### Spring 2011 Physics 101 Hour Exam 1(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 110. The exam period was 90 minutes; the mean score was 79.2; the median score was 82. Click here to see page1 page2 of the formula sheet that came with the exam.

### QUESTION 1**

Amelia is using the pulley system shown in the picture to hold a 45 kg mass stationary. The pulleys and the ropes can be considered massless and free to move without friction.

What is the magnitude of the force F provided by Amelia's hand necessary to keep the 45 kg mass stationary?

(a)   221 N
(b)   442 N
(c)   662 N
(d)   882 N
(e)   1100 N

### QUESTION 2*

Two boxes are initially stationary and connected by a string over a frictionless pulley, as shown in the figure. At time t = 0, the string just above box 1 is cut so that box 2 drops downward due to gravitational acceleration. How long does it take for box 2 to drop by 1 meter?

(a)   0.21 s
(b)   0.45 s
(c)   0. 85 s
(d)   1.25 s
(e)   1.56 s

### QUESTION 3*

Paul is pulling on the rope to keep the block of mass m from sliding down a frictionless incline of angle Φ from the horizontal, as shown in the picture. The wheel is frictionless. If the rope is let go, the block will slide down the incline. What would be the magnitude of acceleration?

(a)   g
(b)   g cosΦ
(c)   g sinΦ

### QUESTION 4*

This question and the following one concern the same physical situation.

A cart is moving along the x-coordinate. It's x-coordinate as a function of time is measured every second and is shown below.

The average velocity between 0 and 7 seconds is

(a)   negative.
(b)   positive.

### QUESTION 5**

The average acceleration between 0 and 7 seconds is

(a)   negative.
(b)   positive.

### QUESTION 6*

This question and the following two concern the same physical situation.

A cellular motor protein called myosin V can carry cargos and move on a straight cellular track called actin filament. Researchers in the Physics Department at the University of Illinois measured the position of a single myosin V molecule along an actin filament a function of time as shown below.

What is the magnitude of the average velocity between 0 second and 70 second? (1 nm is 10-9 m.)

(a)   1.5 nm/s
(b)   2.7 nm/s
(c)   5.9 nm/s
(d)   7.4 nm/s
(e)   12.1 nm/s

### QUESTION 7*

What is the instantaneous velocity at time 30 s ?

(a)   0 nm/s
(b)   - 40 nm/s
(c)   + 40 nm/s
(d)   - 400 nm/s
(e)   + 400 nm/s

### QUESTION 8*

Is the molecule experiencing constant acceleration?

(a)   yes
(b)   no

### QUESTION 9*

This question and the following one concern the same physical situation.

A steel block of mass 3 kg is sitting atop a horizontal table with coefficient of static friction μs = 0.35 and coefficient of kinetic friction μk = 0.25 . Professor Ha is pushing the book as shown in the figure.

If the block is at rest, how hard can he push on the block before it will start moving?

(a)   2.0 N
(b)   4.1 N
(c)   5.7 N
(d)   7.4 N
(e)   10.3 N

### QUESTION 10**

Once the book is traveling at 4 m/s, he stops pushing. How much farther does the book move before coming to rest?

(a)   3.3 m
(b)   2.3 m
(c)   1.8 m
(d)   1.5 m
(e)   1.2 m

### QUESTION 11**

A train is moving to north at the speed of 200 km/hour (see figure). A person on the train shoots a toy cannon and the speed of the projectile leaving the toy cannon is 100 km/hour. If the toy cannon was facing toward 45° to the left relative to the front of the train, what is the speed of the projectile relative to the ground?

(a)   137 km/hr
(b)   200 km/hour
(c)   223 km/hour
(d)   280 km/hour
(e)   300 km/hour

### QUESTION 12***

This question and the following one concern the same physical situation.

Consider two Blocks, Block A = mass M, and Block B = mass m, connected with a weightless string through a weightless and frictionless pulley as shown in the figure. Block A is on a table which is horizontal. The coefficient of static friction between Block A and the table is μs = 3/5. Let g be the acceleration of gravity.

Assume that m = M/3 and that the masses are static. What is the friction force f on Block A from the table?

(a)   f = Mg / 5
(b)   f = Mg / 3
(c)   f = 3Mg / 5
(d)   f = 2Mg / 3
(e)   f = Mg

### QUESTION 13*

Suppose the mass m of Block B is increased to be a very large value (say, m = 1000M). The blocks start to move and the kinetic friction between Block A and the table is μk = 1/5. What is the acceleration of Block A?

(a)   The acceleration is much smaller than g.
(b)   The acceleration is almost g.
(c)   The acceleration is much larger than g.

### QUESTION 14**

This question and the following two concern the same physical situation.

A box of mass M is sitting on a scale in an elevator. Initially, the elevator is stationary (is not moving relative to the earth). Let g be the acceleration of gravity.

When the elevator starts to move, the reading of the scale becomes 1.1 times M (that is, 1.1M). What can you say about the motion of the elevator immediately after it starts to move?

(a)   The elevator is going upward.
(b)   The elevator is going downward.
(c)   Not enough information to tell the direction.

### QUESTION 15**

Let us be quantitative. What is the magnitude of the acceleration a of the elevator?

(a)   |a| = g / 20
(b)   |a| = g / 10
(c)   |a| = g / 7
(d)   |a| = g / 4
(e)   |a| = g / 3

### QUESTION 16**

Suppose the elevator is initially moving at a constant velocity. Then, you feel the elevator starts to accelerate. You observe that the reading of the scale becomes 1.3 times M (i.e. 1.3M). Which of the following statements is true about the motion of the elevator before the acceleration starts?

(a)   The elevator is going upward.
(b)   The elevator is going downward.
(c)   There is not enough information to tell the direction.

### QUESTION 17*

This question and the following two concern the same physical situation.

There are three boxes on a frictionless horizontal surface as illustrated in the following figure. The masses of the two left boxes are M (identical) and the mass of the rightmost box is 12 kg. A person pushes the leftmost box to the right with a force F. Consequently, all the boxes are accelerated to the right.

The rightmost box of mass 12 kg pushes the middle box with a force whose magnitude is 240 N. What is the magnitude of the force the middle box exerts on the rightmost box?

(a)   12 kg
(b)   24 kg
(c)   120 N
(d)   240 N
(e)   480 N

### QUESTION 18**

The magnitude of F is 540 N. What is the value of M?

(a)   M = 2.0 kg
(b)   M = 4.0 kg
(c)   M = 4.5 kg
(d)   M = 6.5 kg
(e)   M = 7.5 kg

### QUESTION 19*

A ball of mass m is thrown with initial velocity that makes a 45° angle with the horizontal, with initial speed V. The coordinates of the starting point are (0, 0) as illustrated below. What is the x-coordinate XH of the highest point the ball can reach? (g is the usual acceleration of gravity near the Earth's surface.)

(a)   XH = V2 / 2g
(b)   XH = V2 / g
(c)   XH = 2V / g
(d)   XH = V / g
(e)   XH = V / 2g

### QUESTION 20*

You are going at constant speed, but not constant direction. Therefore the force acting on you is zero. Is this true or false?

(T)   True
(F)   False

### QUESTION 21**

This and the following question concern the same physical situation.

A small coin of mass M is just inside the edge of a rotating horizontal circular stage of radius R as illustrated below. The coefficient of static friction between the coin and the stage floor is μs.

What is the angular speed ω of the circular stage beyond which the coin cannot stay on the edge of the stage?

(a)
(b)
(c)
(d)
(e)

### QUESTION 22*

The angular speed exceeds the limit obtained in the preceding problem, and the coin falls off the stage. Choose the right trajectory immediately after the coin leaves the stage from the following three top view sketches observed by a stationary person.

(a)
(b)
(c)

### QUESTION 23*

This and the following question concern the same physical situation.

A cart is pushed up a ramp with force F in the presence of friction. At some point the force is suddenly turned off. It reaches some height, then rolls down. (Our sign convention is the positive direction is up.)

The velocity is:

(a)   v > 0 on the way up, zero at the top, and then v > 0 on the way down.
(b)   v > 0 on the way up, zero at the top, and then v < 0 on the way down.
(c)   v < 0 on the way up, zero at the top, and then v < 0 on the way down.

### QUESTION 24***

The acceleration is:

(a)   a > 0 on the way up, zero at the top, and then a > 0 on the way down.
(b)   a > 0 on the way up, zero at the top, and then a < 0 on the way down.
(c)   a < 0 on the way up, zero at the top, and then a< 0 on the way down.
(d)   a < 0 at all times but of greater magnitude when going up than coming down.
(e)   a < 0 at all times but of greater magnitude when going down than coming 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.