Actual Exam Will Be Given Wednesday, March 8, 1995, at 1:00 PM
Actual Exam will have 25 Multiple Choice Questions and 3 Short Answer Questions
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 the midterm examination.
Problem 1:
A speedboat is pulling a water-skier with a rope, exerting a large forward force on her. The skier is traveling forward in a straight line path at a constant speed. The net force experienced by the skier is
(A) in the backward direction.
(B) in the forward direction.
(C) zero.
(D) in the upward direction.
Problem 2:
A television set uses a magnetic field to deflect its electron beam horizontally. This magnetic field is able to steer the electron beam because
(A) a moving charged particle can experience a force when it passes through a magnetic field.
(B) an electron always travels in the direction of the magnetic field.
(C) an electron always travels at right angles to the direction of the magnetic field.
(D) a stationary charged particle can experience a force in a magnetic field.
Problem 3:
Ball bearings permit a wheel to turn freely on an axle without creating any heat because they form a mechanical system that involves
(A) no static friction.
(B) no sliding friction.
(C) no friction of either type.
(D) no electricity.
Problem 4:
If you measure the force experienced by an electrically-charged bead placed inside an operating microwave oven, you will find that the force
(A) reverses directions several billion times per second.
(B) is proportional to the mass of the bead.
(C) is proportional to the velocity of the bead.
(D) is proportional to the amount of air inside the microwave oven.
Problem 5:
A gymnast doing a double back flip leaps off the floor with her arms and legs extended and then pulls herself into a very compact position. In her compact shape, she rotates very rapidly and completes two full rotations before opening back up to land on the floor. During the time that she is not touching the floor, the one aspect of her motion that is constant is her
(A) angular momentum.
(B) momentum.
(C) velocity.
(D) angular velocity.
Problem 6:
If the local radio station WXYZ at 1020 on the AM dial wants to increase the strength of its radio signal so that it can reach a wider audience, it should
(A) increase the average amount of electrical charge that oscillates up and down its antenna but keep the frequency of oscillation the same.
(B) increase the frequency of oscillation of the electrical charge that travels up and down its antenna but keep the average amount of charge constant.
(C) increase the average amount of electrical charge that oscillates up and down its antenna and increase the frequency of oscillation.
(D) increase the temperature of the antenna by running more current through the heater coil.
Problem 7:
Even when you are driving at a constant 60 miles-per-hour along a straight, level road, your car's engine must be running. As the engine turns the car's wheels, friction between the ground and the tires exerts a forward force on the car. The car needs this forward force from the ground because
(A) an object that is moving requires a net force to keep it moving. In the absence of any net force, objects are motionless.
(B) air drag (air resistance) exerts a backward force on the car.
(C) an object's velocity points in the direction of the net force on that object.
(D) the car has a velocity and is thus accelerating. In order to accelerate, the car must be experiencing a net force.
Problem 8:
A small current of high-voltage electrons can carry the same amount of power as a large current of low-voltage electrons. Cross-country power transmission systems operate with a small current of very high-voltage electrons because this arrangement
(A) minimizes the fraction of total electrical power wasted as heat in the transmission wires.
(B) eliminates the need for transformers at either end of the cross-country transmission system.
(C) makes it possible to deliver electrical energy to the consumer continuously without requiring a complete electrical circuit. Only a single wire is needed to carry the power.
(D) is less likely to give anyone a shock than a system in which a large current of low-voltage electrons carries the power.
Problem 9:
At the end of the power cord of a table lamp is a plug. A typical plug has two metal prongs that you insert into the two slots of an electrical outlet. The plug has two separate metal prongs because
(A) electrons flow toward the lamp through one prong and return from the lamp through the other prong.
(B) alternating current means that only one wire is actively transporting electrons at any given time. For half the cycle electrons flow toward the lamp through one prong and the other prong has no current through it. For the next half cycle, the roles reverse.
(C) a single prong offers the potential of a bad connection, which would cause it to overheat and possibly start a fire. Electric current flows toward the lamp through both prongs simultaneously, reducing the risk of fire.
(D) one prong carries the electric current and the other prong serves as a grounding wire, enhancing the safety of the lamp.
Problem 10:
In some motors, the spinning rotor is made of nonmagnetic aluminum and has no electrical connections. This rotor becomes magnetic as the motor operates on alternating current because
(A) the changing magnetic environment around the rotor induces electrical currents in the rotor.
(B) the positive and negative charges in the rotor separate as the rotor spins.
(C) alternating current is inherently magnetic while direct current is not.
(D) aluminum is easily magnetized when exposed to a nearby magnetic pole.
PART II: SHORT ANSWER QUESTIONS
Please give a brief answer in the space provided. Part II is worth 33% of the grade on the midterm examination.
Problem 1:
Many doors are equipped with simple mechanical devices that close them automatically when no one is holding the door open. One of the simplest automatic door closers is a spring that connects the top corner of the door to the door frame. As you open the door, the spring stretches beyond its equilibrium length. The system now contains stored mechanical energy.
(A) When you hold this spring-equipped door open and stationary, what is the
net torque on the door?
(B) Use words like "force" and "distance" to show that, when you let go of the
door and it begins to close, work is done by one object on the other:
(C) What object contains most of the system's original energy in the moments
just before the door shuts completely and latches?
(D) More sophisticated door closers bring the door closed very gently and
quietly. This type of door closer still contains a spring, but it also contains
a damping device that always oppose the door's motion. What kind of force must
the damping device use and what becomes of the system's original mechanical
energy?
Problem 2:
A blender is a common kitchen appliance. It consists of a glass or plastic pitcher with a rotating blade at the bottom. The pitcher sits in a base containing an electric motor. When you push the on button, the motor spins very rapidly and turns the blade. The spinning blade stirs and liquefies the contents of the pitcher.
(A) If you put an ice cube into the pitcher and push the on button, the blade
spins and chops the ice cube into small fragments. The bottom of the pitcher is
smooth and the ice cube is slippery, so no outside forces keep the ice cube
from moving and staying ahead of the spinning blade. Still the ice cube stays
put and the blade slices through it. What holds the ice cube in place?
(B) The blender plugs into an electrical outlet. When you push the on button,
the blender mixes. At a particular moment during its operation, which way is
electrical current flowing through each of the two wires in the blender's power
cord?
(C) The blender contains a universal motor and can actually run on either AC or
DC electrical power. If you reverse the blender's plug, so that the two prongs
trade places in the outlet, will the motor and blade continue turning in the
same direction or will they now turn in the opposite direction?
(D) You leave a beverage mixing in the blender while you answer the telephone.
When you return, you find that the beverage has become quite warm. How did the
spinning blade heat up the liquid?