Physics 201 MWF10:20 Fall 1999 (Ford)

Exam II Chapts. 5-7 in Cutnell and Johnson 4th ed.


Multiple Choice questions. Circle the correct answer. No work needs to be shown.


1. A rock of mass m is tied to a string and whirled in a horizontal circle of radius 0.5 m. Call the tension in the string T_A. An identical rock with the same mass m is tied to a longer string and whirled in a horizontal circle of radius 2.0 m. Call the tension in this string T_B. In each case the string is nearly parallel to the ground. (3 pts) If the speeds of the two rocks are the same, which statement about the tensions T_A and T_B is correct:

(a)  T_A > T_B
(b)  T_B > T_A
(c)  T_A = T_B


(3 pts) If instead the rocks have different speeds but each one makes one complete revolution in 1.0 s, which statement about the tensions T_A and T_B is correct:

(a)  T_A > T_B
(b)  T_B > T_A
(c)  T_A = T_B


(3 pts) 2. A box slides 2.0 m along a horizontal surface. During this motion the work done by the gravity force on the box is:

(a)  positive
(b)  negative
(c)  zero


(3 pts) 3. A box slides 2.0 m up a ramp that is inclined at 37° above the horizontal. During this motion the work done by the gravity force is:

(a)  positive
(b)  negative
(c)  zero



(3 pts) 4. An elevator goes from the first floor of a building to the fifth floor and returns to the first floor. For the entire round trip the net work done by gravity is:

(a)  positive
(b)  negative
(c)  zero


(3pts) 5. Two objects A and B are launched from the top of the same building as shown. Neglect air resistance. The initial speeds v_0A and v_0B are the same but A is launched in a direction 37° above the horizontal and B is launched in a direction 37° below the horizontal. Let v_fA and v_fB be the speeds of the two objects just before they strike the ground. Which of the following statements are correct:

(a)  v_fA > v_fB
(b)  v_fB > v_fA
(c)  v_fA = v_fB


(3 pts) 6. You are standing in a canoe on a pond. The canoe is at rest relative to the bank. The water is not moving and you can neglect any horizontal force that the water exerts on the canoe. Consider the system that consists of you and the canoe. Your mass is 40 kg and the canoe's mass is 80 kg. Initially the center of mass of the system of you plus the canoe is 30 m from a tree on the bank. You walk from one end of the canoe to the other and in doing so you move 2 m closer to the tree. After you have moved, how far is the center of mass of the system from the tree?

(a)  28 m
(b)  32 m
(c)  30 m
(d)  29 m
(e)  31 m


On the following four problems show all your work. Partial credit will be given if earned. Write your answers in the blanks provided.


(19 pts) 7. A small block of mass 0.03 kg is placed on the wall of a hollow cylinder that is rotating about a vertical axis. The cylinder has an inner radius R = 2.0 m and the block and the cylinder make 0.5 rev in one second.

a) What is the magnitude of the normal force the cylinder exerts on the block?

Ans. ___________________

b) If the block is to remain at the same vertical height and not slide down the wall of the cylinder, what must be the minimum coefficient of static friction between the block and the cylinder?

Ans. _________________


(20 pts) 8. A block of mass 2 kg slides along a track, from point A to point B, as shown in the sketch. Point A is 8 m above the ground and point B is 6 m above the ground. At point A the block has speed 2.0 m/s and at point B it has speed 6.0 m/s.

a) As the block moves from point A to point B, how much work is done on it by friction?

Ans. _________________

b) At point B the track is in the shape of an arc of a circle of radius 6 m. As the block slides through point B, what is the magnitude of the normal force exerted on it by the track?

Ans. __________________

(20 pts) 9. A boy is at rest on a skateboard on a level surface. The total mass of the boy and the skateboard is 40.0 kg. The boy catches a 8.0 kg ball that is traveling horizontally with speed v₀. Catching the ball sets the boy into motion and he rolls up a ramp a vertical distance of 3.0 m before rolling back down. There is no friction between the skateboard and the surface or the ramp.

a) What is the speed of the boy just after he catches the ball?

Ans. _________________

b) What is the speed v₀ of the ball just before it is caught?

Ans. __________________


(20 pts) 10. A mine car whose mass is 450 kg is rolling at a velocity of 3.0 m/s to the right on a horizontal track. The friction force exerted on the car can be neglected. A 350 kg chunk of coal has a velocity of 1.2 m/s at 37° below the horizontal when it leaves a chute 3.0 m above the bottom of the car, as shown. The chunk of coal lands in the car.

a) Determine the velocity (magnitude and direction) of the car after the chunk of coal has come to rest relative to the car.

Ans. magnitude ____________________

direction (left or right) ___________________

b) Is kinetic energy conserved in the collision between the coal and the car? (Just answer yes or no).

Ans. _____________________

c) Is the vertical component of momentum conserved in the collision between the coal and the car? (Just answer yes or no).

Ans. _____________________

d) Is the horizontal component of momentum conserved in the collision between the coal and the car? (Just answer yes or no).

Ans._____________________