### Spring 2000 Physics 101 Hour Exam 2(25 questions)

The grading button and a description of the scoring criteria are at the bottom of this page. Basic questions are marked by a single star *. More difficult questions are marked by two stars **. The most challenging questions are marked by three stars ***.

This exam consists of 25 questions; true-false questions are worth 2 points each, three-choice multiple choice questions are worth 3 points each, five-choice multiple choice questions are worth 6 points each. The maximum possible score is 111. When the exam was given, the minimum "A" score was 99; the minimum "B" was 85; the minimum "C" was 72; the minimum "D" was 55. The mean was 92.3; the median was 96. Click here to see the formula sheet that came with the exam.

### QUESTION 1**

A constant net force F is applied to block 1 (mass=M) and block 2 (mass=2M), both initially at rest, for the same amount of time Dt. Which block will end up with the higher momentum?

(a)   block 1
(b)   block 2
(c)   They end up with the same momentum.

### QUESTION 2*

A ball of mass M is initially at rest. It explodes into 3 pieces, all with the same mass M/3. One piece moves in the +x direction with speed v. Another piece moves in the –x direction with speed v/3. What is the speed and direction of the third piece?

(a)   v/3 in +x direction
(b)   v/3 in –x direction
(c)   2v/3 in +x direction
(d)   2v/3 in –x direction
(e)   impossible to answer with the information given

### QUESTION 3*

A hoop, a solid disk, and a sphere all have the same radius R and mass M, and they all roll without slipping on a horizontal surface with the same center-of-mass velocity v. Which of the objects has the largest total kinetic energy?

(a)   hoop
(b)   disk
(c)   sphere

### QUESTION 4*

This and the next question are about the following situation:

Block A of mass M initially moves in the +x direction with velocity v. Block B of mass 3M is initially at rest. The two blocks collide, but do not stick together. After the collision Block B moves in the +x direction with velocity v/2. The surface is frictionless.

What is the momentum of block B after the collistion?

(a)   Mv/2
(b)   Mv
(c)   3Mv/2
(d)   2Mv
(e)   3Mv

### QUESTION 5*

What is the velocity of block A after the collision?

(a)   v/2 in +x direction
(b)   v/2 in –x direction
(c)   0

### QUESTION 6*

A ball of mass 0.15 kg is thrown directly against a wall with an initial speed of 18 m/s. It bounces straight backward (in the opposite direction) with a final speed of 10 m/s. The contact time between the ball and wall is 1.5 x 10-3 s. What is the magnitude of the average force exerted on the ball by the wall during the contact time?

(a)   2800 N
(b)   5100 N
(c)   6667 N
(d)   12000 N
(e)   18667 N

### QUESTION 7***

A block of mass M slides down a ramp of height h0 and collides with an identical block that is initially at rest. The two blocks stick together and slide up a different ramp, reaching a maximum height h1. All surfaces are frictionless.

What is the height h1?

(a)   h0
(b)   2h0
(c)   4h0
(d)   h0/2
(e)   h0/4

### QUESTION 8*

This and the next question are about the following situation:

Block 1 of mass 2 kg initially moves in the +x direction with velocity 4 m/s. Block 2 of mass 1 kg initially moves in the –x direction with velocity 6 m/s. The two blocks collide and stick together.

In what direction do the two blocks move after the collision?

(a)   +x
(b)   –x
(c)   They are at rest.

### QUESTION 9*

Suppose instead that block 2 is initially moving in the +y direction with velocity 6 m/s. The two blocks collide and stick together. What angle will the two blocks move with respect to the x axis after the collision?

(a)   25°
(b)   37°
(c)   45°
(d)   53°
(e)   62°

### QUESTION 10*

This and the next two questions are about the following situation:

A car is traveling with speed V down a straight road. The wheels have a radius of 0.20 m and roll without slipping on the ground. The initial angular velocity of the wheels is w0=100 rad/s.

What is the initial speed V of the car?

(a)   5 m/s
(b)   10 m/s
(c)   15 m/s
(d)   20 m/s
(e)   25 m/s

### QUESTION 11*

The brakes are applied and the car is brought to a stop uniformly in 30 turns of the wheels. What is the magnitude of the angular acceleration of the wheels while the car is stopping? (Recall that one turn = 2p radians.)

### QUESTION 12*

If the initial speed of the car was doubled, but the angular acceleration of the wheels was kept the same as above, how many turns N would the wheels make as the car stopped?

(a)   N = 30
(b)   N = 60
(c)   N = 120

### QUESTION 13*

This and the next question are about the following situation:

A uniform plank of length 6 m and weight 450 N rests horizontally on two supports, as shown in the drawing, with 2 m of the plank hanging over the right support.

To what distance x can a person who weights 900 N walk on the overhanging part of the plank before it just begins to tip?

(a)   0.3 m
(b)   0.5 m
(c)   0.7 m
(d)   0.8 m
(e)   0.9 m

### QUESTION 14**

If the 900 N person stands directly over the right support (x = 0), what is the magnitude of the normal force FN exerted on the plank by the left post?

(a)   FN = 34 N
(b)   FN = 63 N
(c)   FN = 45 N
(d)   FN = 97 N
(e)   FN = 112 N

### QUESTION 15*

A uniform horizontal bar of weight 100 N and of length 1 m is pinned to a vertical wall at one end and supported by a string at the other end. In case 1, as shown in the figure, the string makes an angle of 45 degrees with the horizon, while in case 2, this angle is 30 degrees.

How does the tension in the string compare in each case?

(a)   T1>T2
(b)   T1<T2
(c)   T1=T2

### QUESTION 16*

The picture shows two different dumbbell shaped objects. Object A has two balls of mass m separated by a distance 2L, and object B has two balls of mass 2m separated by a distance L. Which of the objects has the largest moment of inertia for rotations around the x-axis? (Assume the balls can be treated as point masses).

(a)   object A
(b)   object B
(c)   Both objects have the same moment of inertia about the x axis.

### QUESTION 17*

This and the next two questions are about the following situation:

A uniform disk of mass M = 3 kg and radius R = 0.08 m spins with an initial angular velocity of w0 = 2000 rad/s.

An external torque t is applied to the disk that slows it with a constant angular acceleration of a = -65 rad/s2. What is the magnitude of the applied torque?

(a)   t = 1.94 N-m
(b)   t = 0.26 N-m
(c)   t = 2.18 N-m
(d)   t = 1.35 N-m
(e)   t = 0.62 N-m

### QUESTION 18*

How long does it take the disk to stop after the torque is applied?

(a)   43 s
(b)   31 s
(c)   28 s
(d)   22 s
(e)   16 s

### QUESTION 19**

What is the magnitude of the total work done on the disk as it stops?

(a)   Wtot = 1/4 MR2w02
(b)   Wtot = 1/2 MR2w02
(c)   Wtot = 0

### QUESTION 20*

This and the next two questions are about the following situation:

Starting from rest, a 4 kg ball (solid sphere) rolls without slipping down a ramp. At the bottom of the ramp the total kinetic energy of the ball is 100 J, and speed of the ball is V1.

The total kinetic energy of the ball is due to both rotation and translation (i.e. KETOT = KETRANS + KEROT). How do the magnitudes of these compare?

(a)   KETRANS > KEROT
(b)   KETRANS < KEROT
(c)   KETRANS = KEROT

### QUESTION 21*

What was the initial vertical height H of the ball above the bottom of the ramp?

(a)   H = 1.48 m
(b)   H = 2.12 m
(c)   H = 2.55 m
(d)   H = 3.04 m
(e)   H = 3.22 m

### QUESTION 22**

Now suppose the ramp is oiled so that there is no friction between it and the ball (in other words, the ball slides down without rolling). If the ball is released at rest from the same height as above, how will its speed V at the bottom of the ramp compare to V1?

(a)   V > V1
(b)   V < V1
(c)   V = V1

### QUESTION 23*

This and the next two questions are about the following situation:

A system is composed of two identical disks. Initially, the bottom disk is rotating with angular velocity w0 around a fixed frictionless axle, and the top disk is at rest. The top disk is now dropped onto the bottom one, and eventually both disks rotate with the same final angular velocity wf (around the same axis). There is no net external torque acting on the two-disk system.

Compare the final angular momentum of the system, LF, to the initial angular momentum of the system, LI.

(a)   LF = LI
(b)   LF = LI / 2
(c)   LF = LI / 4

### QUESTION 24*

Compare wf and w0. (Hint: The moment of inertia of the two disks spinning together is twice as big as the moment of inertia of a single spinning disk.)

(a)   wf = w0
(b)   wf = w0 / 2
(c)   wf = w0 / 4

### QUESTION 25**

Compare the final rotational kinetic energy of the system, KEF, to the initial rotational kinetic energy of the system, KEI.

(a)   KEF = KEI
(b)   KEF = KEI / 2
(c)   KEF = KEI / 4

Scoring of True-False (T,F) Questions:
• If you mark the correct answer, you get 2 points.
• If you mark the wrong answer or if you mark neither answer, you get 0 points.
Scoring of Multiple Choice I (a,b,c) Questions:
• If you mark the correct answer, you get 3 points.
• If you mark a wrong answer or if you mark none of the answers, you get 0 points.
Scoring of Multiple Choice II (a,b,c,d,e) Questions:
• If you mark one answer and it is the correct answer, you get 6 points.
• If you mark two answers and one of them is the correct answer, you get 3 points.
• If you mark three answers and one of them is the correct answer, you get 2 points.
• If you do not mark the correct answer, or if you mark more than three answers, or if you mark none of the answers, you get 0 points.