### Fall 2008 Physics 211 Hour Exam 1(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 114. The exam period was 90 minutes; the mean score was 93.2; the median score was 95. Click here to see the formula sheet that came with the exam.

### QUESTION 1*

This and the next two questions are about the following situation.

A car undergoing uniform acceleration, a = 4 m/s2, is travelling at a velocity of 40 m/s as it enters a tunnel. When the car exits the tunnel, its velocity is 72 m/s. How long is the tunnel?

(a)   7 m
(b)   90 m
(c)   197 m
(d)   378 m
(e)   448 m

### QUESTION 2*

For how long is the car in the tunnel?

(a)   0.2 s
(b)   1.0 s
(c)   4.1 s
(d)   7.0 s
(e)   8.0 s

### QUESTION 3*

The average velocity of the car while in the tunnel is

(a)   closer to Vin than Vout.
(b)   half way between Vin and Vout.
(c)   closer to Vout than Vin.

### QUESTION 4*

This and the next question are about the following situation.

Mr. Stick lives on the planet Teflon where the force due to gravity is different than that of Earth. Mr. Stick drops a small water balloon off the top of a building (V0 = 0 m/s). He releases the balloon at a height H = 10 m above the surface of Teflon. The balloon hits the ground 1.8 seconds after it is dropped and has a velocity V1 on impact. (Neglect Teflonian air resistance.) What is the acceleration due to gravity on the planet Teflon?

(a)   3.5 m/s2
(b)   6.2 m/s2
(c)   8.3 m/s2
(d)   9.8 m/s2
(e)   11.6 m/s2

### QUESTION 5*

Mr. Stick threw a 2nd balloon upwards with initial velocity 5 m/s that hit the ground with a velocity V2. Which one of the following is true?

(a)   V2 > V1
(b)   V2 = V1
(c)   V2 < V1

### QUESTION 6*

This and the next question are about the following situation.

The two massless springs have the same length L0 when not compressed or stretched. The stiffness of each spring is k1 and k2, respectively. Mass M1 hangs from spring 1 and it reaches equilibrium at position L1. Mass M2 hangs from spring 2 and it reaches equilibrium at position L2.

If k2 = 2 k1 and M2 = 2 M1, which one of the relationships below is correct? (a)   L2 = 2 L1
(b)   L1 = 2 L2
(c)   L1 = L2
(d)   L2 = 4 L1
(e)   L1 = 4 L2

### QUESTION 7***

If k2 = k1 and M2 = 3 M1, which one of the relationships below is correct:

(a)   L2 = 3 L1
(b)   L1 = 3 L2
(c)   L2 = 2 L1 - 3 L0
(d)   L2 = 3 L1 - 2 L0
(e)   L1 = 3 (L2 - L0)

### QUESTION 8*

This and the next two questions are about the following situation. A mass m = 7.1 kg is hung over a massless pulley. Two masses, each with mass M = 15.3 kg are attached to m with a massless rope as shown in the drawing below. The desktop and the pulley are frictionless.

The tension T1 is greater than the tension T2.

(T)   True
(F)   False

### QUESTION 9**

What is the relationship between the tensions T2 and T3?

(a)   T2 > T3
(b)   T2 = T3
(c)   T2 < T3

### QUESTION 10*

What is the acceleration a of the hanging mass m?

(a)   a = 0.0835 m/s2
(b)   a = 1.40 m/s2
(c)   a = 1.85 m/s2
(d)   a = 5.69 m/s2
(e)   a = 10.42 m/s2

### QUESTION 11*

This and the next question are about the following situation.

A mass m is tied to a string of length R = 0.5 m and set in uniform circular motion in the vertical plane, as shown in the left figure. The angular velocity of the motion is ω = 3.1 rad/s. The string tension is largest at

(a)   the top of the circle.
(b)   the bottom of the circle.
(c)   it is the same at all points on the circle.

### QUESTION 12*

When the mass is at the bottom of the circle, the string suddenly breaks so that the mass slides (without rolling) on a rough, horizontal surface with μk = 0.32, as shown. How far does the mass slide before it comes to rest? (a)   0.10 m
(b)   0.25 m
(c)   0.31 m
(d)   0.38 m
(e)   0.49 m

### QUESTION 13*

This and the next question are about the following situation.

Block m1 (3 kg) is hanging over the edge of a table and is attached to block m2 (17 kg) by a massless string that runs over a frictionless pulley as shown in the figure. The table has static and kinetic coefficients of friction of 0.35 and 0.2 respectively. Block m2 is also attached to a wall by an ideal, massless spring with a spring constant of 100 N/m. The force of m1 on the earth is equal in magnitude to the force of the earth on m1.

(T)   True
(F)   False

### QUESTION 14*

What is the maximum distance the spring can be compressed from its equilibrium position and have the system remain at rest?

(a)   0.0 m
(b)   0.075 m
(c)   0.121 m
(d)   0.152 m
(e)   0.289 m

### QUESTION 15*

This and the next two questions are about the following situation: A tennis ball of weight W = 0.5 N is attached to a rope and swung in a vertical circle. The rope is L = 1 m in length. When the ball is at its highest point, the tension in the rope is measured to be zero.

What is the tangential velocity of the ball at its highest point?

(a)   3.1 m/s
(b)   4.4 m/s
(c)   8.8 m/s
(d)   19.6 m/s
(e)   79.3 m/s

### QUESTION 16*

Which one of the following is a valid expression for the magnitude of the tension in the string when the ball is at the lowest point?

(a)   W
(b)   W (1+v2/gL)
(c)   W(1-v2/gL)

### QUESTION 17*

Imagine that the string breaks. Immediately afterwards the acceleration of the ball is g.

(T)   True
(F)   False

### QUESTION 18**

This and the next two questions are about the following situation.

A student is walking in a straight line along Green St. at a constant velocity of 2 m/s. She tosses a pen into the air at an angle of 60° with respect to the vertical in her reference frame. A stationary observer on the other side of the street (directly across from her) observes that the pen goes straight up vertically and then falls to the ground. Assume the student is walking in the positive x-direction. As measured by the student who tossed the pen, what is the x-component of the velocity of the pen?

(a)   vx,s = -4 m/s
(b)   vx,s = -2 m/s
(c)   vx,s = 0 m/s
(d)   vx,s = +2 m/s,
(e)   vx,s = +4 m/s

### QUESTION 19*

As measured by the stationary observer, what is the x-component of the velocity of the pen?

(a)   vx,o = -2 m/s
(b)   vx,o = 0 m/s
(c)   vx,o = +2 m/s

### QUESTION 20**

As measured by the stationary observer, what is the y-component of the velocity of the pen? (a)
(b)
(c)
(d)
(e)

### QUESTION 21*

This and the next question are about the following situation. A space telescope of mass mt = 10,000 kg is in a stable orbit above the Earth at an altitude h = 3630 km. The radius and mass of the Earth are RE = 6370 km and ME = 6 × 1024 kg, respectively. Newton's gravitational constant is G = 6.672 × 10-11 N m2/kg2.

Once in orbit, what is the force of gravitational attraction between the space telescope and the Earth?

(a)   0 N
(b)   800 N
(c)   5,000 N
(d)   30,000 N
(e)   40,000 N

### QUESTION 22* If the force of gravitational attraction between the space telescope and the Earth is denoted F, then which of the following expresses the tangential speed v of the telescope in its orbit?

(a)
(b)
(c)

### QUESTION 23** Two identical blocks, A and B, of mass M, are stacked in an elevator as shown to the right. The elevator accelerates upwards with acceleration a . What is the magnitude of the force that block A exerts on block B?

(a)   M (g+a)
(b)   M (g-a)
(c)   2 M (g+a)

### QUESTION 24*

This and the next question are about the following situation:

A block of mass M = 13.4 kg is supported on a frictionless ramp by a spring having a constant k = 145 N/m. When the ramp is horizontal, as in view a) below, the equilibrium position of the mass is at x = 0. The angle of the ramp is then changed to 25°, as in view b) below. What is the new equilibrium position of the block, x1?

(a)   0.12 m
(b)   0.19 m
(c)   0.38 m
(d)   0.41 m
(e)   0.56 m

### QUESTION 25*

If the block is pulled further down the ramp, to a position x2 = 0.6 m, and released from rest, what is the magnitude of the net force acting on the block immediately after its release?

(a)   17.6 N
(b)   31.4 N
(c)   44.7 N
(d)   59.2 N
(e)   125.6 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.