Physics 121 Practice Exam, Fall 2009

Questions from previous years' exams

You may need to refer to table 10.2 (rotational inertia) on the last page

1. A person holds a 50-kg mass above their head at a height of 2 m above the floor for 10 seconds. The power required to do this is

A. 10 W

B. 50 W

C. 500 W

D. 1000 W

E. zero

Solution for Problem 1

2. In each case below, a baseball is thrown with the same initial speed and from the same initial height. The directions of the initial velocity are different as shown by the vectors. In which case does the ball have the largest final speed, just before it hits the ground?

A. they all have the same speed

B. case B

C. case C

D. case D

E. case E

Solution for Problem 2

3. Car #1 is at rest on a level, frictionless track. Car #2 collides elastically with the first, and the two move off separately but in the same direction. What can you say about masses of the two cars?

A. there is not enough information given to compare masses.

B. the mass of car #1 is much less than the mass of car #2

C. the mass of car #1 is much greater than the mass of car #2

D. the masses are equal

E. masses are irrelevant because the problem is one-dimensional

Solution for Problem 3

4. The angular speed (in rad/s) of a minute hand of an analog watch is:

A. p/1800

B. p/60

C. p/30

D. 2p

E. 1/3600

Solution for Problem 4

5. A 120-lb woman is sitting in a stationary, massless raft on calm water. Her 40-lb child is on a separate raft 100 ft away and the two rafts are tied together by a rope. The woman pulls in the rope until the two rafts meet at a common point. Neglecting all drag forces, how far does the child's raft move?

A. 100 ft

B. 75 ft

C. 67 ft

D. 50 ft

E. the child's raft does not move because he is an obstinate boy

Solution for Problem 5

6. Two hockey pucks, A and B, are at rest on a horizontal, frictionless ice surface. The mass of puck A is two times larger than the mass of puck B. Both pucks are pushed with the same continuous, constant force from the start to the finish line. Which of the following statements is true for the kinetic energies of the pucks at the finish line?

A. K_A = K_B

B. K_A < K_B

C. K_A > K_B

D. not enough information given to compare K_A and K_B

Solution for Problem 6

7. For the above problem, which statement is true regarding the momenta of the pucks at

the finish line?

A. p_A = p_B

B. p_A > p_B

C. p_A < p_B

D. not enough information given to compare p_A and p_B

Solution for Problem 7

8. Consider four round objects with the same radius that are rolling with the same translational speed along a level surface that gradually curves upward. The objects and their masses are: (i) a solid disk with mass m_sd=1.5 kg, (ii) a hollow sphere with mass m_hs=0.2 kg, (iii) a solid sphere with mass m_ss=1.0 kg, (iv) a hoop with mass m_sd=0.5 kg. Rank these four objects in order of increasing maximum height that they travel up the inclined surface (Note - you may need to refer to Table 10.2 on the last page.)

A. (i), (ii), (iii), (iv)

B. (iv), (iii), (ii), (i)

C. (iv), (ii), (i), (iii)

D. (ii), (iii), (iv), (i)

E. they all go up to the same height

Solution for Problem 8

9. For the time when the four objects are rolling along the level surface in problem 8, rank them in order of increasing angular speed about their center of mass.

A. (i), (ii), (iii), (iv)

B. (iv), (iii), (ii), (i)

C. (iv), (ii), (i), (iii)

D. (ii), (iii), (iv), (i)

E. they all have the same angular speed

Solution for Problem 9

10. The physics department acquires a new anemometer for measuring wind speed (see figure). It consists of four small cups mounted on the ends of thin rods 16 cm long. The anemometer has a rotational inertia of 0.014 kg ^. m², and it is spinning freely (with no wind or friction in the bearings) at 60 rpm. After a light rainshower where water droplets fall vertically and collect on the cups (but not the rods), the rotation rate slows to 50 rpm. What is the total mass of water that collected on all four cups? (Hint: with no torque, angular momentum should be conserved.)

A. .014 kg

B. .002 kg

C. 0.50 kg

D. 0.11 kg

E. Water mass cannot be determined

Solution for Problem 10

11. Suppose you have your bicycle upside-down for repairs. One wheel is situated with the 40-g valve stem located 50 cm from the axle, and 30^o below the horizontal as shown below. What is the magnitude and direction of the resulting torque about the axle due to the weight of the valve stem?

Solution for Problem 11

12. A child is playing with a ball in the street, and she throws the ball horizontally with a speed of 10 m/s toward the front of a car that is speeding towards her at 32 m/s. What is the speed of the ball after it rebounds elastically from the car?

Solution for Problem 12