Name __________________________________

Student I.D. _____________________________

Physics 105N

How Things Work

Fall 1997

Friday Oct. 10, 1997

There are 26 problems. Do all of them.

Write your name clearly above, along with your ID number.

For the multiple choice questions, circle the one best answer.

Any ambiguity will result in the question being marked incorrect.

For the short answer questions, be concise.

Use g = 22 mph per second or 9.8 m/s per second.

1. A baseball began at rest and has been falling toward the Earth for 1 second, as shown. At that moment, the Earth vanishes. The baseball

1. continues in the same direction with a speed of 22 mph.
2. slowly reverses direction, until it's going 22 mph in the other direction
3. slows to a stop in one second, then remains stopped
4. slows to a stop in one second, then begins moving in the other direction
5. continues accelerating at 22 mph per second

2. Gravity on the moon is only about one-sixth of that on Earth. Because of that, walking on the moon, as compared to the Earth, is easier in which of the following ways?

1. When you stand still, the force upward on the bottom of your feet is less.
2. When moving, making a sharp turn to the left or right takes less effort.
3. When you jump up, you go higher.
4. When you jump up, you spend a longer time above the surface.

1. III only
2. III and IV
3. I, III and IV
4. I, II, III and IV
5. II only

2. You drive your car into a lake with the windows rolled up, and it sinks into the water. You want to open the door, but you find this difficult primarily because

1. The cold water has made the rubber seals brittle.
2. You are closer to the center of the Earth, so you feel heavier.
3. The pressure of the water outside pushing on the door is much greater than the air pressure inside the car.
4. The buoyancy force on the car is great enough that you are pinned down to your seat.
5. As the car descends you are pinned to the ceiling.

3. If you place a helium balloon in a container of helium gas as shown, it will

1. rise, because the buoyancy force is greater than the weight of the balloon.
2. rise, because the top of the container protects the balloon from the air above it.
3. be "neutrally buoyant," that is, it will neither rise nor fall
4. fall, because the buoyancy force is too small to support the weight.
5. fall, because the balloon expands and displaces more of the surrounding gas.

5. You hold a new glass bottle of root beer upright in one hand and pound its bottle cap downward hard with a rubber mallet. As a consequence of this action,

(a) The sudden acceleration will cause the liquid in the bottle to float above the gas in the bottle for several seconds.

(b) The surge in pressure near the top of the bottle will cause the bottle cap to vibrate loudly.

(c) Liquid colliding with the bottom of the bottle will knock the bottom out of the bottle.

(d) The sudden expansion of gas inside the bottle will cause the top layer of root beer to freeze.

6. Which of these motions is not like the others?

1. zero velocity
2. constant, non-zero velocity
3. zero acceleration
4. constant, non-zero acceleration

6. You practice driving your car over a hill as shown. There is a scale under the road surface at the top of the hill, which reads 3000 pounds when you drive on top of the hill and stop. Now you drive over the hill at different speeds, but always maintaining contact with the road. The reading on the scale as you drive over it is

1. 3000 pounds always, since gravitational force is independent of speed.
2. more than 3000 pounds at low speeds, less than 3000 pounds at high speeds.
3. more than 3000 pounds at high speeds, less than 3000 pounds at low speeds.
4. always less than 3000 pounds.
5. always more than 3000 pounds.

6. In the loop-the-loop shown at right, a frictionless roller coaster car of weight 500 pounds is started from the same height as the top of the loop. As it moves through the loop

1. it will fall off the track before reaching the top.
2. it will have zero speed when it reaches the top, then roll down the other side.
3. it will have zero speed when it reaches the top, then reverse and roll down the same side.
4. it will push on the top of the track with a force greater than zero but less than 500 pounds.
5. it will push on the top of the track with a force greater than 500 pounds.

6. You and your bicycle together have a weight of 300 Newtons. You ride up each incline. To do this your body burns

1. 30,000 Joules on the shallow ramp, 15,000 Joules on the steep ramp.
2. 30 Joules on the shallow ramp, 60 Joules on the steep ramp.
3. 3000 Joules on the shallow ramp, 3000 Joules on the steep ramp.
4. 300 Joules on the shallow ramp, 600 Joules on the steep ramp.
5. (3000) x (9.8) Joules on the shallow ramp, (3000) x (9.8) Joules on the steep ramp.

6. Now you coast down each ramp. How do the times to reach the bottom and the speeds at the bottom compare?

1. The speed is less, and the time greater, for the shallow ramp.
2. The speeds are the same for each ramp, but the time is greater for the shallow ramp.
3. The speed is greater for the steep ramp, and the time is less.
4. The speeds are the same, and the times are the same.

6. On the ramp at the right, the weight of 100 Newtons can only move vertically. It cannot slide down the ramp. There is no friction. With what force must the person push to raise the weight at a constant speed?

1. 100 Newtons
2. 60 Newtons
3. (500) ¸ (3) Newtons
4. (400) ¸ (3) Newtons
5. 75 Newtons

6. When you push on the outside edge of a door, in a direction that is towards the hinges, why will the door not rotate?

1. The door pushes back with an equal and opposite force.
2. The torque on the door is zero because the lever arm is zero.
3. The torque on the door is zero because the force is parallel to the lever arm.
4. The normal force from your hand is too large.
5. The torque due to gravity cancels out the torque due to your push.

6. The object to the right consists of a long pencil with a methane-filled balloon at one end, and an air-filled balloon at the other. Under which location will it most likely balance on your finger?

1. A
2. B
3. C
4. D
5. E

6. There are two 10 kg children sitting at the left end, and one 20 kg child sitting at the right end. You glance at the seesaw and notice that it's position is a little counterclockwise of horizontal, but it's rotating clockwise. As it passes through the horizontal position it will

1. stop at the horizontal position and remain there.
2. continue rotating clockwise at the same angular speed until the right side hits the ground.
3. rotate faster and faster clockwise until the right side hits the ground.
4. stop at the horizontal position, then begin rotating counterclockwise until the left side hits the ground.
5. rotate clockwise with an ever-increasing angular acceleration.

6. Now one of the 10 kg children goes home. You, the playground master, start the seesaw horizontal and give the left side a quick punch downwards, so that the seesaw is rotating counterclockwise. Possible subsequent motions of the seesaw are that it

1. rotates at constant angular speed counterclockwise until the left side hits the ground.
2. rotates counterclockwise slower and slower until the left side hits the ground.
3. slows down, stops, and begins rotating clockwise at a constant angular speed
4. slows down, stops, and begins rotating clockwise at ever-increasing angular speeds.

1. I only
2. I and II
3. III and IV
4. II and IV
5. I and III
6. All are possible

6. Now the 10kg child that went home returns and sits above the pivot. Where must the 20 kg child sit to balance the seesaw, as measured from the left end?

1. 6 2/3 meters
2. 7.5 meters
3. 8.0 meters
4. 10.0 meters (i.e. same place)
5. 3.0 meters

6. You put a little water in an empty soda can and heat it until steam emerges from the opening. You then quickly invert the can into a pan of cool water. The soda can will

1. implode (be crushed)
2. explode (expand)
3. rise about six inches into the air, and land upright
4. rise about six inches into the air, and land inverted
5. not do anything (unfortunately) and burn the lecturer's hands

6. You push a book to the right across a table and then let go, after which the book slows down and comes to rest. If you could see the internal workings of the book with a microscope at a given moment, which best describes what you would see after the book is at rest?

1. most of the molecules would be moving to the right, with a few moving to the left
2. all molecules would be moving either up or down
3. for every molecule moving up, there would be one moving down, and for every molecule moving to the right, there would be one moving to the left
4. most of the molecules would be stopped, but a few would be moving to the right
5. all of the molecules would be stopped

6. Blood is flowing through two different veins, each of which has the same pressure difference between the ends, and the same length. The cross-sectional areas are different: 1 square millimeter (small vein), and 2 square millimeters (big vein). The blood flow rate will be

1. Twice as great in the big one, and fastest near the walls
2. Twice as great in the big one, and fastest at the center
3. Four times as great in the big one, and fastest near the walls
4. Four times as great in the big one, and fastest at the center
5. Sixteen times as great in the big one, and fastest near the walls
6. Sixteen times as great in the big one, and fastest at the center

6. In your apartment, you weigh two closed refrigerators on your bathroom spring scale and find that they each weigh 100 pounds. You notice that each has the same volume, but one is twice as tall and half as wide. You then place them on the same spring scale at the bottom of a deep swimming pool. You could find that

1. the scale reads 100 pounds for each, since gravity is unaffected by the water
2. the scale reads 120 pounds for the short fridge, and also 120 pounds for the tall fridge
3. the scale reads 80 pounds for the short fridge, and 90 pounds for the tall fridge
4. the scale reads 90 pounds for the short fridge, and 80 pounds for the tall fridge
5. the scale reads 80 pounds for the short fridge, and also 80 pounds for the tall fridge

6. You are directly approaching your friend on an ice-skating rink, so there's negligible friction. You each have the same speed, and you each weigh the same on the locker room scale. After the collision, you grab each other, so that you're stuck together and move as one unit. Which of the following is true of your motion and energy?

1. You are stopped, and your kinetic energy is transformed into gravitational potential energy
2. You continue moving in the direction of the most massive person, and your kinetic energy is the same
3. You continue moving in the direction of the most massive person, and some of your kinetic energy is transformed into heat (thermal energy)
4. You are stopped, and your kinetic energy is transformed into heat (thermal energy)
5. You are stopped, and your kinetic energy is the same

6. If you sit in a good swivel chair with your feet off the floor, the chair will turn slightly as you move about but will immediately stop moving when you do. Why can't you make the chair spin without touching something?

23. You're taking a step aerobics class. In front of you is a small platform that you step onto and off of during the course of the exercises. Most of the time, one foot remains stationary on the platform and you use it to lift your body up and down.

(A) As you step up onto the platform, your leg does work on your body. What characteristics of you and the platform determine how much work your leg must do?

(B) If you were on the moon, how would the amount of work your leg must do change?

(C) If you let yourself drop back down to the ground, rather than lowering yourself gently, you may injure the leg you land on. Wearing padded shoes reduces your risk of injury. Do they change the force that your leg experiences? Why or why not?

(D) The platforms are all identical and are made of a sturdy plastic that acts like a stiff spring. When you step up onto your platform, it distorts downward by about 4 millimeters. You're curious about the weight of the person to your right so you watch the platform as that person steps onto it. It distorts downward by 6 millimeters. How much does that person weigh?

24. Why does putting sand in the trunk of a car help to keep the rear wheels from skidding on an icy road?

 

 

 

 

 

25. Some fish move extremely slowly and it's hard to tell whether they are even alive. However, if a fish is floating at a middle height in your aquarium and not at the top or bottom of the water, you can be pretty certain that it's alive. Why?

 

 

 

 

 

26. If you close your eyes in a subway train moving smoothly at constant speed, can you tell which way you're moving? If so, how; if not, why not?