Spring 2005 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 ***.

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 95. The exam period was 90 minutes. The mean score was 70.1; the median was 73. Click here to see page1 page2 of the formula sheet that came with the exam.

QUESTION 1*

This and the next two questions concern the same situation:

Two identical disks slide on a horizontal frictionless surface. Disk 1 is initially traveling in the positive x-direction at a speed v1. Disk 2 is initially traveling in the positive y-direction at a speed v2. The disks collide and stick together as shown in the figure below. The final speed of the two disks is vf = 4 m/s, and they are traveling at an angle θ = 20°.

What is the initial speed v1 of disk 1 before the collision?

(a)   v1 = 7.5 m/s
(b)   v1 = 9.1 m/s
(c)   v1 = 11.3 m/s

QUESTION 2*

What is the initial speed v2 of disk 2 before the collision?

(a)   v2 = 0 m/s
(b)   v2 = 1.1 m/s
(c)   v2 = 2.7 m/s

QUESTION 3*

The total mechanical energy of the two disks was conserved during the collision.

(T)   True
(F)   False

QUESTION 4*

This and the next three questions concern the same situation:

A block with an initial speed v0, is at the start of the hilly path shown in the figure below. There is no friction between the block and the path, except on the 12-m long stretch between points B and points C, where the kinetic friction coefficient is μ = 0.43.

What is the minimum initial speed of the block so that it passes the 9 m hill at point A?

(a)   v0 = 7.2 m/s
(b)   v0 = 10.0 m/s
(c)   v0 = 13.3 m/s
(d)   v0 = 16.1 m/s
(e)   v0 = 19.9 m/s

QUESTION 5*

Now, let the initial speed of the block be v0 = 20 m/s.

What is the speed of the block when it reaches point B?

(a)   10 m/s
(b)   20 m/s
(c)   40 m/s

QUESTION 6**

What is the maximum height, H, the block can reach at point D?

(a)   H = 4.4 m
(b)   H = 9.9 m
(c)   H = 12.2 m
(d)   H = 15.3 m
(e)   H = 23.3 m

QUESTION 7**

Let H be the answer to the previous question. If a second block identical to the first one were placed at rest at point C, and it is struck by the first block as it moves along the track, with the two blocks sticking together after their collision, the maximum height reached by the two blocks would be

(a)   smaller than H/2.
(b)   equal to H/2.
(c)   larger than H/2.

QUESTION 8**

This and the next three questions concern the same situation:

Two identical putty balls are suspended from the ceiling by two massless strings as shown in the figure below. The two strings have the same length. Ball 1 is held at rest at point A, 0.4 m above ball 2, which is freely hanging. Ball 1 is then released, collides with ball 2 at point B and and sticks to it. Both balls then go up together and reach a maximum height h at point C.

When ball 1 travels from point A to point B, the work done by the tension in the string is

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

QUESTION 9*

When ball 1 travels from point A to point B, the work done by gravity is

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

QUESTION 10*

What is the speed of ball 1 just before it collides with ball 2?

(a)   0 m/s
(b)   1.7 m/s
(c)   2.8 m/s

QUESTION 11***

What is the maximum height h reached by the two balls at point C?

(a)   0.1 m
(b)   0.2 m
(c)   0.4 m
(d)   0.8 m
(e)   1.6 m

QUESTION 12*

A 1 kg mass is located at position x=1 m. A 2 kg mass is located at position x=2 m, and a 3 kg mass is located at position x=3 m. What is the center of mass of these three masses?

(a)   x = 2.0 m
(b)   x = 2.3 m
(c)   x = 2.5 m

QUESTION 13*

This and the next two questions concern the same situation:

A block of mass 0.25 kg moves at a speed of 1.5 m/s on a frictionless horizontal surface. The block then hits a wall and bounces back. After the collision with the wall, the block is traveling at the same speed and in the opposite direction compared to before the collision.

Calculate the change in the kinetic energy of the block?

(a)   ΔKE = 0 J
(b)   ΔKE = 0.28 J
(c)   ΔKE = 0.56 J

QUESTION 14*

What is the magnitude of the impulse of the wall on the block?

(a)   0 kg m/s
(b)   0.38 kg m/s
(c)   0.53 kg m/s
(d)   0.75 kg m/s
(e)   0.97 kg m/s

QUESTION 15*

If the average force exerted by the wall on the block were 250 N, the block and the wall were in contact during

(a)   0.002 sec
(b)   0.003 sec
(c)   0.004 sec

QUESTION 16*

This and the next two questions concern the same situation:

A uniform beam 4 meters long is attached to the ceiling by a string 1.5 meters from the left end of the beam. The left end of the beam is attached to the floor by another string. A 7 kg block is hanging from the right end as shown in the figure.

What is TF the tension in the string attached to the floor if the beam is massless?

(a)   TF = 68.6 N
(b)   TF = 114 N
(c)   TF = 172 N

QUESTION 17*

What is the tension in the string attached to the floor, if the beam has a mass of 11 kg?

(a)   TF = 150 N
(b)   TF = 176 N
(c)   TF = 208 N

QUESTION 18**

If the 7 kg mass is moved to the left, so that it is attached 0.5 meters from the right end of the beam, TC the tension in the string attached to the ceiling will

(a)   increase.
(b)   remain unchanged.
(c)   decrease.

QUESTION 19*

This and the next three questions concern the same situation:

An 8 kg weight is attached to a 6 kg weight over a pulley as shown in the figure. The pulley is a uniform disk with moment of inertial I = 2 kg m2 and radius R = 0.4 meters as shown in the figure. The blocks start from rest, and after the 8-kg block has fallen a distance h, it is observed to be traveling at 3 m/s.

What is the mass of the pulley?

(a)   15 kg
(b)   20 kg
(c)   25 kg

QUESTION 20*

Calculate Kp the kinetic energy in the pulley when the 8 kg block is moving at 3 m/s.

(a)   Kp = 11 J
(b)   Kp = 18 J
(c)   Kp = 23 J
(d)   Kp = 36 J
(e)   Kp = 56 J

QUESTION 21***

What is the height h that the 8 kg block has fallen when it is traveling at 3 m/s?

(a)   h = 0.46 m
(b)   h = 1.5 m
(c)   h = 3.2 m
(d)   h = 6.1 m
(e)   h = 8.2 m

QUESTION 22**

If the pulley was replaced with another pulley with identical mass, but a larger radius, how fast would the 8 kg block be traveling after falling the same distance h?

(a)   > 3 m/s
(b)   = 3 m/s
(c)   < 3 m/s

QUESTION 23**

A torque t is applied to a uniform disk of mass M and radius R for a rotation of 180°. The same torque is applied for a rotation of 180° to a uniform hoop with the same mass and radius of as the disk. Compare Wdisk the work done on the disk, with Whoop the work done on the hoop.

(a)   Wdisk > Whoop
(b)   Wdisk = Whoop
(c)   Wdisk < Whoop

QUESTION 24***

This and the next question concern the same situation:

Two masses are attached to a two-meter massless beam that has been bent in the center and which is balancing on a fulcrum as shown in the figure. The left side of the beam is horizontal and has a 1 kg mass suspended from the end. The right side of the beam, makes an angle of 35° above the horizontal, and has an unknown mass suspended as shown in the figure. The system is in equilibrium

Calculate m the mass of the object attached to the right side of the beam.

(a)   1 kg cos(35°)
(b)   1 kg sin(35°)
(c)   1 kg
(d)   1 kg / sin(35°)
(e)   1 kg / cos(35°)

QUESTION 25**

This same system is now rotated and held in place such that the right side of the beam is horizontal, and the left side makes an angle of 35° above horizontal as shown in the diagram below. When released, the beam will

(a)   rotate counter clockwise (the 1 kg mass will go down).
(b)   remain stationary.
(c)   rotate clockwise (the 1 kg mass will go higher up).

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.