem 25:
You and your friend are riding a carousel. Your horses are side-by-side and your friend is closer to the center of the carousel than you are. As the carousel turns steadily, which of the following is true?
(A) Your friend experiences a larger inward acceleration than you do. [22.0% picked]
(B) You and your friend experience equal inward accelerations, but your speed is larger than your friend's speed. [33.9% picked]
(C) You experience a larger inward acceleration than your friend does. [19.7% picked]
(D) You and your friend experience equal inward accelerations, but your friend's speed is larger than your speed. [24.3% picked]
Answer: (C) You experience a larger inward acceleration than your friend does. [19.7% picked]
Why: TBA
Problem 26:
You heat your metal water bottle on a stovetop until it is filled with hot air at atmospheric pressure. You then seal the bottle and set it on the kitchen counter to cool. Assume the bottle doesn't change size (volume) throughout this process. During the cooling process, what happens to the density and pressure of the air in the bottle?
(A) The density and pressure decrease. [5.1% picked]
(B) The density increases, but the pressure decreases. [47.5% picked]
(C) The density and pressure remain constant. [5.1% picked]
(D) The density remains constant, but the pressure decreases. [42.4% picked]
Answer: (D) The density remains constant, but the pressure decreases. [42.4% picked]
Why: TBA
Problem 27:
You are riding a train that is moving straight ahead at high speed. The train comes to a curve in the tracks and begins to turn toward the left. You find yourself pressed against the right side of your seat. What force, if any, is pushing you toward the right?
(A) There is no force pushing you toward the right. [86.2% picked]
(B) The force of your momentum is pushing you toward the right. [8.7% picked]
(C) A friction force from the seat is pushing you toward the right. [0.9% picked]
(D) A support force from the seat is pushing you toward the right. [4.1% picked]
Answer: (A) There is no force pushing you toward the right. [86.2% picked]
Why: TBA
Problem 28:
After heavy rains, the water flowing down a nearby river is turbulent, also known as "white water." During a drought, however, the water flowing down that same river is laminar; it appears smooth and clear. Explain the difference.
(A) In the deep water after the rains, the extra weight produces turbulence. In the shallow water during the drought, buoyancy produces laminar flow. [9.6% picked]
(B) In the fast-moving water after the rains, inertia dominates the flow. In the slow-moving water during the drought, viscosity dominates the flow. [84.9% picked]
(C) After the rains, the water has a smaller density. During the drought, the water has a larger density. [0.5% picked]
(D) After the rains, the water has a larger viscosity. During the drought, the water has a smaller viscosity. [5.0% picked]
Answer: (B) In the fast-moving water after the rains, inertia dominates the flow. In the slow-moving water during the drought, viscosity dominates the flow. [84.9% picked]
Why: TBA
Problem 29:
Some fire extinguishers use spray carbon dioxide. The mass of a carbon dioxide molecule is about 50% greater than the mass of an average air molecule. At room temperature, a liter of carbon dioxide
(A) has the same number of particles as a liter of air, but the carbon dioxide weighs more than the air it displaces and it sinks to the floor. [86.2% picked]
(B) has more particles than a liter of air, but the carbon dioxide weighs the same as the air it displaces and can hover motionless in the middle of the room. [2.8% picked]
(C) has fewer particles than a liter of air, but the carbon dioxide weighs more than the air it displaces and it sinks to the floor. [8.7% picked]
(D) has fewer particles than a liter of air, but the carbon dioxide weighs the same as the air it displaces and can hover motionless in the middle of the room. [2.3% picked]
Answer: (A) has the same number of particles as a liter of air, but the carbon dioxide weighs more than the air it displaces and it sinks to the floor. [86.2% picked]
Why: TBA
Problem 30:
You kick a soccer ball through the air. From the ball's perspective, air is flowing northward toward the ball. Describe the air pressures on the ball's south side and east side (see figure).
(A) The south side pressure is less than atmospheric pressure. The east side pressure is greater than atmospheric pressure. [12.4% picked]
(B) The south side pressure is less than atmospheric pressure. The east side pressure is less than atmospheric pressure. [2.8% picked]
(C) The south side pressure is greater than atmospheric pressure. The east side pressure is less than atmospheric pressure. [81.2% picked]
(D) The south side pressure is greater than atmospheric pressure. The east side pressure is greater than atmospheric pressure. [3.7% picked]
Answer: (C) The south side pressure is greater than atmospheric pressure. The east side pressure is less than atmospheric pressure. [81.2% picked]
Why: TBA