Homework #1

1. Bloom2 1.TB.01. [116853] A rock is dropped from a great height. Ignore air resistance. If the acceleration due to gravity is 32 ft/s2, then after 3.0 s the rock's speed is

32 ft/s2

96 ft/s

144 ft/s

288 ft/s

2. Bloom2 1.TB.02. [116854] A rock is dropped from a great height. Ignore air resistance. If the acceleration due to gravity is 32 ft/s2, then after 3.0 s the rock's acceleration is

96 ft/s2

32 ft/s2

144 ft/s2

288 ft/s2

3. Bloom2 1.TB.03. [116855] A rock is dropped from a great height. Ignore air resistance. If the acceleration due to gravity is 32 ft/s2 , then after 3.0 s the rock has fallen

32 ft

144 ft

96 ft

288 ft

4. Bloom2 1.TB.08. [116860] A book is lifted straight up. During this process, the work done by gravity is

positive.

negative.

zero.

Not enough information is given to determine.

5. Bloom2 1.TB.09. [116861] A 120 ft long ramp is used to lift a wheeled box to a height of 6.0 ft. Compared to a direct 6.0 ft lift, the force needed to push the box along the ramp is

20 times as large.

1/20 as large.

unchanged.

Not enough information is given to determine.

6. Bloom2 1.TB.10. [116862] A 120 ft long ramp is used to lift a wheeled box to a height of 6.0 ft. Compared to a direct 6.0 ft lift, the work performed in pushing the box along the ramp is

20 times as large.

unchanged.

1/20 as large.

Not enough information is given to determine.

7. Bloom2 1.TB.15. [116867] A rock on Earth has mass M and weight W. One the Moon, the acceleration due to gravity is 1/6 what it is on Earth. On the moon, the object's mass and weight are, respectively,

M and W

M and W/6

M/6 and W

M/6 and W/6

8. Bloom2 1.TB.28. [116880] Suppose that a shoelace passes without friction through 5 holes on each side of your shoe. If the tension in that shoelace is 10 N, how much total force is the shoelace exerting on the right side of your shoe?

25 N

100 N

10 N

50 N

9. Bloom2 1.TB.33. [116885] Two steel balls, one of which weighs twice as much as the other, roll off of a horizontal table with the same speed. In this situation,

the lighter ball impacts the floor at about half the horizontal distance from the base of the table than does the heavier.

both balls impact the floor at approximately the same horizontal distance from the base of the table.

the heavier ball impacts the floor at about half the horizontal distance from the base of the table than does the lighter.

the heavier ball hits considerably closer to the base of the table than the lighter, but not necessarily half the horizontal distance.

10. Bloom2 1.TB.37. [116889] You and a friend are wondering how your weights compare. You do not have a scale, but you do have a piece of rope. To compare your weights you throw the rope over the branch of a tree and each hang from one end of the rope. To your surprise, when you both pick your feet up the rope remains motionless indicating that your weights are exactly the same (neglecting any friction between the rope and the branch). From this observation you know that the tension in the rope is

equal to four times your weight.

equal to your weight.

equal to half your weight.

equal to your weight plus your friend's weight (or twice your weight).

11. Bloom2 2.TB.01. [117064] The left end of a see-saw accelerates downward, as you view it. By the right-hand rule, the torque on the see-saw is

away from you.

toward you.

down.

counterclockwise.

12. Bloom2 2.TB.04. [117067] The moment of inertia of a long thin rod is maximum when it is spinning about an axis

down the center of the rod lengthwise.

through the end of the rod, perpendicular to its length.

through the center of the rod, perpendicular to its length.

The moment of inertia is a constant, so the axis you choose does not matter.

13. Bloom2 2.TB.06. [117069] When cutting cardboard with scissors, why is it a good idea to move the cardboard as close as possible to the scissors' pivot point?

There is a bigger torque near the pivot.

There is a constant torque, so a small lever arm gives a large force.

There is a bigger lever arm near the pivot.

There is a larger force due to the smaller torque.

14. Bloom2 2.TB.07. [117070] While a cart is moving, the friction between the cart's wheels and the road is

sliding.

static.

both static and sliding.

There is no friction.

15. Bloom2 2.TB.12. [117075] Power is

force times time.

work per time.

force times distance.

mass times velocity.

16. Bloom2 2.TB.13. [117076] Momentum is

force times time.

mass times velocity.

force times distance.

work per time.

17. Bloom2 2.TB.15. [117078] Impulse is

how energy is transferred.

how momentum is transferred.

how work is done.

how torque is done.

18. Bloom2 2.TB.16. [117079] Work is

how momentum is transferred.

how energy is transferred.

how work is done.

how torque is done.

19. Bloom2 2.TB.30. [117093] After clearing the bar in the high jump, you land softly on a giant mattress. Landing on the mattress is much more comfortable than landing on a sand heap of equal size because

you transfer less momentum to the mattress in coming to a stop than you would have transferred to the sand heap in coming to a stop.

the force that the mattress exerts on you to stop your descent is much less than the force that the sand heap would have exerted on you.

you transfer more momentum to the mattress in coming to a stop than you would have transferred to the sand heap in coming to a stop.

your velocity is less as you land on the mattress than it would have been if you'd landed on the sand heap.

20. Bloom2 2.TB.32. [117095] You are riding on a playground swing. As you swing forward, there is a moment when you are directly below the pivot that supports the swing. At that moment, you are

accelerating forward.

accelerating upward.

not accelerating at all.

accelerating backward.

21. case 1-2 [121146] Diving boards and platforms offer a nearly ideal opportunity in which to experience the various laws of motion. When you jump off the high diving board, you are a falling object and, if you can keep your presence of mind as you fall, you can learn something about physics. Imagine yourself diving off a platform 10 m about the water below.

a. If you walk very slowly off the platform, so that you fall directly downward, roughly how long will it take for you to reach the water? Look at Fig. 1.2.2 and make a reasonable estimate.

b. In the situation described in part a, about how fast will you be traveling downward when you reach the water? Estimate your velocity from Fig 1.2.2.

c. If you jump upward as you leave the platform, so that you begin with a modest upward velocity, will your downward velociy when you hit the water be more or less than in part b?

d. You leave the platform simultaneously with a friend. She walks slowly off the platform and you jump to give youself a modest initial upward velocity. Who will reach the water first?

e. You leave the platform simultaneously with a friend. He walks slowly off the platform and you run off the platform, so that your initial velocity is in the horizontal direction. Who will reach the water first?

f. In the situation described in part e, who hits the water with the largest speed or are your speeds equal?

22. case 1-7 [121144] You have recently taken up track and field as a way to keep in shape. You soon begin to notice how simple physical laws appear in many of the events.

a. You notice that great sprinters have extremely strong legs. Why is it so important that a sprinter be able to push back hard on the starting blocks at the beginning of a race?

b. You find that throwing a heavy metal shot is far more difficult than throwing a baseball. Weight isn't the whole problem. Even if you try to throw the shot horizontally or downward, so that weight is not an issue, you have great difficulty getting the shot to move quickly. Why?

c. As you land on the soft foam pad beneath the pole vault, you realize that its job is to bring you to rest by accelerating you upward gradually with only modest support forces. If there were no pad there, only concrete, what would the the acceleration and support forces be like during your landing?

d. You cross the finish line at the end of a race. The net force on your body points in what direction as you slow down?

e. In the long jump, you run rapidly down a path and then leap into the air. You find that the best distance comes from pushing yourself upward rather than forward during the leap. Why is it so important to have a large upward component of velocity at the start of a leap?

23. case2-1 [121049] You're seated in a crowded bus that has stopped at a bus stop. Two people board the bus, one wearing inline skates (roller skates- i.e., no friction). They both have to stand in the middle of the aisle and neither one holds onto anything.

a. The bus driver is new and the bus lurches forward from the bus stop. Which way does the person wearing inline skates move in relationship to the bus?

b. The bus is accelerating forward as it pulls away from the the bus stop. Why doesn't the person wearing inline skates accelerate with the bus?

c. The person wearing rubber-soled shoes remains in place as the bus starts moving. What provides the force that causes that person to accelerate with the bus?

d. Once the bus has reached a constant velocity (it's traveling along a straight, level road at a steady pace), the person wearing inline skates is able to stand comfortably in the aisle without rolling anywhere. What is the net force on the person wearing skates?

e. The driver accidentally runs into a curb and the bus stops so abruptly that everything slides forward, including the person wearing rubber-soled shoes. In stopping quickly, the bus experiences an enormous backward acceleration. Why doesn't the person wearing rubber-soled shoes accelerate backward with the bus?

24. case 2-5 [121143] A popular exercise machine in a modern weight room simulates the act of climbing stairs. You stand on two pedals and move your feet, and hidden machinery inside the device makes a whirring sound. It takes a lot of force to push each pedal downward, but that pedal rises upward easily as you lift your foot. As long as you keep moving your feet up and down at the right pace, you remain stationary. If you slow your pace, you begin to sink downward. If you speed up, you begin to rise upward.

a. When you're moving your feet up and down at the right pace, so that you remain stationary, how much total downward force are you exerting on the two pedals?

b. As you push your foot and the pedal downward, you exert a large downward force on the pedal. Between your foot and the pedal, which is doing (positive) work on which?

c. As you lift your foot upward, the pedal exerts a small upward force on your foot. Between your foot and the pedal, which is doing (positive) work on which?

d. On the average, what is doing (positive) work on what (you and the machine)?

e. Clearly, energy is being transferred out of you and into the machine. The machine converts that energy into a form that can be carried away in the air. What is that final form of energy?