Physics
141, Midterm Exam #1
Thursday October
4, 2012
8.00 am –
9.30 am
Do not turn the
pages of the exam until you are instructed to do so.
Exam rules: You may
use only a writing instrument while
taking this test. You may not consult any calculators, computers,
books, nor each other.
1. Answer the multiple-choice questions (problems 1 – 10) by marking your answer on the scantron form. For each multiple-choice question (problems 1 – 10), select only one answer. Questions with more than one answer selected will be considered incorrect. If your student ID is not listed properly on the Scantron form (in the bubbles on the top-left corner), the form will not be processed and you loose points for all multiple-choice questions.
2. Problems 11, 12, and 13 must be answered in the blue exam booklets (answer questions 11 and 12 in booklet 1 and question 13 in booklet 2.) The answers need to be well motivated and expressed in terms of the variables used in the problem. You will receive partial credit where appropriate, but only when we can read your solution. Answers that are not motivated will not receive any credit, even if correct.
At the end of the exam, you need
to hand in your exam, the blue exam booklets, and the scantron form. All items must be clearly labeled with
your name, your student ID number, and the day/time of your recitation. If any of these items are missing, we
will not grade your exam, and you will receive a score of 0 points.
Name: __________________________________________________
ID number:
______________________________________________
Recitation Day/Time: ______________________________________
INTENTIONALLY LEFT BLANK
Useful Relations:
Problem 1 (2.5 points)
Consider the motion of a fan cart on a frictionless track. You observe that the cart moves to the left, gradually slowing down, stops, and moves to the right, speeding up. Which of the following six graphs can represent the velocity of the cart as function of time? Assume the usual coordinate system where +x points towards the right. Ignore friction and drag forces.
Problem 2 (2.5 points)
A ball moves in the direction of the arrow
labeled c in the Figure. The ball is struck by a stick that
briefly exerts a force on the ball in the direction of the arrow labeled e.
Which arrow best describes the direction of 
, the change in the ballŐs linear momentum?
1. a
2.
b
3. c
4. d
5. e
6. f
7. g
8. h
Problem 3 (2.5 points)
A comet travels in an elliptical path around a star, in the direction shown in the Figure. Which arrow in the Figure indicates the direction of the change in the linear momentum of the comet when it is located at position 5 along its orbit?
1. 
a
2. b
3. c
4. d
5. e
6. f
7. g
8. h
Problem 4 (2.5 points)
List the four basic forces in order of strength (start with the strongest force and end with the weakest force)
1. Strong > Electromagnetic > Weak > Gravitational.
2. Strong > Electromagnetic > Gravitational > Weak.
3.
Electromagnetic > Strong > Gravitational
> Weak.
4. Electromagnetic > Strong > Weak > Gravitational.
Problem 5 (2.5 points)
How many times did the Yankees become AL East champions?
1.
0 times.
2. 4 times.
3. 8 times.
4. 12 times.
5. 17 times.
6. 18 times.
7. 23 times.
8. 37 times.
Problem 6 (2.5 points)
A cart rolls on a frictionless track. A fan is mounted on the cart, and when the fan is turned on, there is a constant force acting on the cart. The fan is turned on and you hold the cart stationary. You then take your hand away and the cart moves forward, in the +x direction. After travelling a long distance along the track, you quickly stop and hold the cart.
PROBLEM CONTINUED ON NEXT PAGE
Problem 6 (continued)
a. The following four graphs display the x component of the net force acting on the cart as function of time. Which of these graphs matches the motion we just described?
b. The following four graphs display the position of the cart along the track as function of time. Which of these graphs matches the motion we just described?
PROBLEM CONTINUED ON NEXT PAGE
Problem 6 (continued)
Select one of the answer combinations listed below.
1. Answer question a: Graph 5; Answer question b: Graph 9.
2. Answer question a: Graph 5; Answer question b: Graph 10.
3. Answer question a: Graph 5; Answer question b: Graph 12.
4. Answer question a: Graph 6; Answer question b: Graph 9.
5. Answer question a: Graph 6; Answer question b: Graph 10.
6. Answer question a: Graph 6; Answer question b: Graph 12.
7. Answer question a: Graph 7; Answer question b: Graph 9.
8. Answer question a: Graph 7; Answer question b: Graph 12.
9. Answer question a: Graph 8; Answer question b: Graph 9.
10. Answer question a: Graph 8; Answer question b: Graph 10.
Problem 7 (2.5 points)
A spring has stiffness k. You cut the spring in half. What is the stiffness of the half-spring?
1. 4k
2. 2k
3. k
4. k/2
5. k/4
Problem 8 (2.5 points)
Consider the decay of a neutron that is at rest in your reference frame. Which of the decays shown schematically in the Figure below are impossible? The arrows shown in the Figure represent the linear momenta of the decay products.
1. (a)
2. (b)
3. (c)
4. (d)
Problem 9 (2.5 points)
You hang a 10 kg mass from a copper wire, and the wire stretches by 8 mm. Then you hang the same mass from a second copper wire, whose length is the same but whose cross-sectional area is half as large. By how much will the second wire stretch?
1. 2 mm
2. 4 mm
3. 8 mm
4. 16 mm
5. 32 mm
Problem
10 (2.5
points)
Which of the following statements is not correct?
1.
The momentum principle tells us that 
.
2. The principle of relativity states that physical laws work in the same way for observers in uniform motion as for observers at rest.
3.
The momentum principle tells us that 
.
4. The velocity of an object is related to its momentum in the following manner
Problem 11 (25 points)
Consider the Earth to be a sphere of radius R and uniform density r. Suppose that a hole was drilled along the diameter of the Earth, straight through the center. After the hole is drilled, the air is pumped out of it.
Consider a mass m dropped into the hole.
a. What is the net force exerted by the Earth on this mass when it is located a distance r from the center of the Earth?
b. What type of motion will result from this type of force?
c. How long will it take until the mass m reaches the center of the Earth?
Problem 12 (25 points)
There is an amusement park ride that some people love and others hate. A bunch of people stand against the wall of a cylindrical room of radius R and the room starts to rotate at higher and higher speed (see Figure below). The surface of the wall is designed to maximize friction between the person and the wall. When a certain critical speed is reached, the floor drops away, leaving the people stuck against the wall as they whirl around at constant speed.
a. If the critical speed is v, what is the minimum value of the static friction coefficient between the wall and the people that will ensure that they do not slide down the wall when the floor drops?
b. What happens to the minimum value of the static friction coefficient when the speed doubles to 2v?
Problem 13 (25 points)
Two blocks with masses M1 and M3, connected by a rod of mass M2, are sitting on a frictionless surface, as shown in the Figure below. A constant external force F, directed towards the left, is applied to the right-hand side of the right block. The motion of the system is such that you can treat the system non-relativistically.
a. What is the acceleration of masses?
b. What is the compression force in the rod at its right end?
c. What is the compression force in the rod at its left end?
INTENTIONALLY LEFT BLANK
INTENTIONALLY LEFT BLANK