Problem 1:

When you switch on a flashlight, current flows through the batteries from their

(A) negative ends to their positive ends and through the bulb from higher voltage to lower voltage. [64.9% picked]

(B) positive ends to their negative ends and through the bulb from lower voltage to higher voltage. [3.2% picked]

(D) negative ends to their positive ends and through the bulb from lower voltage to higher voltage. [17.1% picked]

Answer: (A) negative ends to their positive ends and through the bulb from higher voltage to lower voltage. [64.9% picked]

Why: The batteries pump charge against its natural direction of flow; they pump that charge from lower voltage (–) to higher voltage (+). The bulb allows charge to travel in its natural direction of flow; that charge flows from higher voltage to lower voltage.

Problem 2:

When a flashlight experiences a short circuit, the current in the flashlight bypasses

(A) the batteries. [1.8% picked]

(B) the coil. [14.4% picked]

(D) the lightbulb. [72.5% picked]

Answer: (D) the lightbulb. [72.5% picked]

Why: A short circuit occurs when the power-consuming component in a circuit is bypassed. In this case, a conducting path allows current in the circuit to skip the lightbulb. The circuit then has nothing left in it that is designed to consume electric power safely, so the other parts of the circuit begin to consume power unsafely.

Problem 3:

You are in a bicycle race and your friend is right beside you. You reach over and push her forward with your hand. She responds by

(A) pushing you backward, but less hard than you push her forward. [8.6% picked]

(B) yelling at you, but she doesn't push on you at all. [0.5% picked]

(D) pushing you backward equally hard. [88.3% picked]

Answer: (D) pushing you backward equally hard. [88.3% picked]

Why: As recognized by Newton's 3rd law, your force on your friend causes your friend to push back on you with an equal force in the opposite direction. That the two of you are moving makes absolutely no difference.

Problem 4:

You are making popcorn on the stove and one of the popped kernels zooms straight up into air. At the moment it reaches its peak height, its velocity is

(A) zero and its acceleration is zero. [19.4% picked]

(B) downward and its acceleration is downward. [5.0% picked]

(D) upward but its acceleration is downward. [4.5% picked]

Answer: (C) zero and its acceleration is downward. [70.7% picked]

Why: The popcorn kernel is a falling object, one that was initially traveling upward but is always accelerating downward. The kernel's velocity changes smoothly from its initial upward value to its eventual downward value (right before it hits the floor). At the moment the velocity reaches zero, the kernel stops rising and starts descending, so that is the highest point on the kernel's trajectory.

Problem 5:

You have a negatively charged plastic rod. If you hold that rod near an electrically neutral balloon,

(A) it will electrically polarize the balloon and they will repel one another. [9.5% picked]

(B) it will electrically polarize the balloon and they will attract one another. [86.9% picked]

(D) it will exert no electrostatic forces on the balloon because the balloon has equal numbers of positive and negative charges. [0.9% picked]

Answer: (B) it will electrically polarize the balloon and they will attract one another. [86.9% picked]

Why: The balloon has no net charge, but it does contain countless individual positive and negative charges. Those individual charges rearrange subtly when you bring the negatively charged rod nearby: the positive charges shift toward the rod and the negative charges shift away from it. That rearrangement is the electrical polarization of the balloon. Since the balloon's attractive positive charges are now slightly closer to the negatively charged rod than the balloon's repulsive negative charges are to the rod, there is a modest net attraction between the polarized balloon and the negatively charged rod.

Problem 6:

Two of your friends are pulling on opposite ends of a rope in the game of "tug of war". At this moment, your friend on the left is winning: both of them are moving to the left at constant velocity. The net force on the rope

(A) is zero. [53.2% picked]

(B) points toward the left. [41.4% picked]

(D) points straight down. [0.9% picked]

Answer: (A) is zero. [53.2% picked]

Why: The people and the rope are all moving at constant velocity, so they are each experiencing net forces of zero. The rope is being pulled to the right by one friend and equal hard to the left by the other friend. Those two forces sum to zero and the rope is therefore not accelerating.

Problem 7:

A popular fundraising toy is a "wishing well" — a funnel-shaped plastic device in which coins roll around many times before eventually dropping through the hole at the bottom of the funnel. While a coin is circling the well, it is accelerating

(A) backward, in the direction opposite its velocity. [4.1% picked]

(B) forward, in the direction of its velocity. [10.4% picked]

(D) in the direction that reduces its total potential energy as quickly as possible. [71.2% picked]

Answer: (D) in the direction that reduces its total potential energy as quickly as possible. [71.2% picked]

Why: Objects always accelerate in the direction that reduces their total potential energies as quickly as possible. That's because forces and potential energies are intimately connected to one another. In the case of the wishing well, it's pretty difficult to figure out all the forces acting on a coin as it rolls around the funnel-shaped device, so the potential energy approach is quite helpful. The coin accelerates in the direction that reduces its potential energy as quickly as possible and that's a nearly horizontal acceleration toward the center of the well. The main potential energies the coin has are its gravitational potential energy (associated with the coin's weight) and the elastic potential energies of the coin and wheel as they dent one another slightly (associated with the support force on the coin).

Problem 8:

Two billiard balls are rolling forward on a horizontal table, but the red ball is traveling twice as fast as the green ball. The two balls roll off the edge of the table simultaneously. The red ball hits the ground

(A) before the green ball. [6.3% picked]

(B) at the same time as the green ball, but the red ball lands four times as far from the table as the green ball. [20.3% picked]

(D) at the same time as the green ball, but the red ball lands twice as far from the table as the green ball. [63.5% picked]

Answer: (D) at the same time as the green ball, but the red ball lands twice as far from the table as the green ball. [63.5% picked]

Why: The two balls fall together in the vertical direction because they are both falling balls that start together with zero vertical component of velocity. The accelerate downward equally and acquire downward velocity equally, so they hit the ground simultaneously. The red ball was traveling faster horizontally, so its horizontal motion continues to carry it ahead of the green ball. Both balls coast in the horizontal direction, but the red ball has twice the green ball's horizontal component of velocity and travels twice as far horizontally by the time the two hit the ground.

Problem 9:

You have two electrically neutral objects: a cotton shirt and a plastic box. You touch them together without rubbing and then carefully separate them. As a result of this contact,

(A) both objects will remain electrically neutral because there was no static friction. [14.9% picked]

(B) one object will have a positive charge and the other will have a negative charge. [59.9% picked]

(D) both objects will remain electrically neutral because there was no sliding friction. [22.5% picked]

Answer: (B) one object will have a positive charge and the other will have a negative charge. [59.9% picked]

Why: Contact between those two initially neutral objects will certainly cause some transfer of charge because the shift and box are chemically different and must have some difference in affinity for electrons. Although we don't know which object transferred (positive) charge to the other, we do know that charge is conserved so that one object will end up with a net positive charge and the other will end up with an equal amount of negative charge.

Problem 10:

When you toss your basketball onto a bookshelf, it bounces off the wall at the back of the shelf and soon falls to the floor. Getting the ball to stay on the shelf is almost impossible because the ball is unable to transfer

(A) momentum to the wall or shelf. [36.9% picked]

(B) velocity to the wall or shelf. [3.6% picked]

(D) mass to the wall or shelf. [6.3% picked]

Answer: (C) energy to the wall or shelf. [52.3% picked]

Why: Velocity is not a conserved quantity, so transfering it makes no sense. Mass is a conserved quantity, but it can't be transferred; the basketball's mass never changes. That leaves only momentum and energy as possibilities here. But the basketball has no trouble at all transferring momentum to the wall or shelf, and it transfers a huge amount of momentum to them. Momentum is transferred via an impulse, the product of force and time. The basketball gives the wall and shelf a big impulse and that's how it reverses its direction during the bounce. What the basketball can't transfer to the wall or shelf is energy -- the wall and shelf don't move when the basketball hits them, so the basketball can't do work on them and can't transfer energy to them.

Problem 11:

You and your friends are riding street luges—essentially sit-down skateboards—down a hill. There are two paths from the top of the hill to the bottom: both are smooth ramps, but one is twice as long as the other. You take the short ramp and your friend takes the long ramp. Neglecting friction and air resistance,

(A) you accelerate twice as quickly as your friend, but your speeds are the same at the bottom of the hill. [50.5% picked]

(B) you and your friend accelerate equally, but when you are both at the bottom of the hill, your speed is greater than your friend's speed. [27.0% picked]

(C) you accelerate slightly less than twice as quickly as your friend, but your speeds are the same at the bottom of the hill. [12.2% picked]

(D) you accelerate slightly less than twice as quickly as your friend, but when you are both at the bottom of the hill, your speed is greater than your friend's speed. [9.9% picked]

Answer: (A) you accelerate twice as quickly as your friend, but your speeds are the same at the bottom of the hill. [50.5% picked]

Why: Each luger's acceleration is equal to the net force that person experiences divided by that person's mass. That ratio increases as the steepness of the slope increases. A luger's final speed, however, depends only on how much their altitude decreases. That's because the luger converts gravitational potential energy into kinetic energy during the descent and the steepness of the descent doesn't affect how much gravitational potential energy becomes kinetic energy.

Problem 12:

Your hairdryer is designed to operate properly on 120 V electric power. If you mistakenly plug it into a 230 V electric power outlet in Europe, it will carry

(A) about twice as much electric current and receive about twice as much power. [14.4% picked]

(B) about twice as much electric current and receive about four times as much power. [39.2% picked]

(D) about four times as much electric current and receive about four times as much power. [5.9% picked]

Answer: (B) about twice as much electric current and receive about four times as much power. [39.2% picked]

Why: The hairdryer obeys Ohm's law, so the current that flows through it is propotional to the voltage drop between its ingoing and outgoing wires. Doubling that voltage drop results in a doubling of the current flowing through the hairdryer. The power delivered to the hairdryer is equal to the voltage drop times the current, so it roughly quadruples.

Problem 13:

You walk on a carpet and accumulate a large static charge. You touch a metal pot that's resting on an insulating plastic countertop and transfer some of your charge to the pot. When you remove your hand from the pot, the pot's charge is

(A) on its outer surface. [66.2% picked]

(B) distributed throughout the metal. [17.6% picked]

(D) on both its outer and inner surfaces. [4.5% picked]

Answer: (A) on its outer surface. [66.2% picked]

Why: Metal objects conduct current and allow charge to move. The pot will allow its charge to move until that charge has minimized its total potential energy. That will occur when all of the charge is on the outside the metal and thus as far apart as possible.

Problem 14:

A transformer has 100 turns in its primary coil and 50 turns in its secondary coil. If you plug the primary coil into a 120 volt AC electrical outlet, the secondary coil will supply

(A) 70 volt AC electric power [10.4% picked]

(B) 170 volt AC electric power [6.3% picked]

(D) 60 volt AC electric power. [69.4% picked]

Answer: (D) 60 volt AC electric power. [69.4% picked]

Why: A transformer's secondary (output) voltage is equal to its primary (input) voltage times the ratio of its secondary coil turns to its primary coil turns. The more turns there are in the secondary coil, the more work the transformer's electric field can do on the charges flowing through the secondary coil and the higher the output voltage. In this case, the ratio is 0.5, so the ouput voltage is half the input voltage.

Problem 15:

You walk on a carpet and accumulate a large static charge. If you reach out to a doorknob with your finger, you'll receive a strong shock. Instead, you reach out to the doorknob with a sharp metal needle in your hand, pointing that needle toward the doorknob. As the needle moves toward the doorknob, you

(A) an extra-strong shock. [4.5% picked]

(B) receive no shock at all. [56.3% picked]

(D) a weak shock. [36.5% picked]

Answer: (B) receive no shock at all. [56.3% picked]

Why: The sharp needle will develop a corona discharge as it approaches the doorknob. That's because the electric field at the sharp tip will become so strong that it will begin to push charges off the needle and onto passing air molecules. The needle will spray your charge harmless at the doorknob and you won't experience a shock.

Problem 16:

To transmit electric power through extremely long transmission lines with maximum energy efficiency, you should send that power as

(A) a small electric current at a small voltage difference. [4.1% picked]

(B) a small electric current at a large voltage difference. [73.9% picked]

(D) a large electric current at a large voltage difference. [5.9% picked]

Answer: (B) a small electric current at a large voltage difference. [73.9% picked]

Why: The wires of a power distribution system waste energy in proportion to the square of the current flowing through those wires. To maximize power delivery efficiency, you should minimize the current while maintaining a large overall power transmission by using high voltages.

Problem 17:

Your heavy backpack hangs from a hook in the middle of your closet door and it affects the door's motion. The backpack

(A) makes it easier to change the door's angular velocity. [2.3% picked]

(B) increases the door's angular velocity. [3.6% picked]

(D) makes it harder to change the door's angular velocity. [77.9% picked]

Answer: (D) makes it harder to change the door's angular velocity. [77.9% picked]

Why: The backpack adds mass to the door at a distance from the door's pivot, so it also increases the door's rotational mass. The door becomes more resistant to angular accelerations. It's angular velocity can still take any value, but changing its angular velocity has become more difficult.

Problem 18:

Which of the following has/have a magnetic field?

(A) An electron and an electric current. [24.3% picked]

(B) An electrically polarized ball. [18.0% picked]

(D) An electric charge and an electric current. [45.9% picked]

Answer: (A) An electron and an electric current. [24.3% picked]

Why: An electron is intrinsically magnetic--it has a north pole and a south pole. An electric current is also magnetic, with its magnetic field circling the current.

Problem 19:

You watch your friend ride upward in a glass elevator at constant velocity. He is holding a heavy package in his hands. From your perspective, he is doing

(A) positive work on the package. [26.1% picked]

(B) negative work on the package. [3.2% picked]

(D) zero work on the package because the package is stationary in his hands. [45.0% picked]

Answer: (A) positive work on the package. [26.1% picked]

Why: If your friend's hands are pushing up on the package and the package is moving upward, your friend is doing work on the package. It doesn't matter whether your friend is lifting the package by himself or allowing the elevator to lift him while he holds the package. Either way, he's doing work on the package.

Problem 20:

The car you are driving skids off the road in bad weather and collides with a tree head on. Fortunately, the airbag inflates and protects your head from injury. By coming to a stop in the airbag instead of on the steering wheel, your head transferred

(A) the same amount of momentum to the car but over a longer period of time and with a smaller force. [67.6% picked]

(B) more momentum to the car but with a smaller force. [2.7% picked]

(D) the same amount of momentum to the car in the same period of time but with a smaller force [25.7% picked]

Answer: (A) the same amount of momentum to the car but over a longer period of time and with a smaller force. [67.6% picked]

Why: Your head comes to a stop, so it transfers all of its forward momentum to its environment. But the rate at which it transfers momentum is greatly reduced by hitting the airbag rather than the steering wheel. So while the total impulse is the same either way, the airbag makes that impulse a small force times a long time while the steering wheel would make that impulse a large force times a short time.

Problem 21:

You're doing laundry and there is a positively charged sock stuck to a negatively charge shirt. As you pull them apart, the voltage difference between them

(A) increases. [37.8% picked]

(B) stays the same because voltage is conserved. [5.9% picked]

(D) stays the same because electric charge is conserved. [16.7% picked]

Answer: (A) increases. [37.8% picked]

Why: TBA

Problem 22:

You're sitting still on a bale of hay at the back of a motionless wagon. A tractor suddenly jerks the wagon forward and you find yourself on the ground behind it. You're no longer on the wagon because

(A) of your weight--you are not as heavy as the wagon, so you accelerated more quickly than it. [2.3% picked]

(B) of your momentum--the wagon pushed you backward and gave you backward momentum. [9.0% picked]

(D) of your inertia--the wagon accelerated out from under you. [85.1% picked]

Answer: (D) of your inertia--the wagon accelerated out from under you. [85.1% picked]

Why: TBA

Problem 23:

You are riding an escalator in a store and are moving uphill from the 3rd floor to the 4th floor at constant velocity. The force that the escalator is exerting on your feet is directed

(A) straight up and is equal in amount to your weight. [64.0% picked]

(B) uphill and is equal in amount to your weight. [25.7% picked]

(D) uphill and is smaller in amount than your weight. [2.3% picked]

Answer: (A) straight up and is equal in amount to your weight. [64.0% picked]

Why: TBA

Problem 24:

When you connect an electromagnet to a source of alternating current, its magnetization reverses direction repeatedly. An electric charge near that AC electromagnet

(A) experiences zero force because the AC electromagnet's magnetic field averages to zero. [8.6% picked]

(B) experiences zero force because it is not affected by magnetic fields. [12.6% picked]

(D) experiences a force because the AC electromagnet's changing magnetic field produces an electric field. [69.8% picked]

Answer: (D) experiences a force because the AC electromagnet's changing magnetic field produces an electric field. [69.8% picked]

Why: TBA

Problem 25:

A skateboarder rolls swiftly up the side of a giant bowl and flies high into the air. While the skateboarder is the air above the bowl and neglecting any effects due to the air, the skateboarder's

(A) momentum is constant. [27.5% picked]

(B) velocity is constant. [13.5% picked]

(D) angular momentum is constant. [56.8% picked]

Answer: (D) angular momentum is constant. [56.8% picked]

Why: TBA

Problem 26:

If you put a magnetic compass in a uniform magnetic field, the compass needle will experience a

(A) force in the direction opposite the magnetic field. [12.6% picked]

(B) force in the direction of the magnetic field. [29.3% picked]

(D) torque unless the needle is aligned with the magnetic field or aligned opposite the magnetic field. [41.9% picked]

Answer: (D) torque unless the needle is aligned with the magnetic field or aligned opposite the magnetic field. [41.9% picked]

Why: TBA

Problem 27:

In contrast to a permanent magnet, a piece of pure iron

(A) is non-magnetic and cannot be magnetized. [6.3% picked]

(B) can be magnetized by external influences but will spontaneously demagnetize when it's left alone. [84.2% picked]

(D) can only have north poles. [1.4% picked]

Answer: (B) can be magnetized by external influences but will spontaneously demagnetize when it's left alone. [84.2% picked]

Why: TBA

Problem 28:

The voltage difference between the two ends of a metal wire is proportional to

(A) 1 divided by the current passing through that wire. [27.5% picked]

(B) the net charge of that wire. [11.7% picked]

(D) 1 divided by the net charge of that wire. [14.4% picked]

Answer: (C) the current passing through that wire. [45.5% picked]

Why: TBA

Problem 29:

A wind turbine is spinning clockwise at a constant angular velocity and its generator is producing electric power. The wind exerts a clockwise torque on the turbine

(A) and the turbine exerts a torque half that large on the generator. [9.0% picked]

(B) so the net torque on the turbine is clockwise. [27.5% picked]

(D) and the turbine exerts a torque twice that large on the generator. [15.3% picked]

Answer: (C) but the generator exerts an equal but opposite torque on the turbine so the net torque is zero. [46.8% picked]

Why: TBA

Problem 30:

You are building a decorative mobile—a complicated sculpture that hangs from a single string—and you want it to balance. You should attach the string to the sculpture's center of gravity so that the sculpture will

(A) have a constant angular acceleration. [11.7% picked]

(B) have as large a rotational mass as possible. [7.2% picked]

(D) experience zero torque due to gravity. [63.5% picked]

Answer: (D) experience zero torque due to gravity. [63.5% picked]

Why: TBA

Problem 31:

A battery charger recharges a battery by

(A) removing positive charges from the battery's positive terminal and negative charges from its negative terminal. [3.2% picked]

(B) connecting a wire between the battery's positive terminal and its negative terminal. [3.6% picked]

(D) pushing current through the battery from its positive terminal to its negative terminal. [62.2% picked]

Answer: (D) pushing current through the battery from its positive terminal to its negative terminal. [62.2% picked]

Why: TBA

Problem 32:

You insert the two-pronged plug a toaster into an ordinary AC electrical outlet and turn the toaster on. As the toaster is operating, the voltage difference between the two prongs

(A) alternates between positive and negative, but the current flowing through the toaster remains constant. [15.8% picked]

(B) remains constant and the current flowing through the toaster remains constant. [7.7% picked]

(D) alternates between positive and negative, and the current flowing through the toaster reverses directions repeatedly. [40.1% picked]

Answer: (D) alternates between positive and negative, and the current flowing through the toaster reverses directions repeatedly. [40.1% picked]

Why: TBA

Problem 33:

When the wheel of a bicycle is rolling on the pavement, it is experiencing

(A) sliding friction and the pavement may be exerting a horizontal force on the wheel. [23.0% picked]

(B) static friction and the pavement is exerting zero horizontal force on the wheel. [22.5% picked]

(D) sliding friction and the pavement is exerting zero horizontal force on the wheel. [7.2% picked]

Answer: (C) static friction and the pavement may be exerting a horizontal force on the wheel. [46.8% picked]

Why: TBA

Problem 34:

Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand takes less energy from you because

(A) it doesn't move downward as you push downward on it, so you do almost zero work on it. [42.8% picked]

(B) it stops the downward motion of your foot faster and thus absorbs less of your momentum. [38.7% picked]

(D) it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand. [13.5% picked]

Answer: (A) it doesn't move downward as you push downward on it, so you do almost zero work on it. [42.8% picked]

Why: TBA

Problem 35:

In a famous ice skating trick, a skater begins spinning with her arms out and then pulls her arms tightly against her body. She is then spinning extremely fast on the tip of one skate. Her angular velocity increases because her angular momentum

(A) is constant but her rotational mass decreases. [68.9% picked]

(B) is constant but her momentum decreases. [2.3% picked]

(D) increases but her rotational mass remains constant. [20.7% picked]

Answer: (A) is constant but her rotational mass decreases. [68.9% picked]

Why: TBA

Problem 36:

If you carry positive charge from a region of high voltage to a region of low voltage, that charge will

(A) become a smaller positive charge as a result of that move. [37.8% picked]

(B) become negative charge as a result of that move. [11.7% picked]

(D) do negative work on you during that move. [25.7% picked]

Answer: (C) do work on you during that move. [24.3% picked]

Why: TBA

Problem 37:

A man climbs out of his car just as his wife is about to insert the gasoline pump nozzle into its tank. The car had been electrically neutral, but the man walks away with a huge amount of positive charge on his wool sweater. The nozzle is electrically connected to the entire earth. The electric field in the space between the car and the gasoline nozzle points toward

(A) the nozzle's left. [2.3% picked]

(B) the nozzle's right. [1.8% picked]

(D) the car. [59.0% picked]

Answer: (D) the car. [59.0% picked]

Why: TBA

Problem 38:

When you supply the primary coil of a transformer with alternating current from the power company and connect the secondary coil of that transformer to a lightbulb, the lightbulb lights up. The transformer is transferring

(A) electric charge from the primary circuit to the secondary circuit. [13.1% picked]

(B) power from the primary circuit to the secondary circuit. [36.0% picked]

(D) both current and voltage from the primary circuit to the secondary circuit. [41.4% picked]

Answer: (B) power from the primary circuit to the secondary circuit. [36.0% picked]

Why: TBA

Problem 39:

You've just finished the eighth 300-page book for your anthropology class and you toss it across your dorm room to the bookshelf. After the book leaves your hand and neglecting any effects due to the air, the net force on the book while it is in the air is

(A) downward and forward (toward the bottom of the bookshelf). [23.9% picked]

(B) directly downward. [29.3% picked]

(D) directly forward (toward the bookshelf). [2.7% picked]

Answer: (B) directly downward. [29.3% picked]

Why: TBA

Problem 40:

You have two "alkaline" batteries, one is a small AAA battery and the other is a large D battery. Compared to the AAA battery, the D battery provides

(A) a larger voltage difference between its terminals and it can pump more charge before it runs out of energy. [13.1% picked]

(B) a larger voltage difference between its terminals but it can pump the same amount of charge before it runs out of energy. [15.8% picked]

(C) a smaller voltage difference between its terminals but it can pump much more charge before it runs out of energy. [4.1% picked]

(D) the same voltage difference between its terminals but it can pump more charge before it runs out of energy. [66.2% picked]

Answer: (D) the same voltage difference between its terminals but it can pump more charge before it runs out of energy. [66.2% picked]

Why: TBA