### Fall 2003 Physics 101 Hour Exam 2(26 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 102. When the exam was given, the mean was 76.4; the median was 79. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1*

This and the next question concern the same situation:

A box is pulled across a horizontal surface at a constant speed by a horizontal rope with tension T = 30 newtons. The coefficient of friction between the box and surface is μk = 0.05.

Calculate the work done by friction as the box moves 4 meters to the right.

(a)   -120 J
(b)   -30 J
(c)   -1.5 J

### QUESTION 2*

What is the mass of the box?

(a)   31 kg
(b)   61 kg
(c)   121 kg
(d)   25 kg
(e)   27.4 kg

### QUESTION 3*

A ball is thrown up with initial velocity v. The work done be gravity as the ball is coming down is

(a)   positive.
(b)   zero.
(c)   negative.

### QUESTION 4*

This and the next three questions concern the same situation:

Two 0.025 kg balls are released at rest from a height h above the ground. Both balls are traveling at 5 m/s just before they hit the ground. Ball 1 is a steel ball, and bounces back up to the original height. Ball two is made of clay and sticks to the floor.

Which ball exerts a greater impulse on the floor?

(a)   clay ball
(b)   same
(c)   steel ball

### QUESTION 5*

From what height h were the balls dropped?

(a)   1.28 m
(b)   2.15 m
(c)   2.89 m

### QUESTION 6**

If the average force on the steel ball during the collision is 15 N, what was the duration of the collision?

(a)   0.011 s
(b)   0.017 s
(c)   0.025 s

### QUESTION 7**

If instead of being dropped, the steel ball is given an initial horizontal velocity, the average force of the floor on the ball during the collision will be

(a)   smaller than 15 N.
(b)   equal to 15 N.
(c)   larger than 15 N.

### QUESTION 8*

This and the next two questions concern the same situation:

A box starts from rest and slides down a hill of height h. Friction is negligible. When it reaches the point marked A, it collides with a stationary box with three times its mass. The two boxes stick together and slide to the right. The speed of the first box is 15 m/s just before the collision.

What is the height h of the hill?

(a)   3.8 m
(b)   6.2 m
(c)   9.7 m
(d)   11.5 m
(e)   17.3 m

### QUESTION 9**

Compare Kinitial, the kinetic energy of the two boxes just before the collision with Kfinal, the kinetic energy of the two boxes after the collision.

(a)   Kinitial > Kfinal
(b)   Kinitial = Kfinal
(c)   Kinitial < Kfinal

### QUESTION 10**

What is the velocity of the two boxes after the collision?

(a)   15 m/s
(b)   7.5 m/s
(c)   5 m/s
(d)   3.75 m/s
(e)   0 m/s

### QUESTION 11**

This and the next three questions concern the same situation:

Two disks are on a frictionless plane as shown. The first disk, with mass M is initially at rest. The second disk with mass 2M has an initial velocity of 6 m/s in the y direction before colliding with the first disk.

After the collision, the disk with mass 2M is deflected 15° and has speed 3 m/s as shown in the figure. (Ignore the effects of gravity, all the pieces are moving in a horizontal plane, and you may neglect friction).

What is the x component of the velocity of disk 1 (mass M) after the collision?

(a)   0.78 m/s
(b)   1.55 m/s
(c)   3.10 m/s
(d)   6.20 m/s
(e)   12.4 m/s

### QUESTION 12**

What is the y component of the velocity of disk 1 (mass M) after the collision?

(a)   0.78 m/s
(b)   1.55 m/s
(c)   3.10 m/s
(d)   6.20 m/s
(e)   12.4 m/s

### QUESTION 13**

Compare I1, the magnitude of the impulse on disk 1, with I2 the magnitude of the impulse on disk 2.

(a)   I1 < I2
(b)   I1 = I2
(c)   I1 > I2

### QUESTION 14**

Which of the following were conserved in the collision?

(a)   only kinetic energy
(b)   only momentum
(c)   both kinetic energy and momentum

### QUESTION 15*

This and the next three questions concern the same situation:

In yet another experiment gone wrong, a rocket car built by Dr. Olaf Whatanut exploded into three pieces. The first two pieces have a mass M1 = 95 kg , and M2 = 50 kg, respectively. The device was originally at rest. Piece one was observed heading exactly North West at 150 mph. Piece 2 was observed heading exactly North East at 285 mph. Dr. Whatanut is inside piece 3, which has a total mass of M3 = 250 kg. (Ignore the effects of gravity, all the pieces are moving in a horizontal plane, and you may neglect friction).

The total kinetic energy of the system after the collision is zero.

(T)   True
(F)   False

### QUESTION 16*

The total momentum of the system after the collision is zero

(T)   True
(F)   False

### QUESTION 17*

In what direction should the rescue party look for Dr. Whatanut?

(a)   southeast
(b)   southwest
(c)   due south

### QUESTION 18**

How fast is Dr. Whatanut traveling?

(a)   54 mph
(b)   60 mph
(c)   81 mph
(d)   92 mph
(e)   125 mph

### QUESTION 19**

This and the next question concern the same situation:

A street lamp of mass M = 5 kg is suspended from the end of a rod of length L = 1.0 m and mass m = 1.5 kg. The rod is supported by a cable attached to its midpoint and by a pivot at the wall. The cable makes an angle θ = 45° with the rod, and has negligible mass.

What is the magnitude of the tension in the cable?

(a)   T = 130 N
(b)   T = 140 N
(c)   T = 150 N
(d)   T = 160 N
(e)   T = 170 N

### QUESTION 20**

The position of the rod supporting the lamp is now changed, as shown below. The cable is still attached at the midpoint of the rod and the angle between the rod and the cable is unchanged (θ = 45°).

The tension in the cable in this new situation is

(a)   larger than in the previous case.
(b)   the same as the previous case.
(c)   smaller than the previous case.

### QUESTION 21**

Four identical point masses are arranged at the corners of a rectangle. Rotation may occur about each of the two axes shown, both of which pass through the center of mass.

The relationship between the moments of inertia for rotations about the axes shown is:

(a)   IA > IB
(b)   IA = IB
(c)   IA < IB

### QUESTION 22*

This and the next question concern the same situation:

A torque wrench is used to tighten nuts (such as the ones that hold the wheels on a car) to a specific value of torque.

If a torque of 85 Nm is required and the wrench handle is 1.5 m long, how much force must be applied to the end of the wrench?

(a)   128 N
(b)   85 N
(c)   57 N

### QUESTION 23*

Let F be the answer to the previous question. If you were to use a wrench with a shorter handle, the force required to obtain the same torque would be

(a)   smaller than F.
(b)   equal to F.
(c)   larger than F.

### QUESTION 24*

This and the next question concern the same situation:

A large crane used in construction uses a counter weight M to balance the weight m that is to be moved. The center of mass (CM) of the uniform beam is located 2.25 m from the support as shown. The mass of the beam is 300 kg,

If m = 1200 kg, what mass should M be such that the crane is in static equilibrium in the horizontal position?

(a)   1500 kg
(b)   3340 kg
(c)   3938 kg
(d)   6680 kg
(e)   3900 kg

### QUESTION 25*

A larger mass m could be supported in equilibrium with the same value of M if

(a)   M was moved closer to the support point.
(b)   M was moved further from the support point.

### QUESTION 26**

A block is attached to a disk-shaped pulley. The mass of the pulley is M = 0.10 kg and its radius is R = 0.4 m. The mass of the block is m = 0.25 kg. (Moment of Inertia of a disk I = ½MR2)

If the block starts a rest, what is the speed of the block after it drops by 5 m?

(a)   11.3 m/s
(b)   9.9 m/s
(c)   9.0 m/s
(d)   8.1 m/s
(e)   7.2 m/s

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.