### Spring 2010 Physics 211 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 108. The exam period was 90 minutes; the mean score was 86.3; the median score was 90. Click here to see the formula sheet that came with the exam.

Unless otherwise stated, ignore air resistance and assume the acceleration of gravity is g = 9.81 m/s2 vertically downward.

### QUESTION 1*

This and the next question are about the following situation.

A 211 student, who is initially at rest, wants to catch a Frisbee. When the Frisbee passes over her head it is moving at a speed of 4 m/s, and this is when she starts to run in the same direction as the Frisbee, accelerating at a rate of 1 m/s2. The Frisbee is decelerating at a rate of 1.5 m/s2. (You may neglect the vertical motion of the Frisbee).

How long Δt will it take the student to catch the Frisbee?

(a)   Δt = 3.20 s
(b)   Δt = 4.15 s
(c)   Δt = 5.32 s
(d)   Δt = 6.18 s
(e)   Δt = 6.97 s

### QUESTION 2*

Which one of the following graphs best represents the position (x) of the student and the Frisbee as a function of time, choosing x = 0 to be the initial location of the student and t = 0 to be the time when the students starts to run.

(a)
(b)
(c)

### QUESTION 3*

Which one of the following statements best describes the motion of an object that has a constant net force acting on it?

(a)   Its acceleration must be in the same direction as the force.
(b)   Its velocity must be in the same direction as the force.
(c)   Its velocity is constant and is in the same direction as the force.

### QUESTION 4*

A baseball player wants to hit a home run over the far wall of a stadium. He hits the ball 1 meter above the ground so that its speed is 38.2 m/s and such that it makes an angle of 30° with respect to the horizontal.

What is the tallest wall the player's ball can clear 120 m away?

(a)   2.98 m
(b)   3.22 m
(c)   5.06 m
(d)   5.74 m
(e)   6.28 m

### QUESTION 5*

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

Two blocks, of mass M1 = 8 kg and M2 = 3 kg are in contact with each other on a frictionless floor. A horizontal force F = 72 newtons is applied to block M1 as shown.

What is the force F1on2 of the block of mass M1 on the block of mass M2?

(a)   F1on2 = 0.0 N
(b)   F1on2 = 5.1 N
(c)   F1on2 = 19.6 N
(d)   F1on2 = 51.3 N
(e)   F1on2 = 72.0 N

### QUESTION 6*

If the force F were doubled and the mass M1 were doubled but M2 is held the same, the force F1on2 would

(a)   decrease.
(b)   stay the same.
(c)   increase.

### QUESTION 7*

Suppose now that F = 72 N and M1 = 8 kg again, but a coefficient of kinetic friction mk = 0.1 between the floor and each of the blocks is introduced.

The magnitude of the total force acting on M2 is now

(a)   bigger than it was without friction.
(b)   smaller than it was without friction.
(c)   the same as it was without friction.

### QUESTION 8**

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

Two stones are thrown simultaneously from a height of 500 m. The first stone is thrown vertically downward at a speed of 10 m/s, while the second stone is thrown vertically upward at a speed of 10 m/s.

Immediately after the two stones are thrown, the difference in their speeds ( |vdown| - |vup| ) will

(a)   increase with time.
(b)   decrease with time.
(c)   stay the same.

### QUESTION 9**

During the time interval that both stones are in flight the difference in their velocities will

(a)   increase.
(b)   decrease.
(c)   stay the same.

### QUESTION 10*

How long Δt will it take the second stone to get back down to the level from which it was thrown?

(a)   Δt = 2.04 s
(b)   Δt = 3.02 s
(c)   Δt = 4.62 s
(d)   Δt = 5.38 s
(e)   Δt = 6.02 s

### QUESTION 11*

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

A block of mass M = 2 kg is on a stationary inclined plane inclined with an angle θ = 30°. A horizontal rope is attached to the block and is pulled to the right with tension T. The tension remains horizontal even in the event that the block moves down the plane. The coefficient of static friction between the block and the inclined plane is μs = 0.7 and the coefficient of kinetic friction is μk = 0.5 .

Which of the following is the correct free-body diagram for the block?

(a)
(b)
(c)

### QUESTION 12**

What is Tmax the maximum value of T for which the block can be held in place with static friction?

(a)   Tmax = 0.18 N
(b)   Tmax = 0.37 N
(c)   Tmax = 0.63 N
(d)   Tmax = 1.71 N
(e)   The block remains held in place with static friction for all values of T.

### QUESTION 13*

If θ were increased, the value of Tmax found in the previous problem would

(a)   decrease.
(b)   stay the same.
(c)   increase.

### QUESTION 14**

This question and the next one are about the following situation.

A projectile is fired from a submarine traveling horizontally at 20 m/s with respect to the water as shown in the figure below. According to an observer on the submarine, the projectile is fired at 45° with an initial velocity of 60 m/s. After firing the projectile, the submarine continues to travel at 20 m/s.

According to an observer watching from a boat that is stationary with respect to the water, what will be the angle θ that the projectile makes with respect to the horizontal when it is launched?

(a)   θ = 30.9°
(b)   θ = 34.2°
(c)   θ = 45°
(d)   θ = 62.1°
(e)   θ = 71.6°

### QUESTION 15**

A tugboat captain also sees the submarine fire the projectile, but to him is looks like the projectile is moving straight up (i.e. 90° above the horizontal). What is the velocity of the tugboat relative to the water?

(a)   0 m/s
(b)   22.4 m/s in the opposite direction the submarine is moving
(c)   22.4 m/s in the same direction the submarine is moving
(d)   43.7 m/s in the opposite direction the submarine is moving
(e)   43.7 m/s in the same direction the submarine is moving

### QUESTION 16*

This question and the next one are about the following situation.

Block m1 (5 kg) is hanging over the edge of a frictionless table and is attached to a block m2 (20 kg) by a massless string that runs over a frictionless, massless pulley as shown in the figure. Block m2 is also attached to a wall by an ideal, massless spring with a spring constant of 130 N/m that has a relaxed length of X0.

By how much is the spring compressed or stretched relative to its relaxed length X0 if the system is in equilibrium?

(a)   The spring is compressed by 0.377 meters.
(b)   The spring is compressed by 0.141 meters.
(c)   The spring is neither compressed nor stretched.
(d)   The spring is stretched by 0.141 meters.
(e)   The spring is stretched by 0.377 meters.

### QUESTION 17**

Suppose the spring is compressed a distance 0.3 m from its relaxed length X0 and then the system is released. What is the acceleration a of m2 right after release.

(a)   a = 0.00 m/s2
(b)   a = 3.52 m/s2
(c)   a = 4.18 m/s2
(d)   a = 5.92 m/s2
(e)   a = 9.81 m/s2

### QUESTION 18*

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

A student twirls a tennis ball of mass m at the end of a string of length l in a vertical plane.

What is the minimum speed, v, required to just keep the string taut when the ball is at the top of its travel?

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

### QUESTION 19**

Suppose the ball is traveling at speed vh when the string is horizontal. What is the tension T in the string?

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

### QUESTION 20*

Suppose the string is cut when the ball is at the top of its travel and its velocity is vt. After the string is cut

(a)   the horizontal component of the velocity remains constant and the vertical component increases in magnitude.
(b)   the horizontal and vertical components of the velocity remain constant.
(c)   the horizontal component of the velocity decreases in magnitude and the vertical component remains constant.

### QUESTION 21*

Again suppose the string is cut when the ball is at the top of its travel and its velocity is vt. After the string is cut, the only forces on the ball are

(a)   the ball's weight and the centripetal force in the vertical direction.
(b)   the ball's weight.
(c)   There are no forces on the ball after the string is cut.

### QUESTION 22*

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

A satellite is put into a uniform circular orbit around the earth. The radius of the satellite's orbit is Rs = 4.2 × 107 m (measured from the center of the earth). The satellite has a mass of 45 kg.

At the instant shown above, the direction of the satellite's acceleration vector is.

(a)
(b)
(c)

### QUESTION 23*

What is the speed of the satellite?

(a)   2.3 km/s
(b)   3.1 km/s
(c)   5.7 km/s
(d)   6.2 km/s
(e)   9.8 km/s

### QUESTION 24*

If the mass of the satellite were doubled but the radius of its orbit was kept the same, its speed would

(a)   increase.
(b)   decrease.
(c)   stay the same.

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.