Physics 105 - How Things Work - Fall, 2002

Midterm Examination

Given Friday, October 11, from 1:00 PM to 1:50 PM

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 springboard diver leaps upward from the springboard, rises dramatically to a peak height, and than drops impressively into the water below the board. Neglect any influences of air or the atmosphere. During this trip, the diver experiences

Problem 2:

You are part of a team playing tug-of-war with another team. Each team is pulling on one end of a rope and trying to drag the other team into a patch of mud in the middle of the horizontal playing field. Your team is lighter than the other team, but the two teams are about equally strong and no one is making any progress. While the two teams are essentially motionless, the frictional force the other team experiences from the ground is

Problem 3:

A two-inch fire hose can carry over 200 times as much water per second as a half-inch garden hose, even though both hoses have the same length and are fed with water at the same pressure. The basic reason for this difference is that the fire hose

Problem 4:

You are trying to knock over a heavy bottle by hitting it with a 100-gram object and transferring momentum to it. You have two such objects: an almost perfectly elastic "bouncy ball" and an almost perfectly dead beanbag. If you throw the two objects equally fast, then the bouncy ball will transfer

Problem 5:

You've missed a turn on a go-cart track and you're headed for disaster. You can crash into either a brick wall or a haystack. After a rapid review of all the physics you know, you decided to aim for the haystack. Good choice. Although either obstacle would have brought you to a complete stop, the haystack extracted

Problem 6:

A steady, horizontal stream of water from a hose strikes a wall. As the water hits the wall, its velocity is forward, its acceleration is

Problem 7:

When you first step onto a bathroom spring scale, the value it reports increases rapidly to more than your actual weight. It takes a second or two before the scale gives an accurate reading of your weight. The reason the scale reads too high at first is that

Problem 8:

You are trying to loosen an ordinary screw that someone has jammed into your dormitory wall. Try as you may, you can't get that screw to turn at all. While you are twisting as hard as possible on the head of the screw, the net torque that the screw is experiencing is

Problem 9:

When a spinning skater pulls his arms in close to him, his rate of rotation increases dramatically. During this process, his moment of inertia

Problem 10:

You can bicycle up a particular hill in one of several gears, making the bicycle easier or harder to pedal. In going from rest at the bottom of the hill to rest at the top of the hill, which of the following quantities will be the same regardless of which gear you use?

Problem 11:

If you swing a full bucket of water over your head quickly enough, the water will stay in the bucket even when that bucket is upside-down. If you swing the bucket more slowly, you'll get wet. One way to understand these different behaviors is to note that

Problem 12:

Scuffing the soles of your shoes on the sidewalk wears them out. You do much better to plant each sole firmly against the pavement, move your body forward, and then lift that sole cleanly back off the pavement. Keeping each sole in place as it touches the sidewalk dramatically reduces its wear because then the sole doesn't

Problem 13:

The drought has dried up your favorite water skiing lake, so you have no choice but to ski on the highway behind a car. It's fun, but it sure isn't good for your skies. A rope attached to the car is pulling you straight ahead along a level, horizontal road at a constant speed of 60 miles per hour. The net force on you is

Problem 14:

Consider two automobile accidents in which identical cars are driven into a solid wall at 30 mph and 60 mph, respectively. Compared to the car traveling 30 mph, the car 60 mph transfers

Problem 15:

When you jump on a trampoline, you stretch an elastic surface up and down. The two of you (you and the trampoline) have a combined total potential energy that changes as you bounce around. The point at which your combined total potential energy is lowest is when you are located

Problem 16:

You drop a ball from rest on a concrete floor and let it bounce twice. The first time it rises to 80% of its original height before descending again. The second time it rises to

Problem 17:

Airports often provide you with moving sidewalks to help you get from one place to another quickly and easily. Suppose you are riding one of these sidewalks up a gentle hill to your departure gate. You are traveling at a constant velocity, forward and slightly upward. Neglect any air resistance. During this time, the sidewalk is pushing you

Problem 18:

You find that when you load up the door to your refrigerator with bottles of soda, the door becomes relatively hard to open or shut. It doesn't respond quickly to your pushes or pulls. To minimize this effect, you should put the soda bottles as close as possible to the

Problem 19:

Which of the following controls can cause an automobile to accelerate: the accelerator pedal, the brake pedal, and the steering wheel.

Problem 20:

A unicycle has only a single wheel and its rider sits above that wheel, pedaling furiously to stay upright. When she is traveling straight forward at a steady pace, she can maintain stability by always placing her overall center of gravity

Problem 21:

You shoot an arrow straight up, using a bow. Naturally, you run away before it comes back down. While the arrow is heading upward, but no longer touching the bow, the net force on it is

Problem 22:

You have three identical one-liter containers. One is filled with solid lead, the second is filled with liquid water, and the third is filled with gaseous helium. You push all three containers to the bottom of a tank of water and hold them there. Which container experiences the largest upward buoyant force?

Problem 23:

You're seated in the last car of a roller coaster train and being pulled over the lip of the first plunging drop. As you go over this lip, you feel nearly weightless. This experience of weightlessness is more severe for you than it was for people in the first car of the train because you

Problem 24:

Through a strange mishap during a flight to Miami Beach, a scuba tank full of extremely high pressure air falls out of an airplane. It has been falling for a while and is now just 10 feet above the ground. The air inside the tank is moving much faster than the air outside the tank and it also has much greater pressure. Why doesn't this arrangement violate Bernoulli's equation?

Problem 25:

While some fires can be fought with hoses connected directly to fire hydrants, other fires require that the water first pass through the pumps inside a fire truck. Adding energy to the water with a pump allows that water to

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:

What could be more fun than designing outrageous stunts for a comedy western movie? With your vast understanding of physics, you're earning the big bucks as one of Hollywood's best. You are going to end your current film by having the heroine save the hero from a hanging. As this scene begins, the hero is standing still on a trapdoor high above the ground and there is a rope around his neck. Suddenly the trapdoor opens and the hero has nothing beneath his feet.

(A) The rope is limp at first. The hero is experiencing what (1) net force, (2) velocity, and (3) acceleration? (report specific values for these three quantities)

(B) The heroine has secretly replaced the stiff rope with a stretchy elastic cord. Instead of pulling tight suddenly, the cord tenses gradually as the hero descends. At what point in this descent does the hero reach his maximum downward velocity?

(C) The heroine gallops up on a white horse, passing right under the descending hero. He slows to a stop just as she reaches him and she cuts the cord from his neck. At the moment he stopped descending (and she cut him free), was the hero accelerating vertically and, if so, which way?

(D) The elastic cord snaps upward violently after being cut and it knocks the evil judge off the platform. Clearly, the cord has enormous kinetic energy at the end of the scene. What form was that energy in at the beginning of the scene?

Problem 2:

You are playing soccer with your friends.

(A) As you kick the ball and it accelerates forward, how does the force you exert on the ball compare to the force the ball exerts on your foot?

(B) While the ball is in the air after you kick it, which of the following physical quantities remains constant? (1) the ball's momentum, (2) the ball's angular momentum about its center of mass, (3) the ball's velocity, (4) the ball's acceleration, (5) the ball's total energy. Neglecting air resistance.

(C) Still neglecting air resistance, describe the horizontal forces on the airborne ball during the period after it leaves your foot until just before it bounces on the ground 50 feet away.

(D) You kick the ball twice. The first time, the ball was stationary when you kicked it and the second time, the ball was moving toward you quickly when you kicked it. The motion of your leg was identical in each case. Which time did you kick the ball farther? Explain BRIEFLY.

Problem 3:

You are riding in a large helium-filled balloon that is passing over a city park. Down below, there is a huge picnic going on and you are watching the festivities.

(A) You are maintaining a constant altitude of about 100 feet above the ground. Compare the total weight of the balloon and its contents to the weight of the air that these objects are displacing. BRIEFLY justify your answer.

(B) You use a pump and a rigid gas storage tank to control the vertical motion of the balloon. To make your balloon descend, you pump helium out of the balloon and into the tank. To make your balloon rise, you let helium flow out of the tank and into the balloon. Why does this scheme work?

(C) Your friend in the balloon lowers a long tube over the side of the balloon and its end dips into a huge container of lemonade 100 feet below. He begins to suck on the tube and lemonade begins rising toward his mouth. Can he succeed in drinking lemonade this way? BRIEFLY justify your answer.

(D) Your friend then pours a stream of water out of a cup onto one of the picnic tables far below. Neglect any air resistance or friction. Identify the transformations of energy that occur in this stream as it descends and hits the table's surface.