Midterm Examination 2

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.
(B) Once the center of gravity is above the rotation axis, tipping causes the boat's total potential energy to decrease.
(C) Once the center of gravity is above the rotation axis, tipping causes the boat's total mass to decrease.
(D) Once the center of gravity is above the rotation axis, tipping causes the boat's total weight to decrease.
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.
(B) As the balloon pushed oxygen backward out of the balloon's opening that oxygen pushed the balloon forward.
(C) The air behind the balloon pushed the balloon forward.
(D) The buoyant force due to the atmosphere's pressure gradient pushed the balloon forward.
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.
(B) The water pressure and water speed are both larger in the ground floor shower.
(C) The water pressure and water speed are both larger in the second floor shower.
(D) The water pressure is larger in the second floor shower and the water speed is larger in the ground floor shower.
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).
(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.
(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.
(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.
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.
(B) downhill, your feeling of acceleration is directed downhill, and your apparent weight is directed vertically downward.
(C) downhill, your feeling of acceleration is directed uphill, and your apparent weight is smaller in amount than your actual weight.
(D) vertically downward, your feeling of acceleration is directed vertically upward, and your apparent weight is vertically upward.
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.
(B) Since the birdfeeder's total energy always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium.
(C) Since the birdfeeder's momentum always increases whenever it is displaced from its equilibrium, it tends to accelerate back toward that equilibrium.
(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.
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.
(B) turned on, with their exhaust direct away from the distant star.
(C) turned off.
(D) turned on, with their exhaust direct perpendicular to the ship's 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
(B) Westward
(C) Eastward
(D) Southward
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.
(B) Air in the wide duct moves slower, requiring less kinetic energy per liter, and it experiences weaker viscous interactions with the duct.
(C) Air from the wide duct emerges into the space outside the duct at a lower pressure than air emerging from the narrow duct.
(D) Air in the wide duct has a higher density and coasts more easily forward.
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.
(B) The pressure is highest farthest from the center of the circle and the plasma is denser than the red blood cells.
(C) The pressure is highest farthest from the center of the circle and the red blood cells are denser than the blood plasma.
(D) The pressure is highest closest to the center of the circle and the red blood cells are denser than the blood plasma.
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.
(B) The log sinks to the bottom of the lake.
(C) The log descends slightly, so that more of it is located beneath the water level.
(D) The log remains at the same height, so that the same fraction is located beneath the water level.
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.
(B) Its average density is less than the density of vinegar and less than the density of oil.
(C) Its average density is more than the density of vinegar and less than the density of oil.
(D) Its average density is less than the density of vinegar and greater than the density of oil.
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.
(B) the sudden acceleration causes the liquid in the bottle to float above the gas in the bottle for several seconds.
(C) liquid colliding with the bottom of the bottle knocks the bottom out of the bottle.
(D) the surge in pressure near the top of the bottle causes the bottle cap to vibrate loudly.
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
(B) 30 km/h
(C) 90 km/h
(D) 60 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.
(B) The balloon's weight decreases, but the buoyant force increases.
(C) Both forces increase, but the buoyant force increases more rapidly than the balloon's weight.
(D) Both forces increase, but the balloon's weight increases more rapidly than the buoyant force.
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.
(B) downward and equal to the acceleration due to gravity.
(C) downward and greater than the acceleration due to gravity.
(D) zero.
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.
(B) maintains constant velocity.
(C) does not accelerate horizontally.
(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.
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.
(B) Water speeds up.
(C) Water near the inside of the bend speeds up and water near the outside of the bend slows down.
(D) Water near the inside of the bend slows down and water near the outside of the bend speeds up.
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.
(B) The water would continue to flow at the same constant velocity.
(C) The water would flow faster than before because of viscous interactions between the water and the pipe.
(D) The water would flow through the pipe in the opposite direction.
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.
(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.
(C) Her forward lean moves her closer to the finish line so that she arrives sooner.
(D) She leans in the direction of her velocity in order to increase that forward velocity.
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.
(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.
(C) The water's pressure is high throughout its trip: from before the nozzle to when it hits the stone surface.
(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.
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.
(B) The rocket falls, accelerating vertically downward because of its weight.
(C) The rocket begins to orbit the Earth because its weight is zero.
(D) The rocket begins to orbit the Earth because it is not going fast enough to travel into the solar system or beyond.
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.
(B) The 8th floor.
(C) The 12th floor.
(D) The 10th floor.
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.
(B) The density is about 100 times atmospheric density and the pressure is about 100 times atmospheric pressure.
(C) The density is about 100 times atmospheric density and the pressure is the same as atmospheric pressure.
(D) The density is the same as atmospheric density and the pressure is about 100 times atmospheric pressure.
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.
(B) You and your friend experience equal inward accelerations, but your speed is larger than your friend's speed.
(C) You experience a larger inward acceleration than your friend does.
(D) You and your friend experience equal inward accelerations, but your friend's speed is larger than your speed.
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.
(B) The density increases, but the pressure decreases.
(C) The density and pressure remain constant.
(D) The density remains constant, but the pressure decreases.
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.
(B) The force of your momentum is pushing you toward the right.
(C) A friction force from the seat is pushing you toward the right.
(D) A support force from the seat is pushing you toward the right.
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.
(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.
(C) After the rains, the water has a smaller density. During the drought, the water has a larger density.
(D) After the rains, the water has a larger viscosity. During the drought, the water has a smaller viscosity.
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.
(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.
(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.
(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.
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.
(B) The south side pressure is less than atmospheric pressure. The east side pressure is less than atmospheric pressure.
(C) The south side pressure is greater than atmospheric pressure. The east side pressure is less than atmospheric pressure.
(D) The south side pressure is greater than atmospheric pressure. The east side pressure is greater than atmospheric pressure.