Physics 141, Midterm Exam #3

Thursday November 27, 2007

8.00 am – 9.30 am

Do not turn the pages of the exam until you are instructed to do so.

You are responsible for reading the following rules carefully before beginning.

Exam rules: You may use only a writing instrument while taking this test. You may not consult any calculators, computers, books, nor each other.

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. 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 and student ID number. If any of these items are missing, we will not grade your exam, and you will receive a score of 0 points.

NOTE: If your student ID is not listed properly on the Scantron form, the form will not be processed and you loose points for all multiple-choice questions.

Name: __________________________________________________

ID number: ______________________________________________

Recitation Day/Time: ______________________________________

Useful Relations:

Moments of inertia of various objects of uniform composition.

Problem 1 (2.5 points)

1

2

3

4

Match the above shown players of the best baseball team in the world with the following names:

A. Robinson Cano

B. Andy Phillips

C. Bobby Abreu

D. Joba Chamberlain

1234 =

1. ABCD

2. ACDB

3. BADC

4. BDAC

5. CADB

6. CABD

7. DBAC

8. DCBA

Problem 2 (2.5 points)

A ball falls straight down in the xz plane. The linear momentum of the ball is shown by the arrow in the Figure. What is the direction of the angular momentum of the ball about the origin of the coordinate system (point A)?

1.

2.

3.

4.

5.

6.

7. 0

Problem 3 (2.5 points)

If an object is traveling at constant speed in a vertical circle, how does the object’s angular momentum change as the object goes from the top of the circle to the bottom of the circle?

1. increases.

2. decreases.

3. stays the same, but the direction of changes.

4. The direction and magnitude of remain the same.

Problem 4 (2.5 points)

A diatomic molecule, such as molecular nitrogen, consists of two atoms, each of mass M, whose nuclei are a distance d apart. What is the moment of inertia of the molecule about its center of mass?

1.

2.

3.

4.

5.

Problem 5 (2.5 points)

A comet orbits the Sun in the xz plane. The linear momentum of the comet is shown by the arrow. What is the direction of the comet’s angular momentum about the Sun?

1.

2.

3.

4.

5.

6.

Problem 6 (2.5 points)

A bullet of mass m, traveling horizontally with a speed v, embeds itself in a block of mass M that is sitting at rest on a nearly frictionless surface. What is the speed of the block just after the bullet embeds itself in the block?

1. v

2.

3.

4.

5.

Problem 7 (2.5 points)

In the Rutherford experiment, what was surprising to the experimenters?

1. Sometimes the alpha particles passed right through the gold foil without being deflected.

2. Sometimes the alpha particles were deflected slightly when they passed through the gold foil.

3. Sometimes the alpha particles bounced back from the gold foil.

Problem 8 (2.5 points)

A ball of mass m₁ hits a stationary target of mass m₂ head-on. The total initial and final kinetic energies are the same. Which of the following statements is false?

1. If m₁ « m₂, the linear momentum of the ball hardly changes.

2. If m₁ < m₂, the ball bounces straight back.

3. If m₁ < m₂, the ball bounces straight back with less kinetic energy than it had originally.

4. If m₁ » m₂, the ball keeps going without a change of direction.

Problem 9 (2.5 points)

Two pucks lie on ice and can slide with little friction. A string is attached to each puck and the string is pulled with a constant force F. The string is wound abound the outer edge of puck 1 but attached to the center of puck 2. They start both from rest. Try to imagine what you would see as they move. What do you think will happen in the next 3 seconds?

1. Puck 1 will move farther than puck 2.

2. Puck 2 will move farther than puck 1.

3. Puck 1 and puck 2 will move the same distance.

Problem 10 (2.5 points)

What is the work done by a constant force F = 5 N on the center of mass of the blocks-spring system shown in the Figure below?

1. 0.03F

2. 0.04F

3. 0.07F

4. 0.08F

5. 0.10F

Problem 11 (25 points) ANSWER THIS QUESTION IN BOOKLET 1.

A ball with an initial speed of v₁ collides elastically with two identical balls whose centers are on a line perpendicular to the initial velocity and are initially in contact with each other (see Figure). The first ball is aimed directly at the contact point between balls 2 and 3 and all motion is frictionless.

a. What is the speed of ball 1 after the collision?

b. What is the speed of ball 2 after the collision?

Express all your answers in terms of the variables provided. Your answers must be well motivated.

Problem 12 (25 points) ANSWER THIS QUESTION IN BOOKLET 1.

A rod of length L and negligible mass is attached to a disk of mass M and radius R. A string is wrapped around the disk, and you pull on the string with a constant force F. Two small balls each of mass m slide along the rod with negligible friction. The apparatus starts from rest, and when the center of the disk has moved a distance d, a length of string s has come off the disk, and the balls have collided with the ends of the rod and stuck there. The apparatus slides on a nearly frictionless table. Here is a view from above: