### Fall 2010 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 113. The mean score was 76.4; the median was 77. The exam period was 90 minutes. Click here to see page1 page2 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 question and the next three concern the same situation:

A block of 10 kg rests on the (horizontal) floor. The coefficient of kinetic friction between the block and the floor is 0.1. You push horizontally on the block with a force F = 30 N.

What is the work done on the block by the force F after the block has moved a distance d = 2 m (in the same direction of the force on the floor).

(a)   20 J
(b)   30 J
(c)   40 J
(d)   50 J
(e)   60 J

### QUESTION 2***

What is the increase of the kinetic energy of the box after it has traveled a distance d = 2 m ?

(a)   20 J
(b)   30 J
(c)   40 J
(d)   50 J
(e)   60 J

### QUESTION 3*

Suppose F is still 30 N but is now angled at 30° downward. Is the work done by F in moving the block through a distance d = 2 m (on the floor) smaller or larger than the work done when F is horizontal?

(a)   smaller
(b)   larger
(c)   the same

### QUESTION 4**

Is total mechanical energy conserved in this system?

(a)   yes
(b)   no

### QUESTION 5*

A 7.2 kg bowling ball is suspended from a 3 m long wire to make a pendulum. The pendulum is pulled to one side, to a height h above its lowest point, and released from rest. What is the speed v of the ball when it reaches its lowest point?

(a)   v = 0
(b)   v = mgh
(c)   v2 = 2gh

### QUESTION 6*

This question and the next two concern the same situation:

Two 80 kg skiers, A and B, race on a pair of trails down a mountain. Both trails drop a vertical distance of 100 m. Skier A chooses a trail that zigzags back and forth and has a total length of 1000 m. Skier B chooses a trail that heads straight down the steepest part of the mountain and has a total length of 250 m. Ignoring friction and air resistance, which skier is moving faster at the bottom of the mountain?

(a)   A
(b)   B
(c)   A and B have the same speed.

### QUESTION 7*

Which skier will get to the bottom first?

(a)   A
(b)   B
(c)   A and B get to the bottom at the same time.

### QUESTION 8**

The skier is hauled back to the top of the mountain by a tow rope. The tow rope operates at a speed of 1 m/s up a frictionless tow track of length 400 m, up a height of 100 m. What is the average power required to lift skier A?

(a)   175 W
(b)   196 W
(c)   229 W
(d)   344 W
(e)   not enough information given

### QUESTION 9**

This question and the next two concern the same situation:

A 1.0 kg hammer moving at 3 m/s strikes an initially stationary 10 kg metal block head-on and bounces straight back at a speed of 0.5 m/s. What is the speed of the block after the collision?

(a)   0.11 m/s
(b)   0.27 m/s
(c)   0.35 m/s
(d)   0.54 m/s
(e)   0.99 m/s

### QUESTION 10**

The collision is

(a)   elastic.
(b)   inelastic.
(c)   completely inelastic.

### QUESTION 11*

Suppose that after the collision the block is moving at 0.3 m/s, and that the hammer is in contact with the block for 0.01 s. What is the average force of the hammer on the block?

(a)   50 N
(b)   100 N
(c)   200 N
(d)   300 N
(e)   800 N

### QUESTION 12*

A hockey puck is moving across the ice with velocity (vx = 3 m/s, vy = 4 m/s) and has a head on, elastic collision with a second, identical puck with velocity (vx = 0,vy = 0), as in the figure. What is the speed of the second puck after the collision? Think before you calculate! This problem should involve minimal calculation.

(a)   0 m/s
(b)   1 m/s
(c)   2 m/s
(d)   3 m/s
(e)   5 m/s

### QUESTION 13**

A comet, initially at rest, explodes into two parts of mass M/4 and 3M/4. If the more massive fragment has velocity (vx = 0, vy = 50 m/s), what is vx for the smaller fragment?

(a)   0 m/s
(b)   25 m/s
(c)   50 m/s
(d)   100 m/s
(e)   150 m/s

### QUESTION 14**

In the laboratory, when you dropped the basketball, the maximum height after each bounce off the floor slowly decreased. This was because:

(a)   mechanical energy was not conserved
(b)   total energy was not conserved
(c)   both (a) and (b)

### QUESTION 15**

A cart is allowed to roll down an incline without friction. The cart has both translational and rotational kinetic energy. The potential energy is defined to be zero at the top, becoming more negative as it rolls down. At the bottom of the incline, the cart's total kinetic energy is

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

### QUESTION 16*

This question and the next one concern the same situation:

There are two identical looking disks A and B. A and B have the same mass MA = MB, but the moment of inertia IA of A around its symmetry axis is larger than that IB of B. A and B are released gently from the same height on the incline as illustrated below.

Suppose the slope is frictionless. Which disk reaches the bottom of the incline first? (Note: MA = MB and IA > IB)

(a)   A
(b)   B
(c)   A and B arrive simultaneously.

### QUESTION 17**

Suppose instead that there is friction and the disks roll without slipping. Which disk reaches the bottom of the incline first? (Note: MA = MB and IA > IB)

(a)   A
(b)   B
(c)   A and B arrive simultaneously.

### QUESTION 18**

This question and the next one concern the same situation:

There is a very light stick of length 3 m. At its one end is an almost point like mass of 5 kg, and at the other end of the stick is another almost point like mass of 9 kg. The stick with two masses is hung from the ceiling with two strings so that it is horizontal as illustrated below. (The dotted line segment in the figure is for the second question below.)

Find the tension T in the right string.

(a)   0 N
(b)   49 N
(c)   69 N
(d)   88 N
(e)   127 N

### QUESTION 19**

If the position of the left string is moved slightly to the right (to the position designated by the dotted line in the figure above), what happens to T ?

(a)   T increases.
(b)   T does not change.
(c)   T decreases.

### QUESTION 20**

A uniform disk of radius R and mass M has an angular momentum L initially around the symmetry axis of the disk. We wish to halve its angular momentum in t seconds. What tangential constant force F should we apply to its rim?

(a)   F = Lt / (2R)
(b)   F = L / (2Rt)
(c)   F = L / (2t)
(d)   F = L / (Rt)
(e)   F = Lt / R

### QUESTION 21***

This question and the next one concern the same situation:

At one end of a massless wire with the partially circular shape shown in the figure is a mass M = 2 kg. The other end is on a fulcrum O, and this balancing toy is stationary. That is, the mass is sitting vertically below the fulcrum O. What is the net torque around the corner A? The angle θ is π/6 and R = 0.5 m.

(a)   -RMg
(b)   -RMg / 2
(c)   0
(d)   RMg / 2
(e)   RMg

### QUESTION 22*

What is the moment of inertia of the balancing toy in the previous problem around an axis through O and perpendicular into the page?

(a)   0
(b)   0.5 kg·m2
(c)   1.0 kg·m2
(d)   1.5 kg·m2
(e)   2.5 kg·m2

### QUESTION 23*

At the rim of a rotating, uniform turntable of mass M is a girl of the same mass M. The girl is initially standing still (relative to the turntable) on its rim. She walks to the center of the turntable and stops (again relative to the turntable). The angular speed of the turntable

(a)   increases.
(b)   decreases.
(c)   stays the same.

### QUESTION 24***

A uniform disk of radius R and mass M is moving across a horizontal floor toward an incline with a translational speed of v (see the illustration).

If the floor and the slope are perfectly frictionless, the disk slides without rolling. The largest vertical displacement of the center of mass of the disk is h as shown in the figure.

If instead the floor and the incline have friction, the disk rolls without slipping. What is the largest vertical displacement of the center of mass, if the translational speed of the disk on the floor is the same as before?

(a)   0.5 h
(b)   0.67 h
(c)   0.75 h
(d)   h
(e)   1.5 h

### QUESTION 25*

A massless rod of length 3 m is supported by a balloon that exerts an upward force F at one end and by a fulcrum at the other end as illustrated below. A mass M = 15 kg is placed 1 m from the balloon end and 2 m from the fulcrum end, as shown in the figure. To maintain the rod horizontal, what force F is required?

(a)   33 N
(b)   67 N
(c)   98 N
(d)   131 N
(e)   164 N

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.