Given Monday, December 9, 1996, from 9:00 AM to 12:00 Noon

PART I: MULTIPLE CHOICE QUESTIONS

Please mark the correct answer for each question on the bubble sheet. Fill in the dot completely with #2 pencil. Part I is worth 67% of the grade on this examination.

Problem 1:

You throw a ball at the wall in front of you and the ball bounces directly back at you. While the ball is touching the wall, the horizontal force(s) the ball is experiencing is (are)

(A) the support force from the wall plus a forward force from the ball's momentum throughout the bounce.

(B) only the support force from the wall.

(C) the support force from the wall plus a force from the ball's momentum that initially points forward, gradually reduces to zero as the ball stops, and then begins to point backward as the ball rebounds.

(D) the support force from the wall plus a forward force from the ball's momentum while it's coming to a stop.

Problem 2:

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

(A) zero and that it is calm.

(B) northward.

(C) zero but that it is swirling rapidly in all directions.

(D) southward.

Problem 3:

You are juggling grapefruits in your hands and are about to toss one far above your head. After it leaves your hand, the grapefruit will experience

(A) an upward force as it rises. This upward force will gradually diminish to zero at the grapefruit's maximum height, after which the grapefruit will experience only the downward force of its weight.

(B) only the downward force of its weight, both before and after it reaches its maximum height.

(C) both an upward force and the downward force of its weight as it rises. The upward force will gradually diminish to zero at the grapefruit's maximum height, after which the grapefruit will experience only the downward force of its weight.

(D) the upward force of its weight as it rises. Once the grapefruit reaches its maximum height, it will begin to experience the downward force of its weight.

Problem 4:

You are swimming at the beach. You swim out past the breaking surf so that you can float among the waves. Each time a wave crest passes, you travel

(A) horizontally toward shore for a distance equal to one wavelength of the passing wave.

(B) in a circle-up, toward shore, down, and away from shore.

(C) horizontally toward shore for a distance that's proportional to the slope of the wave crest.

(D) vertically-directly up and then directly down.

Problem 5:

An incandescent light bulb that has operated for a long time will have a dark spot on the top of the bulb. This dark spot is evidence that the bulb contains inert gas because the dark spot forms when

(A) the inert gas carries light downward from the top of the bulb so that it can be emitted by the filament.

(B) the buoyant force of the inert gas lifts the filament upward so that it brushes against the top of the bulb.

(C) convection in the inert gas carries filament atoms up to the top of the bulb.

(D) microwaves from the filament heat the inert gas and produce light, along with overheating the glass near the top of the bulb.

Problem 6:

Suppose that a shoelace passes without friction through 5 holes on each side of your shoe. If the tension in that shoelace is 10 N, how much total force is the shoelace exerting on the right side of your shoe?

(A) 25 N

(B) 10 N

(C) 50 N

(D) 100 N

Problem 7:

When you rub your hands together vigorously, the temperature of your skin increases because sliding friction

(A) converts the work you do in sliding your hands across one another to thermal energy in your skin.

(B) prevents convection and conduction from removing heat from the skin of your hands.

(C) transfers heat from your shoulders and arms to the skin on your hands.

(D) causes moisture to condense on your skin and to release its latent heat of vaporization as thermal energy.

Problem 8:

If you reach briefly into an oven containing 400° F (204° C) air, that air will feel hot but won't hurt you. However, if you put your fingers in the 212° F (100° C) gaseous steam that's emerging from the spout of a tea kettle, you're likely to get burned. The cooler steam is more likely to burn you than the hotter air because

(A) the water molecules in the steam have much higher temperatures than the steam itself.

(B) water molecules are more massive than air molecules and thus have larger thermal kinetic energies than air molecules, even at these temperatures.

(C) the steam condenses to liquid water and releases an enormous amount of thermal energy in the process.

(D) the transfer of heat is fastest when the temperature difference between two objects is smallest.

Problem 9:

In a cartoon, Bugs Bunny is surfing on a 10 foot wave crest that's rapidly approaching the shore. The wave crest travels onto the shore intact, with Bugs still on top of it, and continues to carry him across level land for the next twenty miles. This sort of thing can't happen in real life because

(A) wave crests are made from local water and there is no water on land from which to build the crest.

(B) friction between the moving wave crest and the land will slow the crest to a stop in about a mile.

(C) the wave crest will begin to roll like a wheel once it reaches the shore and Bugs will be run over by it as it turns.

(D) Bugs is in an unstable equilibrium and won't be able to stay on top of the crest for more than a mile or two.

Problem 10:

You're filling in for Wily Coyote in a Roadrunner cartoon. You're trying to tip a huge boulder off a cliff and onto the poor bird. You have just enough strength to start the boulder rocking rhythmically back and forth. To make the boulder rock farther and farther, you should only push it forward when it's

(A) on the far side of its equilibrium position.

(B) rocking away from you.

(C) on your side of its equilibrium position.

(D) rocking toward you.

Problem 11:

Your car has a flat tire. You get out the jack, place it under the corner of the car, and begin to turn the crank that operates the jack. To make the car rise 1 cm, you must push the jack handle for a distance of 100 cm. Assuming that there is no friction in the jack and ignoring the weight of the jack itself, the work you do on the jack handle is

(A) zero.

(B) equal to the work the jack does on the car.

(C) more than the work the jack does on the car.

(D) less than the work the jack does on the car but more than zero.

Problem 12:

Independent, gaseous water molecules can leave the surface of hot water at 212° F (100° C)

(A) but not the surfaces of room temperature water at 68° F (20° C) or ice at 0° F (-18° C).

(B) and the surface of room temperature water at 68° F (20° C) but not the surface of ice at 0° F (-18° C).

(C) and the surface of ice at 0° F (-18° C) but not the surface of room temperature water at 68° F (20° C).

(D) and the surfaces of room temperature water at 68° F (20° C) and ice at 0° F (-18° C).

Problem 13:

You are floating along in a hot air balloon. You look up and notice that the bottom of the balloon is open. Neglecting any diffusion of molecules, hot air remains inside the balloon despite this opening because

(A) hot air has a lower pressure than cold air, so hot air is drawn into the balloon by the partial vacuum inside it.

(B) hot air has more inertia than cold air and doesn't accelerate easily.

(C) the air pressure inside the balloon's opening is the same as the air pressure outside that opening.

(D) the propane burner located below the opening keeps pushing the hot air back into the balloon.

Problem 14:

Many modern watches use liquid crystals to display the time in digital form. Electric forces are used to orient or disorient the liquid crystal molecules in order to form visible numbers on the face of a watch. If you raise the temperature of a liquid crystal display (LCD) watch by putting it in the oven, the display will temporarily lose its ability to form numbers. That's because the elevated temperature

(A) destroys the orientational order of the liquid crystal molecules in the display.

(B) causes the liquid crystal molecules to begin moving about, instead of remaining stationary the way they do at room temperature.

(C) lowers the masses of the liquid crystal molecules so that they respond too easily to the electric forces of the watch.

(D) raises the masses of the liquid crystal molecules so that they don't respond to the electric forces of the watch.

Problem 15:

You are choosing the filament for a light bulb and want it to emit visible light as efficiently as possible when it becomes very hot. You should make that filament

(A) black

(B) shiny like a mirror

(C) gray

(D) white

Problem 16:

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

(A) experience too much drag while in orbit and would be unable to remain in orbit for more than a day or two.

(B) not have anything beneath the rocket on which to push and would be unable to propel itself upward after it left the launch pad area.

(C) clutter the space around the earth with orbiting debris.

(D) have to have a launch weight that was about 90% rocket fuel.

Problem 17:

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

(A) travel much too far after being hit by a golf club.

(B) curve toward the left after being hit by the club of a right-handed golfer.

(C) curve toward the right after being hit by the club of a right-handed golfer.

(D) not travel as far after being hit by a golf club.

Problem 18:

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

(A) you have much more kinetic energy when you're standing than when you're lying down and this kinetic energy becomes pressure potential energy in the air mattress.

(B) you weigh more when you're standing up than you do when you're lying down.

(C) you have much more gravitational potential energy when you're standing than when you're lying down and this gravitational potential energy becomes pressure potential energy in the air mattress.

(D) 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.

Problem 19:

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

(A) moves upward slightly and floats higher in the water.

(B) continues to float just as it did before.

(C) moves downward slightly and floats lower in the water.

(D) sinks to the bottom of the lake.

Problem 20:

If you take a glass baking dish out of a hot oven and plunge it into cold water, it will probably shatter because the sudden cooling will

(A) reduce the glass's density so abruptly that it will vibrate at its resonant frequency and shatter.

(B) cause its thermal energy to flow out so rapidly that the chemical bonds between the glass molecules become unstable.

(C) cause it to shrink unevenly so that internal forces will tear it apart.

(D) reduce the force of the glass's momentum and convert its thermal energy into a torque that accelerates the glass apart.

Problem 21:

During a Fourth-of-July fireworks display, a small cannon-like device is used to toss each firework high into the air. The ball-shaped firework sits atop a small lump of gunpowder at the closed end of a cylindrical tube. When the gunpowder is ignited, it suddenly turns into hot gas and the enormous pressure this gas exerts on the firework accelerates that firework upward. The firework emerges from the tube's open end and rises rapidly into the sky. As the firework moves upward through the tube, the trapped gas expands behind it. As the gas pushes the firework upward, the temperature of this gas

(A) increases.

(B) stays almost constant even when the firework leaves the tube.

(C) stays almost constant until the firework leaves the tube, at which time the temperature suddenly decreases.

(D) decreases.

Problem 22:

A guitar string that normally vibrates with a fundamental frequency of 110 Hz is also capable of vibrating at 330 Hz. When that higher frequency vibration occurs, the string is vibrating

(A) with only one third of its normal mass.

(B) with only one third of its normal amplitude of oscillation.

(C) as 3 strings that are one third as long as the overall string.

(D) with only one third of its normal tension.

Problem 23:

Which of the following is not a harmonic oscillator?

(A) a tuning fork with its two tines vibrating rhythmically in opposite directions.

(B) a superball bouncing on a ceramic tile floor.

(C) a person bouncing gently up and down at the end of a bungee cord, near the end of the ride.

(D) a vertical flagpole swaying back and forth.

Problem 24:

Suppose that you place a timer and a tiny pile of gunpowder in an absolutely rigid box. You then seal the box completely and insulate it perfectly so that no heat can flow in or out of the box. Soon the timer ignites the gunpowder and it burns to form smoke. Since the box is sealed, the smoke remains inside it. If you look into the box a few minutes later, you will find that the smoke has not converted back into the gunpowder because

(A) there is not enough energy in the box to reform the gunpowder.

(B) there are not enough atoms in the box to reform the gunpowder.

(C) such an event, unburning the smoke to form gunpowder, is extraordinarily unlikely.

(D) the basic laws of motion, otherwise known as Newton's three laws, forbid the unburning of smoke to form gunpowder.

Problem 25:

Panama is a narrow country that's bounded on the west by the Pacific Ocean and on the east by the Atlantic Ocean. When it's high tide on Panama's Atlantic side, it's

(A) midway between high and low tides on Panama's Pacific side, but heading toward low tide.

(B) low tide on Panama's Pacific side.

(C) midway between high and low tides on Panama's Pacific side, but heading toward high tide.

(D) high tide on Panama's Pacific side.

Problem 26:

A person is using a water hose to clean the sidewalk outside a shop. As the stream of water from the hose encounters an object on the sidewalk, the object accelerates away from the onrushing water. This effect is explained by the fact that the stream of water

(A) has kinetic energy which is converted to momentum when it touches the object.

(B) has kinetic energy which is converted to a force when it touches the object.

(C) is at atmospheric pressure while in flight but its pressure rises as it slows down near the object's surface.

(D) forms a continuous chain of material between the person's hand and the object. As the person pushes the end of the hose forward, the object moves forward.

Problem 27:

You are pushing a file cabinet across the floor in a straight line at a steady speed. Which of the following statements about the forces acting on the file cabinet is correct?

(A) The magnitude of the force that you're exerting on the file cabinet must be more than the magnitude of its weight.

(B) If you were to exert twice as much force on the file cabinet, it would slide across the floor at twice its original speed.

(C) The magnitude of the force that you're exerting on the file cabinet must be equal to the magnitude of the force that friction is exerting on it.

(D) The magnitude of the force that you're exerting on the file cabinet must be more than the magnitude of the force that friction is exerting on it.

Problem 28:

As air flows through the horizontal ductwork in a warehouse, that air passes through a fan inside the duct and the air's total energy increases. Since the air's speed doesn't change as the result of going through the fan, you know that the air's

(A) kinetic energy has decreased but its pressure has remained the same.

(B) pressure has increased but its kinetic energy has remained the same.

(C) pressure has decreased but its kinetic energy has remained the same.

(D) kinetic energy has increased but its pressure has remained the same.

Problem 29:

People fighting forest fires carry emergency tents that have shiny aluminum outer surfaces. If there is trouble, a fire fighter can lie under the tent to block the heat from burning trees overhead. The tent helps because

(A) only conduction carries heat downward toward the fire fighter and the aluminum tent conducts that heat harmlessly into the ground.

(B) only radiation carries heat downward toward the fire fighter and the aluminum tent reflects most of that radiation.

(C) only convection carries heat downward toward the fire fighter and the aluminum tent blocks most of the heat carried by convection.

(D) both conduction and radiation carry heat downward toward the fire fighter and the aluminum tent blocks most of that heat.

Problem 30:

During your visit to Mexico City-altitude: 7,300 ft (2,225 m) above sea level-you decide to cook an egg in boiling water. In Virginia, you are used to cooking your eggs for 3 minutes in boiling water, so you do the same in Mexico City. But when you open the egg, you find that it's almost raw. That's because

(A) gravity is essential to cooking and the stronger gravity in Virginia speeds cooking relative to that in Mexico City.

(B) the steam that emerges from the water in Virginia is more dense than that in Mexico City. In Virginia, the steam sinks downward in air and contributes to the cooking of the egg while in Mexico City, the steam rises upward in air and doesn't help cook the egg.

(C) boiling water in Mexico City is much cooler than boiling water in Virginia.

(D) air pressure is essential to cooking and the higher air pressure in Virginia speeds cooking relative to that in Mexico City.

Problem 31:

A racquetball is a soft rubber ball that's filled with air. If you put a racquetball on the floor and stamp on it very hard with your foot, so that the ball flattens into a disk, the temperature of the air inside the ball will

(A) rise.

(B) fall and the air will liquefy.

(C) fall but the air will remain gaseous.

(D) stay the same.

Problem 32:

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

(A) causes the air to flow more rapidly around the right side of the ball than around the left side of the ball.

(B) exerts a torque on the ball that causes its angular momentum to shift toward the right.

(C) causes the air to flow more rapidly around the left side of the ball than around the right side of the ball.

(D) exerts a torque on the ball that causes its angular momentum to shift toward the left.

Problem 33:

Houses often emit creaking sounds as they cool off at night. Since sound carries energy, the house must obtain that energy from somewhere. While this energy arrives in the house as heat during the day, the sound can't appear until the outside temperature drops because

(A) the house's thermal energy increases most rapidly after dark.

(B) sound, like heat, can only flow from hotter objects to colder objects.

(C) building materials become too soft during the day to vibrate with enough efficiency to convert thermal energy into sound energy.

(D) the house's thermal energy can only become work and sound as heat flows from the hotter house to the colder outside air.

Problem 34:

You are in the kitchen with three mixing bowls in front of you. One bowl is metal, the second is glass, and the third is plastic. All three are at exactly the same temperature: the 68° F (20° C) temperature of the room. If you touch the three bowls together,

(A) heat will flow from the plastic bowl to the glass bowl, and from the glass bowl to the metal bowl.

(B) heat will flow from the metal bowl to the glass bowl, and from the glass bowl to the plastic bowl.

(C) heat will flow from the glass bowl to both the plastic bowl and the metal bowl.

(D) no heat will flow between the bowls.

Problem 35:

The giant waves or tsunamis created by earthquakes have extremely long wavelengths. They travel

(A) extremely quickly because they involve only the water near the ocean's surface.

(B) extremely slowly and involve water to great depth in the ocean.

(C) extremely slowly because they involve only the water near the ocean's surface.

(D) extremely quickly and involve water to great depth in the ocean.

Problem 36:

You're studying for an exam on the eighth floor of your luxury apartment building. You look out the window and notice that one of your neighbors is throwing a party on the ground-floor terrace and has placed a huge punch bowl full of an interesting looking beverage directly below your window. You quickly string together 80 drinking straws to form a giant straw that can reach the punch bowl 80 feet below. You dip the straw into the punch and begin to suck. When you use a single drinking straw to drink something, it takes you 0.1 seconds to raise the liquid to your lips. But when you use this giant drinking straw,

(A) you find that you can't raise the liquid to your lips no matter how hard you try.

(B) it takes you 8 seconds (80 times 0.1 second) to raise the liquid to your lips.

(C) it takes you 800 seconds (80 divided by 0.1 second) to raise the liquid to your lips.

(D) it takes you 640 seconds (80 times 80 times 0.1 second) to raise the liquid to your lips.

Problem 37:

If you blow carefully across the top of a drinking straw, it will emit a tone. If you now close off the bottom of the straw with your finger, the tone that it emits will

(A) go up by an octave-its frequency will increase by a factor of 2.

(B) go down by an octave-its frequency will decrease by a factor of 2.

(C) stay the same.

(D) go down by a fifth-its frequency will decrease by a factor of 1.5.

Problem 38:

You are riding a multi-speed bicycle up a steep hill that's 100 m high and want to do as little work as possible on your way up the hill.

(A) Your choice of gears doesn't matter because you'll do the same amount of work climbing the hill no matter which gear you choose.

(B) You should choose the largest crank sprocket (attached to the pedals) and the smallest freewheel sprocket (attached to the rear wheel).

(C) You should choose the smallest crank sprocket (attached to the pedals) and the largest freewheel sprocket (attached to the rear wheel).

(D) You should choose the largest crank sprocket (attached to the pedals) and the largest freewheel sprocket (attached to the rear wheel).

Problem 39:

A helium balloon floats in air because the helium balloon contains

(A) more particles than the air it displaces and these particles push upward more strongly on the top surface of the balloon.

(B) faster-moving helium atoms which use their kinetic energies to displace the force of gravity.

(C) fewer particles than the air it displaces.

(D) the same number of particles as the air it displaces, but each helium atom weighs less than the average air molecule.

Problem 40:

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

(A) accelerating forward.

(B) not accelerating at all.

(C) accelerating backward.

(D) accelerating upward.

Problem 41:

You enter an elevator on the ground floor of a tall building and push the button for the 10th floor. During your trip upward there are times when you feel your normal weight, when you feel relatively heavy, and when you feel relatively light. The order in which you experience those feelings is

(A) normal, then light, then heavy.

(B) light, then normal, then heavy.

(C) heavy, then normal, then light.

(D) normal, then heavy, then light.

Problem 42:

It's a hot summer day and your friend's apartment has no air conditioning. However, a brand new refrigerator lies unopened in the center of the living room. Being a creative person, your friend closes all the doors and windows of the apartment, unpacks the refrigerator, plugs it in, and turns it on. Your friend then opens the refrigerator door and uses a small fan to circulate the air past the refrigerator and throughout the room. After the refrigerator has operated for a few minutes, the average temperature in the room is

(A) lower than it was before.

(B) higher than it was before.

(C) the same as it was before and the temperature is uniform throughout the room.

(D) the same as it was before, but some regions are hotter and others are colder.

Problem 43:

Some satellites orbit the earth at such large distances that they are never in the earth's shadow. These satellites are constantly exposed to full sunlight. With no air around them to take away heat, why don't these satellites continue to grow hotter forever?

(A) They have solar panels that convert the sun's thermal radiation completely into electricity and avoid any need to eliminate heat.

(B) Their temperatures rise until they are able to radiate heat away into space as fast as it arrives from the sun.

(C) They use air conditioners (which NASA calls "thermal stabilization units") to eliminate the excess thermal energy as rapidly as it arrives.

(D) Because they are isolated from the sun by empty space, the sun's heat can't reach them and they don't experience any changes in temperature.

Problem 44:

You are bouncing up and down on a spring board, preparing to dive into the pool. While you're in the air above the board, your acceleration is

(A) zero because you are not touching anything.

(B) upward and constant until you reach the peak, then it becomes downward and constant.

(C) downward and constant.

(D) initially upward but it gradually diminishes to zero as you reach the peak and then it gradually becomes more and more downward.

Problem 45:

The reason for using a harmonic oscillator as a clock's time-keeper is so that the clock will keep accurate time even if the

(A) frequency of the time-keeper's oscillation changes.

(B) stiffness of the time-keeper's restoring force changes.

(C) amplitude of the time-keeper's oscillation changes.

(D) mass of the time-keeper's moving object changes.

Problem 46:

Which of these stars has the hottest surface temperature?

(A) A bluish star.

(B) A yellowish star.

(C) The sun.

(D) A reddish star.

Problem 47:

When you drop a rubber ball on the floor and it bounces, the direction of its velocity reverses because

(A) the ball can't stop moving and the floor blocks its path.

(B) the ball's momentum is conserved.

(C) the ball's energy is conserved.

(D) the floor exerts an upward support force on the ball and this force stops the ball's descent and eventually propels it upward.

Problem 48:

When you jump while standing on a bathroom scale, it briefly reads more than your actual weight. During that moment, it's exerting an upward force on you that is greater than your weight and

(A) your velocity is constant but upward.

(B) your velocity is constant but downward.

(C) you are accelerating upward.

(D) you are accelerating downward.

Problem 49:

You're on an airplane trip to the islands for vacation. Your first flight is on a propeller driven airplane. The air flowing toward the spinning propeller blades doesn't slow down before it encounters those blades. Your second flight is on a turbofan-driven jet airplane. The air flowing toward the spinning fan blades slows down significantly before it encounters those blades. The air's pressure just before it encounters the turbofan's fan blades is

(A) much higher than the air's pressure just before it encounters the propeller's blades.

(B) much lower than the air's pressure just before it encounters the propeller's blades.

(C) equal to atmospheric pressure at the plane's altitude.

(D) the same as the air's pressure just before it encounters the propeller's blades.

Problem 50:

A common glass thermometer contains a red or silver liquid that rises upward in a glass tube as the temperature of the thermometer increases. The liquid moves up the tube because

(A) only the glass contracts as its temperature decreases, so the liquid moves up the tube to occupy more of the tube's internal space.

(B) while both the glass and the liquid contract as their temperatures increase, the glass contracts more rapidly than the liquid does.

(C) the pressure of gas above the liquid decreases as its temperature rises and the liquid is sucked upward in the glass tube.

(D) while both the glass and the liquid expand as their temperatures increase, the liquid expands more rapidly than the glass does.

PART II: SHORT ANSWER QUESTIONS

Please give a brief answer in the space provided. Part II is worth 33% of the grade on this examination.

Problem 1:

A boat floats on the surface of a still lake, supported by the buoyant force. The average density of the boat, including the air it contains, is less than that of the lake water.

(A) What is the net force on the boat?

(B) If the boat were moved upward a few centimeters for some reason and then released, what would happen to the net force on the boat the moment after it was released?

(C) The floating boat is in a stable equilibrium with respect to up and down motion. Explain why this is so.

(D) Does the air exert a buoyant force on the boat? If so, why doesn't the boat float into the sky?

Problem 2:

Since firefighter use water to put out fires, handling that water is half their job. That job becomes particularly difficult when they are fighting a fire in a high-rise building.

(A) The water pressure in a fire hydrant is about 500,000 pascals (about 5 times atmospheric pressure). When the firefighters attached a hose directly to a fire hydrant, they find that water won't flow out of the hose above the 10th floor of the building, located about 50 meters above the sidewalk. Why won't the water flow?

(B) A spectator suggests that the firefighters return to the 8th floor, where water does flow from the hose, and attempt to spray water up the stairwell to the 11th floor. Explain why this strategy will or will not work.

(C) The firefighters find that water leaving a hose with a narrow nozzle on it travels faster than water leaving that same hose with no nozzle. Why does narrowing the opening of the hose speed up the water emerging from it?

(D) When water hits hot material in a fire, it turns to steam. This steam takes up much more space than the water did and displaces air from the room. Without oxygen to sustain it, the fire quickly goes out. But the hot material must be much, much hotter than 100° C, water's boiling temperature, for the water that hits it to turn completely into steam. Why?

Problem 3:

You have fashioned a modest catapult out of a truck's leaf spring-a stiff, elastic strip of springy metal that's shaped like a very large ruler. One end of the strip is bolted to wagon full of sand and the other end of the strip is carved into a bowl. You put an apple in the bowl, bend the strip backward with your foot, and then step out of the way. The apple flies several hundred feet into the air.

(A) When you step on the bowl end of the metal strip, it bends downward 50 centimeters. If someone half your weight were to step on the strip instead of you, how far downward would it bend?

(B) You're holding the bowl-end of the strip down with your foot and the apple in the bowl isn't moving. What forces are acting on the apple?

(C) You step off the bent strip and it quickly straightens out, flinging the apple skyward. As the strip straightens out and pushes the apple upward, which force is larger: the force the apple exerts on the strip or the force the strip exerts on the apple?

(D) As the apple rises, its upward momentum gradually decreases. Neglecting air resistance, what object is the apple transferring its momentum to?

Problem 4:

You are holding one end of a ruler against the side of a table, so that the other end of the ruler extends forward into the air. The free end of the ruler can flex from side to side horizontally. When you pluck the ruler's free end, by pulling it to the left and then letting go, that end vibrates back and forth horizontally.

(A) Why does the ruler's frequency of oscillation decrease when you extend more of it away from the table?

(B) Why doesn't the ruler's frequency of oscillation depend on how hard you pluck it?

(C) What forms does its energy take as the ruler vibrates from side to side?

(D) What eventually happens to the ruler's energy?

Problem 5:

The wheel bearings in your new roller-blades are so good that they're essentially frictionless-the wheels turn without any friction at all. You decide to wear the roller-blades on your feet as you walk your dog on leash. The leash is made of Kevlar, a high-strength polymer that's so light that you can ignore its weight in this problem. The dog is pulling you up a smooth, constant incline at a constant velocity.

(A) You are exerting a downhill force of 30 N on the leash. What force is the dog exerting on the leash?

(B) Suppose that there were a spring scale built into the leash to report how much tension the leash is experiencing. What tension would it report?

(C) If the dog is pulling you uphill, with the help of the leash, why aren't you accelerating uphill?

(D) The dog stops suddenly and the tension in the leash vanishes-the leash stops pulling on you altogether. Nonetheless, you continue uphill briefly. In which direction is the net force on you as you continue uphill and in which direction are you accelerating at that time?

Problem 6:

It's your first day on the staff of the U.S. Patents Office and you're excited about having some new ideas come across your desk.

(A) An inventor comes to you with a small box that's supposed to make batteries obsolete. The inventor claims that the box can produce electricity forever without having to be recharged. You can be sure that this claim is nonsense because

(B) Another inventor comes in with a small motor-like device that is powered by a burning candle. The inventor claims that this device takes the heat from the candle and turns it entirely into work. The device thus doesn't warm the room at all. Your knowledge of the laws of thermodynamics assures you that this claim, too, is nonsense because

(C) As though the entire population of loser inventors was released on you in one day, another clown comes in claiming to have a heat pump that can transfer heat out of a box of corn flakes for as long as you like. The cereal just gets colder and colder. Once again, you know that this is impossible because

(D) Just when you thought you were out of the woods, someone comes in with a coffee mug that automatically reassembles itself if you accidentally break it. You're supposed to put the broken mug in a box and wait. In a minute or two, the mug will be as good as new. You agree with the inventor that this reconstruction trick doesn't violate any of the laws of motion. However, you can still be sure that it won't work because