Midterm #1: Spring 2012

You accidently put the wrong type of batteries in your flashlight. When you turn it on, twice the normal current flows through the lightbulb and that current experiences twice the normal voltage drop as it passes through the lightbulb. The power being delivered to the lightbulb is (A) 4 times normal. (B) 1/2 normal. (C) 2 times normal. (D) 1/4 normal.

(A) 4 times normal.

You have a balloon that is negatively charged and a glass window that is electrically neutral. You observe that the balloon sticks to the window. Why is the balloon attracted to the window? (A) The window's positive charges shift toward the balloon and its negative charges shift away from the balloon, leading to a net attraction between the balloon and the window. (B) Half of the balloon's charges transfer to the window, after which the charges on the window and balloon attract each other. (C) The window acquires a net negative charge and that charge attracts the balloon's negative charge. (D) The window acquires a net positive charge and that charge attracts the balloon's negative charge.

(A) The window's positive charges shift toward the balloon and its negative charges shift away from the balloon, leading to a net attraction between the balloon and the window.

A xerographic copier uses a very thin wire with a large positive voltage to spray electric charges onto the surface of its photoconductor. The thin wire (A) has a strong electric field near it and that strong field pushes charge from the wire onto air molecules. (B) has a weak electric field near it and that weak field easily permits the transfer of charge from the wire to the surface of the photoconductor. (C) heats up quickly and boils off electric charge onto the surface of the photoconductor. (D) is able to move charge quickly enough to produce finely detailed light and dark spots on the copies.

(A) has a strong electric field near it and that strong field pushes charge from the wire onto air molecules.

Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because (A) it barely moves downward as you push downward on it, so you do almost zero work on it. (B) it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand. (C) it stops the downward motion of your foot faster and thus absorbs less of your momentum. (D) its water content gives it more mass and that prevents it from absorbing energy.

(A) it barely moves downward as you push downward on it, so you do almost zero work on it.

A battery charger recharges a battery by (A) pushing current through the battery from its positive terminal to its negative terminal. (B) connecting a wire between the battery's positive terminal and its negative terminal. (C) pushing current through the battery from its negative terminal to its positive terminal. (D) removing positive charges from the battery's positive terminal and negative charges from its negative terminal.

(A) pushing current through the battery from its positive terminal to its negative terminal.

In accordance with Ohm's law, the voltage difference between the two ends of a metal wire is proportional to (A) the current passing through that wire. (B) 1 divided by the net charge of that wire. (C) the net charge of that wire. (D) 1 divided by the current passing through that wire.

(A) the current passing through that wire.

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) the same voltage difference between its terminals but it can pump more charge before it runs out of energy. (B) a smaller voltage difference between its terminals but it can pump much more charge before it runs out of energy. (C) a larger voltage difference between its terminals and it can pump more charge before it runs out of energy. (D) a larger voltage difference between its terminals but it can pump the same amount of charge before it runs out of energy.

(A) the same voltage difference between its terminals but it can pump more charge before it runs out of energy.

Alice and Bob are carrying 50 pound bags of seeds from the sidewalk to your penthouse garden, 100 feet above the sidewalk. Alice carries bags up the stairs. Bob carries bags up a vertical ladder. Compare the work each person does on a bag while moving it from the sidewalk to the garden. [Neglect friction and air resistance, and assume that bags are motionless at the start and finish.] (A) Bob does more work on a bag. (B) Alice and Bob do the same work on a bag. (C) Alice does more work on a bag. (D) The person who completed the task first does more work on a bag.

(B) Alice and Bob do the same work on a bag.

You are at the gym, exercising on a step machine. You have one foot on each of the machine's pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push harder while moving downward than while moving upward. When during this exercise are you transferring energy to the step machine? (A) As the pedals move upward. (B) As the pedals move downward. (C) When the pedals are accelerating. (D) As the pedals move either upward or down.

(B) As the pedals move downward.

You are riding a cable car from a valley to a mountaintop and are traveling straight toward the mountaintop at a steady pace. In which direction is the cable car pushing you? (A) Up and forward, toward a point somewhat above the mountaintop. (B) Directly upward. (C) Directly toward the valley. (D) Directly toward the mountaintop.

(B) Directly upward.

You are wearing roller skates. You board a bus and stand motionless in the aisle of the motionless bus. You are facing forward and not touching anything as the bus starts moving forward. You find yourself heading toward the back of the bus. What is causing you to head toward the back of the bus? (A) The bus is exerting a frictional force on your skates. (B) Inertia. (C) Your weight. (D) The bus is exerting a support force on your skates.

(B) Inertia.

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? (A) The amount of force that you're exerting on the carpet must be more than the amount of its weight. (B) The amount of force that you're exerting on the carpet must be equal to amount of force that friction is exerting on it. (C) The amount of force that you're exerting on the carpet must be more than the amount of force that friction is exerting on it. (D) If you were to exert twice as much force on the carpet, it would slide across the floor twice as fast.

(B) The amount of force that you're exerting on the carpet must be equal to amount of force that friction is exerting on it.

A boy is bicycling up a hill and appears to need some help. As he passes you, you reach out with your hand and exert an uphill force of 10 N on him. When you do this, the boy exerts (A) a downhill force of somewhat less than 10 N on you, because his uphill velocity reduces the force he needs to accelerate uphill. (B) a downhill force of 10 N on you, because forces always come in equal but oppositely directed pairs. (C) no downhill force on you at all, because the force of his momentum is already enough to keep him moving uphill. (D) a downhill force of somewhat more than 10 N on you, because, in addition to the reaction force, he must accelerate your hand downhill.

(B) a downhill force of 10 N on you, because forces always come in equal but oppositely directed pairs.

You're having trouble loosening a rusty bolt with a small wrench, so you borrow a large wrench from your neighbor. Exerting only a modest force on the handle of this new wrench easily unscrews the bolt. The large wrench helps because it (A) has a large acceleration and a large mass, so the force it produces is large, according to the equation F=ma. (B) allows you to exert your force far from the center of rotation, so that you produce a large torque on the bolt. (C) has a large moment of inertia so that it develops a great deal of angular momentum when you exert a force on it. (D) has a large mass so that its inertia allows you to overcome the bolt's velocity and accelerate it around in a circle.

(B) allows you to exert your force far from the center of rotation, so that you produce a large torque on the bolt.

Two bowling balls, one of which weighs twice as much as the other, roll off of a horizontal table together at the same initial velocity. In this situation, (A) both balls hit the floor at approximately the same time, but the heavier ball lands considerably farther from the table than the lighter ball does. (B) both balls hit the floor at approximately the same time and at the same distance from the table. (C) the heavier ball hits the floor first and it lands considerably closer to the table than the lighter ball does. (D) the heavier ball hits the floor first and it lands considerably farther from the table than the lighter ball does.

(B) both balls hit the floor at approximately the same time and at the same distance from the table.

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. (B) receive no shock at all. (C) a weak shock. (D) a strong shock anyway.

(B) receive no shock at all.

You are water-skiing behind your new yacht and the rope from the yacht is pulling you forward. At this moment, you are traveling in a straight line path at a constant speed. The net force you are experiencing is (A) in the forward direction. (B) zero. (C) in the backward direction. (D) in the upward direction.

(B) zero.

Two children are balanced on a seesaw, but one child weighs twice as much as the other child. The heavier child is sitting half as far from the pivot as is the lighter child. Since the seesaw is balanced, the heavier child is exerting on the seesaw (A) a torque that is less than the torque the lighter child is exerting. (B) a force that is equal in amount but oppositely directed to the force the lighter child is exerting. (C) a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting. (D) a force that is less than the force the lighter child is exerting.

(C) a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting.

You're at the lake and watch two children jump off a dock at the same time. They both kick equally hard during their jumps, but one child jumps mostly upward while the other child jumps mostly forward. After they leave the dock, (A) the child who jumps upward reaches the water before the child who jumps forward. (B) the two children reach the water at the same moment and at the same distance from the dock. (C) the child who jumps forward reaches the water before the child who jumps upward. (D) the two children reach the water at the same moment and but the child who jumps forward travels farther from the dock than does the other child.

(C) the child who jumps forward reaches the water before the child who jumps upward.

When a flashlight experiences a short circuit, the current in the flashlight bypasses (A) the switch. (B) the coil. (C) the lightbulb. (D) the batteries

(C) the lightbulb.

You are bicycling along a quiet street when a child runs in front of you to retrieve a toy. You slam on the brakes and lock your wheels. The bicycle skids to a stop. What has become of your kinetic energy? (A) It's still present in you, as it must be because kinetic energy is conserved and can't be created or destroyed. (B) It's now elastic potential energy in the brakes. (C) It's now gravitational potential energy in the wheels. (D) It's now thermal energy in the wheels and ground.

(D) It's now thermal energy in the wheels and ground.

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? (A) Its weight along with an upward force that steadily decreases until the basketball reaches its highest point. After that point, there is only the constant downward force of gravity. (B) A steadily decreasing upward force from the moment it leaves your hands until it reaches its highest point and then a steadily increasing downward force as the basketball returns toward your hands. (C) Its weight along with a steadily decreasing upward force. (D) Its weight.

(D) Its weight.

A cheerleader leaps into the air with her arms and legs extended and then pulls herself into a compact ball to complete a somersault. She opens up again and lands on her feet. During the time that she is not touching the ground, one aspect of her motion that is constant is her (A) angular velocity. (B) velocity. (C) momentum. (D) angular momentum.

(D) angular momentum.

A glass bottle and its adhesive label are both electrically neutral. You hold the bottle and label with insulating rubber gloves and peel the label off the bottle. The label acquires a positive net charge. As you move the label farther away from the bottle, still holding them with your gloves, the voltage of label (A) remains constant, but its net charge decreases. (B) increases and its net charge increases. (C) remains constant, but its net charge increases. (D) increases, but its net charge remains constant.

(D) increases, but its net charge remains constant.

Your favorite marble rolls off the deck and falls for 2 seconds before hitting the ground. After only 1 second of falling, the marble was (A) closer to the ground than to the deck. (B) halfway between the deck and the ground. (C) a few inches closer to the deck than to the ground. (D) much closer to the deck than to the ground.

(D) much closer to the deck than to the ground.

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 both its outer and inner surfaces. (B) on its inner surface. (C) distributed throughout the metal. (D) on its outer surface.

(D) on its outer surface.

While wandering in the dark toward the refrigerator, you accidentally walk into a concrete wall and come to a sudden stop without bouncing back. Fortunately, that wall was covered with a soft fabric wall hanging. Coming to a stop on the soft fabric was more pleasant than coming to a stop on the concrete because you transfer (A) more momentum to the concrete than to the fabric. (B) more energy to the concrete than to the fabric. (C) more velocity to the concrete than to the fabric. (D) the same momentum to either surface, but you transfer it more slowly to the fabric than to the concrete.

(D) the same momentum to either surface, but you transfer it more slowly to the fabric than to the concrete.

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 (A) the team on the left. (B) neither team. It was instead provided by chemical potential energy in the rope itself. (C) both teams. (D) the team on the right.

(D) the team on the right.

You toss your cellphone directly upward and watch it rise to its peak height. At the moment that it reaches that peak height, its velocity is (A) downward and its acceleration is downward. (B) downward and its acceleration is zero. (C) zero and its acceleration is zero. (D) zero and its acceleration is downward.

(D) zero and its acceleration is downward.

When you're using your cellphone on the beach, in which direction is current flowing through its battery and through its computer? (A) Current flows toward higher voltage through the battery and toward lower voltage through the computer. (B) Current flows toward higher voltage through the battery and toward higher voltage through the computer. (C) Current flows toward lower voltage through the battery and toward lower voltage through the computer. (D) Current flows toward lower voltage through the battery and toward higher voltage through the computer.

Answer: (A) Current flows toward higher voltage through the battery and toward lower voltage through the computer.