Problem 1:

The curves on bicycle racetracks are steeply banked, so that the outer edge of each curve is much higher than its inner edge. This banking tips the support force that the track exerts on the bicycle wheel toward the center of each turn. That center-directed or centripetal force on the bicycle is important because it

Answer: (A) helps the bicycle accelerate inward to complete each turn without skidding. [66.1% picked]

Why: A bicycle that is turning is accelerating horizontally toward the inside of the turn and that acceleration must be caused by a horizontal force toward the inside of the turn. The only horizontal forces that a bicycle can obtain from a level racetrack are frictional forces. Since those forces are limited, a bicycle can only accelerate and turn at a modest rate on a level track. But on a banked racetrack, the support force that the racetrack exerts on the bicycle has a horizontal component and can cause the bicycle to accelerate horizontally. There is no limit to how fast a bicycle can accelerate and turn on a properly banked racetrack.

Problem 2:

You are watching children play a game of tug-o-war with a plastic clothesline. The two teams are pulling at opposite ends of the cord and each team is trying to drag the other team into a mud puddle that lies between them. After a few minutes without progress, the team on the right suddenly pulls hard toward the right. The team on the left has anticipated this threat and is able to keep their end of the rope from moving. The right end of the rope stretches toward the right and the rope breaks. Breaking the rope required energy and that energy was provided by

Answer: (D) the team on the right. [43.2% picked]

Why: To transfer energy to the rope, a team must do work on that rope.The team on the right pulls the rope toward the right and the portion of rope in their hands moves toward the right, so they do work on that portion of rope. The team on the left pulls the rope toward the left, but that portion of rope does not move so they do no work on that portion of rope.

Problem 3:

As water in steady state flows straight up in a vertical pipe with a uniform diameter, that water

Answer: (D) transforms its pressure potential energy into gravitational potential energy. [68.7% picked]

Why: When water is in steady state flow in a uniform pipe, its speed must remain constant. If the water were to speed up, gaps would develop in the flow and that would spoil steady state flow. If the water were to slow down, it would smash together and that would spoil steady state flow, too. So the water's speed in that uniform pipe is constant and so is its kinetic energy per drop. Therefore, only the water's pressure potential energy and gravitational potential energy can change. As the water rises upward, it converts its pressure potential energy into gravitational potential energy.

Problem 4:

You toss a basketball straight up. Disregarding any effects of due to the air, what force or forces are acting on the basketball while it is above your hands?

Answer: (C) Its weight. [70.9% picked]

Why: The basketball is in free fall, meaning that the only force acting on it is its weight (the force of gravity on the basketball). That it continues upward for while is the result of inertia, not an upward force. There is no object exerting an upward for on the basketball... it is rising because its nature is to continue doing what it is doing when no force acts on it. Gravity slows its upward motion, but that upward motion continues for a while nonetheless.

Problem 5:

You are out for a bicycle ride on a calm, windless day. You are heading northward on a level road and are experiencing a pressure drag force that pushes you toward the south. This air resistance explains why you have to keep pedaling to maintain your constant speed. If someone were to examine the air that you have left behind you after you have passed through it, they would find that the air's average velocity is

Answer: (D) northward. [10.6% picked]

Why: As you travel northward through the air, the air exerts a drag force on you in the direction opposite your velocity and you consequently exert a force on it in the direction of your velocity, as required by Newton's third law. So the air pushes you southward and you push the air northward. Your northward force on the air transfers northward momentum to it and that northward momentum causes the air to continue moving northward after you have passed by.

Problem 6:

A pitcher throws a curveball. While the spinning baseball is in the air between the pitcher and home plate, the forces acting on the ball are its weight, a buoyant force, drag forces, a lift force, and

Answer: (A) no other force. [67.8% picked]

Why: There is no forward force acting on the baseball after it leaves the pitcher's hand. It is continuing to travel toward home plate because of inertia alone. Another way to explain this situation is to say that the pitcher gave the baseball momentum toward home plate and the baseball carries that forward momentum with it. The baseball gradually gives some of its forward momentum to the air as it travels toward home plate, so the baseball slows down slightly. But the baseball reaches home plate with most of its forward momentum still on board.

Problem 7:

Air mattresses are hollow, air-filled bags that are used as beds while camping or to accommodate occasional visitors at home. When you lie on a fully inflated air mattress, it supports your weight easily and barely dents at all. But if you stand on it, its surface dents severely and you may actually pop the air mattress. Standing on the air mattress causes the air pressure inside it to rise much more than lying on it does because

Answer: (B) you're supported by the mattress's air pressure and as your contact surface with the mattress decreases, the air pressure needed to support you increases. [61.2% picked]

Why: To support your weight, the air mattress must exert an upward force on you that is equal in amount to your weight. The pressure difference between the air inside and outside the mattress produces an upward pressure force on the mattress's skin and the skin, in turn, pushes up on you to support you. If you are being supported by a large area of mattress skin, then the skin needs only a small pressure difference to produce enough upward force to support you. But if you are being supported by a small area of mattress skin, then the skin needs a huge pressure difference to produce enough upward force to support you.

Problem 8:

You are watering the garden with a hose and nozzle. As the water flows through the nozzle, it converts

Answer: (B) pressure potential energy into kinetic energy. [95.6% picked]

Why: When water in steady state flow goes through a nozzle, it must speed up so as not to cause "traffic jams" in the narrowing flow. It must be converting other forms of energy into kinetic energy. Since there is essentially no change in height in going through a nozzle, the water's gravitational potential energy is not changing. Therefore, the water's pressure potential energy must be transforming into kinetic energy.

Problem 9:

When you pull a tablecloth out from under a set of dishes, it's important to pull the cloth as fast as possible because

Answer: (D) the momentum transferred to the dishes is proportional to the time during which the cloth pulls on them. [43.6% picked]

Why: When you pull the tablecloth, a sliding frictional force will act from the cloth on the dishes. The strength of a sliding frictional force is relatively independent of speed, so you can't change it by pulling faster or slower. You therefore can't change how much work that force does on the dishes, since work is force times distance and neither depends on speed. But you can change the amount of momentum that force transfers to the dishes. The faster you pull, the shorter the time over which sliding friction acts on the dishes and the less momentum it transfers to those dishes.

Problem 10:

You are moving into a loft apartment and are now dragging an old carpet across the floor in a straight line at a steady speed. Which of the following statements about the forces acting on the carpet is correct?

Answer: (A) The amount of force that you're exerting on the carpet must be equal to amount of force that friction is exerting on it. [57.7% picked]

Why: The carpet is coasting across the floor at constant velocity, so it is not accelerating and the net force it is experiencing is zero. Therefore, the forward force you exert on the carpet must exactly balance the backward sliding friction force that the floor is exerting on the carpet. To get the carpet started from rest, you must make it accelerate forward briefly; you must pull it forward more strongly than friction from the floor pulls it backward. But once the carpet is moving, you need only balance the backward frictional force with your forward force so that the carpet can coast forward at constant velocity.

Problem 11:

You are paddling yourself forward in a Kayak, across a calm lake. 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

Answer: (A) in the forward direction, toward the front of boat. [90.7% picked]

Why: As you move the paddle backward through the water, the water exerts a forward force of pressure drag on the paddle. The water is acting to slow the paddle's backward motion through the water by pushing the paddle forward. At the same time, the paddle pushes the water backward. Overall, the paddle (and kayak) receives forward momentum and the water receives backward momentum.

Problem 12:

A bicycle is remarkably stable while it's moving forward because, when you start to tip over,

Answer: (D) the bicycle spontaneously steers so that its wheels drive under your center of gravity. [85.5% picked]

Why: To keep a bicycle upright at its unstable equilibrium, that bicycle needs to place its line-shaped base of support beneath its center of gravity. As you head forward, the bicycle steers automatically to do exactly that. It drives its wheels under your overall center of gravity and rescues you whenever you start to tip.

Problem 13:

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

Answer: (C) the air pressure inside the balloon's opening is the same as the air pressure outside that opening. [45.4% picked]

Why: The fact that the air just inside the balloon's open remain there, accelerate neither in nor out of the opening, means that there must be no pressure variations in that region. The pressure just inside the opening and the pressure just outside the opening must be equal. The air in the opening doesn't accelerate and doesn't begin to flow either in or out of the balloon.

Problem 14:

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

Answer: (C) higher than atmospheric pressure. [50.7% picked]

Why: As the water encounters the wall, it bends away from the wall. For the water to bend away from the wall, the water pressure must be higher at the wall's surface than far from the wall's surface (where the pressure is atmospheric). So the pressure at the wall's surface must be higher than atmospheric pressure.

Problem 15:

If a golf ball were smooth rather than dimpled, it would

Answer: (A) not travel as far after being hit by a golf club. [90.3% picked]

Why: The dimples of the golf ball "trip" the boundary layer of air that is flowing around toward the back of the ball. That boundary layer air is struggling to flow into a rising pressure and is also rubbing against the surface of the ball and therefore losing energy. If the boundary layer stays laminar, the innermost layer of that boundary flow will soon come to a stop and trigger turbulence. A huge, turbulent wake will appear behind the ball and the ball will experience severe pressure drag. But because the dimples trip the boundary layer and make it turbulent, the various layers of air within the boundary layer inchange frequently and they work together to help the boundary layer travel toward the back of the ball. The result is a smaller turbulent wake, so the dimpled golfball experiences less pressure drag and flies farther.

Problem 16:

When a modern car crashes into a tree and comes to an abrupt stop, the driver's face and chest collide with an air bag rather than with the steering wheel. The driver's chances of serious injury are reduced by hitting the air bag rather than the steering wheel because the driver transfers

Answer: (A) the same amount of momentum to the air bag as he would to the steering wheel if there were no air bag, but he does so with a smaller force because of the air bag. [85.9% picked]

Why: When the driver's car stop, the driver will transfer all of the driver's forward momentum to either the steering wheel or the airbag. The momentum transfer is the same in either case, but the transfer is slower when done to the airbag than when done to the steering wheel. That slower momentum transfer involves a smaller force and is less likely to cause injury.

Problem 17:

You are riding on a playground swing. As you swing forward, there is a moment when you are directly below the pivot that supports the swing. At that moment, you are

Answer: (C) accelerating upward. [48.5% picked]

Why: As you pass through that lowest point, you are traveling at peak speed and are neither speeding up nor slowing down. But while your speed is constant at that one special point, your direction of travel is changing and so you are accelerating. A momentum before this point, you were heading slightly downward and a momentum after this point, you will be heading slightly upward. Therefore, at this special point, you are changing your direction and accelerating directly upward. Another way to view this special situation is to notice that for one brief momentum, it's as though you are traveling in a circle at a constant speed, with the center of that circle being the pivot for the swing. You are accelerating toward the center of the circle and that's upward.

Problem 18:

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?

Answer: (B) They are both exerting the same amount of force on one another. [93.4% picked]

Why: The force the hammer exerts on the nail and the force that the nail exerts on the hammer must be equal in amount but opposite in direction. They are a Newton's third law pair.

Problem 19:

A block of wood is floating easily on the surface of a lake. As a storm approaches, the density and pressure of the air above the lake decrease. As a result of this decrease in air pressure and density, the block of wood

Answer: (C) moves downward slightly and floats a little deeper in the water. [26.0% picked]

Why: In the initial situation, the wood is supported by a buoyant force that is exactly equal in amount to the fluid that the wood is displacing. That fluid is a mixture of water and air! Because the wood is in equilibrium, the buoyant force on the wood is equal in amount to its weight. But when the air density decreases, the wood no longer displaces its weight in fluid. Instead, it displaces slightly less than its weight in fluid. It therefore experiences a net force downward and it descends. It settles into a new equilibrium in which it displaces slightly more water than before and slightly less of the lower-density air.

Problem 20:

A helium balloon floats in air because the helium balloon contains

Answer: (C) the same number of particles as the air it displaces, but each helium atom weighs less than the average air molecule. [89.0% picked]

Why: The air outside the ballon and the helium inside the balloon have equal pressures and temperatures, so they also have equal particle densities. Each cubic meter of the air and each cubic meter of the helium contain the same number of gas particles. But since each helium atom weighs only about 14% as much as an average air molecule, the helium balloon weighs much less than the air it displaces.

Problem 21:

All current rockets eject stages or booster rockets as they climb from the earth's surface up to orbit. A rocket that didn't eject any stages or booster rockets while climbing from the ground to orbit would

Answer: (B) have to have a launch weight that is about 90% rocket fuel. [59.0% picked]

Why: The earth's gravity is strong enough that it's quite hard for a single chemical rocket to reach orbit. That rocket must eject roughly 90% of its launch weight as exhaust. It is so difficult to build a rocket that is 90% fuel at launch; such a rocket is likely to be too fragile to survive the stresses of launch. Instead, all current rockets reach orbit by staging: the rocket ejects stages (fuel holders) as it makes its way to orbit.

Problem 22:

You're filling a jar of honey from the spigot at the bottom of a large, nearly empty barrel at the grocery store. The honey flows extremely slowly, so the store manager has the barrel refilled. Now the honey flows much more rapidly from the spigot because

Answer: (D) the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases. [84.6% picked]

Why: The honey flows slowly from the nearly empty barrel because the pressure at the spigot is only slightly higher than atmospheric pressure. Once the barrel has been filled with honey and the honey near the spigot must support the addition weigh of all that honey above it, the pressure at the spigot is much larger than atmospheric. With the large pressure difference between inside the spigot and outside the spigot, the honey is able to flow quickly through the spigot despite experiencing substantial viscous drag forces as it flows through the narrow openings.

Problem 23:

A curve ball's path bends to the right as it flies toward home plate because it's experiencing an aerodynamic force to its right. One reason why the ball is experiencing this aerodynamic lift force is that the ball is spinning and that spin

Answer: (D) causes the air to flow faster and farther around the right side of the ball than around the left side of the ball. [39.2% picked]

Why: A curveball curves because it is spinning and because that spin carries the airflow farther around one side of the ball than the other. For the ball to curve toward the right, it must be carrying the air farther around the right side of the ball so that the air leaves the ball at slightly to the the left side of center. That air has been deflected toward the left and required that the ball push it toward the left. The air reacts by pushing the ball toward the right, so that the ball curves toward the right.

Problem 24:

Two glass marbles, one of which weighs twice as much as the other, roll off of a horizontal table. As they leave the table, the lighter marble is traveling twice as fast as the heavier marble. In this situation,

Answer: (A) the lighter marble hits the floor about twice as far from the table as the heavier marble. [69.2% picked]

Why: The faster-moving marble travels twice as fast horizontally as the slower-moving marble, even though the two marbles are falling. They take the same time to fall to the floor, but the faster-moving marble has moved twice as far from the table aas the slower-moving ball when the two marbles hit.

Problem 25:

You throw a snowball straight up into the air. It rises to a peak height and then descends. At the moment it reaches its peak height, the snowball's velocity is

Answer: (C) zero and its acceleration is downward. [84.1% picked]

Why: At the top of its verticle travel, the snowball is momentarily motionless -- it is moving neither up nor down at that momentum. However, the only force acting on the snowball is its weight so it is falling. The net force on the snowball is its downward weight and, since net force causes acceleration, the snowball is accelerating downward (in the direction of the net force). Another way to look at why the snowball is accelerating downward even at that moment of peak height is to note that the moment before this one, the snowball was traveling upward and the moment after this one, the snowball will be traveling downward. The snowball's velocity is therefore changing between these three moments and the snowball must therefore be acceleration. After all, acceleration is the change in velocity with time and the snowball's velocity is changing with time!

Problem 26:

You are on a 10th floor hotel balcony, overlooking the pool. You lower a 100-foot-long straw into the pitcher of lemonade at the bar and try to suck the lemonade up to your mouth. What happens?

Answer: (A) You are unable to suck the lemonade up to your mouth. [75.8% picked]

Why: When you drink lemonade with a normal straw, you are attracting the lemonade with your mouth. Instead, you are reducing the pressure of the gas inside the straw above the lemonade. With atmospheric pressure pushing up on the column of lemonade in the straw and less than atmospheric pressure pushing down on the column of lemonade in the straw, there is an overall upward pressure force on the column of lemonade. When that upward pressure force exceeds the weight of the column of lemonade, the lemonade accelerates upward and begins to move toward your mouth. But the longer the column of lemonade in the straw, the more it weighs and the harder it is for the pressure force to accelerate it upward. Supporting a taller column of lemonade requires that the difference in pressure between atmospheric pressure at the bottom of the column and the reduced pressure inside the top of the straw become larger. But that pressure difference can't exceed atmospheric pressure since the lowest possible pressure at the top of the straw is zero pressure. Once the column of lemonade is more than 10 meters tall, even zero pressure at the top of the straw won't be enough to allow atmospheric pressure to support the column of lemonade. A column of lemonade 100 feet tall (about 30 meters) is far to heavy for atmospheric pressure to support.

Problem 27:

You hold a new 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,

Answer: (C) liquid colliding with the bottom of the bottle knocks the bottom out of the bottle. [90.7% picked]

Why: This demonstration is one I did class. When you pound the top of the bottle with a mallet, the bottle accelerates downward rapid, but the beverage remains inertial and is left behind. The beverage finds itself in the neck of the bottle, where it compresses the tapped air. Once the bottle stops accelerating, the compressed air accelerates the beverage back toward the bottom of the bottle. The beverage picks up speed and collides with the bottom of the bottle. Since the beverage now has substantial downward momentum, it must transfer that momentum to the bottle to push the bottle along with it. It transfers that momentum by developing a huge pressure at the bottom of the beverage. That enormous pressure, known as water hammer, knocks the bottom of the bottle out.

Problem 28:

You stop for a cappuccino at Alderman Café and notice that the tiny white bubbles of steamed milk remain on the surface of the coffee. These air-filled bubbles stay where they are, rather than descending into the coffee or rising into the air, because they are

Answer: (C) less dense than the coffee but more dense than the air above the coffee. [85.5% picked]

Why: The bubbles are at equilibrium, so each bubble must be experiencing an upward buoyant force that exactly cancels its downward weight. That bubble must be displacing a mixture of fluids (coffee and air) that exactly equals its weight. Equivalently, the bubble's average density is equal to the average density of the mixture it displaces. Since the bubble rests at the boundary between the dense coffee and the low-density air, the bubble's average density must be less than that of coffee but more than that of air.

Problem 29:

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 toward the inside of a turn keeps you from flipping over because the overall force that the ground exerts on the wheel

Answer: (A) produces zero torque on you and the bicycle about your combined center of mass. [33.5% picked]

Why: During a turn, the bottom bicycle's wheel experiences two forces: a support force upward and a frictional force toward the inside of the turn. Those two forces together must exert a force directly at the bicycle and rider's overall center of mass in order that they not cause angular acceleration of the bicycle and rider.

Problem 30:

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

Answer: (B) taller. [63.9% picked]

Why: The taller the water and its contents, the more energy each drop of water has available to it. While at the top of the water tower, that energy is in the form of gravitational potential energy. But as the water flows downward to the plumbing system, the gravitational potential energy is transformed into pressure potential energy. Tall water towers are energy storage devices that pressurize the plumb system and that can provide large amounts of power during peak demand. A water toward can be refilled at times when energy is relatively inexpensive, such as late at night.