### Fall 2008 Physics 101 Hour Exam 2(24 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 ***.

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 108. The exam period was 90 minutes. The mean score was 78.6; the median was 81. Click here to see page1 page2 page3 of the formula sheet that came with the exam.

Unless told otherwise, you should assume that the acceleration of gravity near the surface of the earth is 9.8 m/s2 downward and ignore any effects due to air resistance.

### QUESTION 1*

This and the next four questions concern the same situation:

A small block of mass 0.2 kg starts from rest at position O on the frictionless slide shown in the figure. When the block passes the point A, its speed is 1.3 m/s.

What is the difference in height, h, of point O and point A?

(a)   0.086 m
(b)   0.097 m
(c)   0.11 m
(d)   0.12 m
(e)   0.13 m

### QUESTION 2*

The block passes the point B, which is at the same height as point A. What is the kinetic energy of the block at point B?

(a)   0.08 J
(b)   0.11 J
(c)   0.14 J
(d)   0.17 J
(e)   0.20 J

### QUESTION 3*

What is the magnitude of the acceleration of the block at point B? The circular portion has a radius of 0.1 m.

(a)   8.8 m/s2
(b)   9.8 m/s2
(c)   11.8 m/s2
(d)   13.9 m/s2
(e)   16.9 m/s2

### QUESTION 4**

What is the direction of the acceleration of the block at point B

(a)   upward
(b)   horizontal
(c)   downward

### QUESTION 5*

What is the speed of the block when it reaches point C? Assume the height H of point B (and also of point A) from point C is 3h.

(a)   1.5 m/s
(b)   2.6 m/s
(c)   3.7 m/s
(d)   4.8 m/s
(e)   5.9 m/s

### QUESTION 6*

This and the next question concern the same situation:

There are two small space ships A and B of the same mass of 5000 kg. The initial velocity of each of these ships is V = 0. A space walker of total mass 120 kg jumps from space ship A to B. Her speed relative to space ship B is 5 m/s before landing on B.

What is the magnitude of the impulse she imparts to spaceship A when she jumps?

(a)   200 Ns
(b)   300 Ns
(c)   400 Ns
(d)   500 Ns
(e)   600 Ns

### QUESTION 7**

Now, she jumps back to spaceship A from spaceship B with speed 5 m/s relative to spaceship B. What happens to the total kinetic energy of the two spaceships plus the space walker?

(a)   The total energy increases.
(b)   The total energy decreases.
(c)   Insufficient information is given to answer this question.

### QUESTION 8**

This and the next question concern the same situation:

There is an incline with a 35° slope. The slope is rough. You pull a block of mass 3.0 kg upward along the slope, for a distance of 5 m, at a constant speed of 2.2 m/s. You have to exert a force of 20 N parallel to the slope, as shown in the figure.

What is the magnitude of the work done by friction on the block?

(a)   4.3 J
(b)   7.3 J
(c)   15.7 J
(d)   20.5 J
(e)   40.6 J

### QUESTION 9**

When you stop pulling the block, it continues up the slope. What is the magnitude of the work done on the block between the time you stop pulling and the time the block comes to a stop?

(a)   4.3 J
(b)   7.3 J
(c)   15.2 J
(d)   30.5 J
(e)   85.3 J

### QUESTION 10**

This and the next question concern the same situation:

There is a merry-go-round of radius 3 m, which completes one rotation every 2.5 seconds at a constant angular velocity. You are standing on the edge of the merry-go-round.

(a)   10.9 m/s2 outward
(b)   10.9 m/s2 tangent to the edge
(c)   10.9 m/s2 inward
(d)   18.9 m/s2 outward
(e)   18.9 m/s2 inward

### QUESTION 11***

You jump upwards (no horizontal force is applied) and return to the same height 0.4 seconds later. You will

(a)   land inside the merry-go-round.
(b)   land again on the edge.
(c)   land off the merry-go-round.

### QUESTION 12**

This and the next two questions concern the same situation:

Two dry ice pucks slide on a horizontal, frictionless surface. Puck A moves at θ = 120° to Puck B, as shown in the figure. Puck B moves in the negative y direction. The pucks collide near the point marked X in the diagram. After the collision Puck A and Puck B are moving in the negative y and positive x directions, respectively. The initial speed of Puck A is 3 m/s and the initial speed of Puck B is 6 m/s. All the pucks have equal mass of 1 kg.

What is the final speed of puck B?

(a)   2.6 m/s
(b)   3.9 m/s
(c)   4.2 m/s
(d)   5.1 m/s
(e)   9.8 m/s

### QUESTION 13**

What is the final speed of puck A?

(a)   2.2 m/s
(b)   3.9 m/s
(c)   4.2 m/s
(d)   4.5 m/s
(e)   5.4 m/s

### QUESTION 14***

How much mechanical energy was lost in the collision?

(a)   None, energy is conserved.
(b)   9.0 J
(c)   19 J
(d)   27 J
(e)   45 J

### QUESTION 15*

This and the next question concern the same situation:

Consider a block attached by a massless rope to a massive pulley. The block has a mass of 3 kg, and the pulley has a moment of inertia I = 0.01 kg m2 and radius 0.1 m. The block starts from rest. When the block has fallen 2 m the pulley is rotating at 8.0 revolutions per second.

How fast is the block falling?

(a)   1.0 m/s
(b)   2.0 m/s
(c)   5.0 m/s

### QUESTION 16*

What is the rotational kinetic energy of the pulley?

(a)   12.6 J
(b)   13.5 J
(c)   19.8 J

### QUESTION 17**

This and the next two questions concern the same situation:

Consider a flywheel that, starting from rest, is spun up with constant angular acceleration α = 5.0 rad/s2.

How many revolutions does the flywheel go through before it reaches its final speed of 3000 revolutions per minute?

(a)   30.5
(b)   511
(c)   1571
(d)   8137
(e)   12496

### QUESTION 18**

If the answer to the previous questions were R revolutions, and the final speed were doubled, what would the new answer be?

(a)   R / 2
(b)   2 R
(c)   4 R

### QUESTION 19*

How long does the flywheel take to spin up?

(a)   5.0 s
(b)   63 s
(c)   128 s

### QUESTION 20*

This and the next two questions concern the same situation:

A block of mass 7 kg explodes into two blocks, one of mass 3 kg and the other of mass 4 kg, as shown in the Figure. Before the explosion the block has a momentum of 6 kg m/s in the +x direction.

What is the x component of the velocity of the center of mass after the explosion?

(a)   0.86 m/s
(b)   3.2 m/s
(c)   7.7 m/s

### QUESTION 21**

What is the y component of the center of mass velocity after the explosion?

(a)   -0.4 m/s
(b)   0.0 m/s
(c)   +1.3 m/s

### QUESTION 22*

Shortly after the explosion, the 4 kg block is at y = +3.0 m and the 3 kg block is at y = +8.0 m. Both have x = +10 m. What is the y-coordinate of the center of mass?

(a)   0 m
(b)   3.0 m
(c)   5.1 m

### QUESTION 23*

You grasp the end of a spring that is attached to the wall and is initially in its resting position. You pull it out until it is extended 0.1 m from its resting position, then push it in until it is compressed by 0.1 m from its resting position. Finally, you return the spring to its resting position. The spring constant is k = 20 N/m. The total work W done by the spring on your hand is

(a)   W < 0
(b)   W = 0
(c)   W > 0

### QUESTION 24**

A block is allowed to slide down a ramp of length L until it reaches the bottom. The coefficient of kinetic friction between the block and the ramp is μk > 0. The experiment is repeated with the ramp at an angle of 30° and 45° to the horizontal. In which case does friction do the most work?

(a)   30°
(b)   45°
(c)   It does the same amount of work in both cases.

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.