Instructions

Physics 141, Midterm Exam #2

Tuesday October 25, 2016

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 lower-left corner), the form will not be processed and you lose 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.

You are required to complete the following Honor Pledge for Exams. Copy and sign the pledge before starting your exam.

“I affirm that I will not give or receive any unauthorized help on this exam, and that all work will be my own.”

_____________________________________________________________________________

Name: ______________________________________________________________________

Signature: ____________________________________________________________________

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Useful Relations:

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Problem 1 (2.5 points) SCANTRON FORM

Look at the images below. What do you see?

Landing in Beijing.

On the ground in Toronto.

1. Runway 36L in Tokyo.

2. A Boeing 737 from American Airlines.

3. Your instructor’s favorite airplane from his favorite airline.

4. A major problem (look at the smoke).

5. Smog in Beijing.

6. None of the above

Problem 2 (2.5 points) SCANTRON FORM

Which of the diagrams in the following figure corresponds to a system of one electron and one positron that start out far apart, moving straight toward each other with nonzero initial velocities?

1. (a)

2. (b)

3. (c)

4. (d)

5. (e)

6. (f)

Problem 3 (2.5 points) SCANTRON FORM

A chain of metal links is coiled up in a tight ball on a low-friction table. You pull on a link at one end of the chain with a constant force. Eventually the chain straightens out to its full length and you keep pulling until you have pulled your end of the chain a total distance of 4.5 m. By what distance does the center of mass of the chain move?

1. 4.5 m.

2. 7.1 m.

3. 1.9 m.

4. 3.2 m.

5. 5.8 m.

Problem 4 (2.5 points) SCANTRON FORM

The figure below shows the potential energy distribution of a star-planet system. Three different types of motion of the planet are represented by the three energy states indicated (A, B, and C). Which of these states represents the planet in an elliptical orbit?

1. A.

2. B.

3. C.

Problem 5 (2.5 points) SCANTRON FORM

The following figure shows a portion of a graph of energy vs. time for a mass on a spring, subject to air resistance. Identify the three curves as to what kind of energy each represents.

1. A = Kinetic energy, B = Potential energy, C = Total energy.

2. A = Kinetic energy, C = Potential energy, B = Total energy.

3. B = Kinetic energy, C = Potential energy, A = Total energy.

4. B = Kinetic energy, A = Potential energy, C = Total energy.

5. C = Kinetic energy, A = Potential energy, B = Total energy.

6. C = Kinetic energy, B = Potential energy, A = Total energy.

Problem 6 (2.5 points) SCANTRON FORM

The following figure shows a graph of the potential energy vs interatomic distance for a particular molecule. The energy of all bound states are shown in the figure.

You heat a sample of molecules to such a temperature that all atoms are in one of the four bound states. All transitions between the states occur with equal probability. What is the energy of the photon that is emitted by this sample of molecules that has the largest wavelength?

1. 2.1 eV

2. 2.3 eV

3. 1.8 eV

4. 1.1 eV

5. 0.5 eV

6. 0.2 eV

7. 0.3 eV

8. 0.6 eV

9. 1.2 eV

10. 0.9 eV

Problem 7 (2.5 points) SCANTRON FORM

The following figure shows the path of a comet orbiting a star.

At what location on the path has the comet its lowest kinetic energy?

1. A.

2. B.

3. C.

4. D.

5. E.

Problem 8 (2.5 points) SCANTRON FORM

Which energy diagrams in the following figure are appropriate for the following situations?

a) Nuclear states.

b) Electronic states of a single atom.

c) Hadronic states.

1. (1=a), (2=b), (3=c).

2. (1=a), (2=c), (3=b).

3. (1=b), (2=a), (3=c).

4. (1=b), (2=c), (3=a).

5. (1=c), (2=a), (3=b).

6. (1=c), (2=b), (3=a).

Problem 9 (2.5 points) SCANTRON FORM

Consider the energy diagram shown in the figure below. Match the description of a process with the corresponding arrow in the figure.

a) Absorption of a photon whose energy is E₁ – E₀.

b) Absorption from an excited state (a rare event at low temperatures).

c) Emission of a photon whose energy is E₃ – E₁.

d) Emission of a photon whose energy is E₂ – E₀.

1. (1=a), (2=b), (3=c), (4=d).

2. (1=a), (2=c), (3=b), (4=d).

3. (1=a), (2=d), (3=c), (4=b).

4. (1=a), (2=c), (3=d), (4=b).

5. (1=b), (2=a), (3=c), (4=d).

6. (1=c), (2=a), (3=b), (4=d).

7. (1=c), (2=d), (3=a), (4=b).

8. (1=d), (2=c), (3=a), (4=b).

9. (1=d), (2=b), (3=c), (4=a).

10. (1=d), (2=a), (3=b), (4=c).

Problem 10 (2.5 points) SCANTRON FORM

A particle moves inside a circular glass tube under the influence of a tangential force of constant magnitude F, as shown in the figure.

Can we associate a potential energy with this force?

1. Yes.

2. No.

3. Insufficient information available to answer this question.

Problem 11 (25 points) ANSWER IN BOOK 1

Consider and object consisting of two masses M connected by a low-mass spring of spring constant k_s. When you exert an upward force of 2Mg, the object remains at rest, as shown in the on the left in the figure. In this situation, the spring is stretched by a distance s_i from its rest length.

At one point, a larger constant force is applied (F > 2Mg) and the object starts moving up. At some later time, the stretch of the spring has increased to s_f, and the object is located as shown on the right in the figure.

a. At this time, what is the speed of the center of mass of the object?

b. At this time, what is the vibrational kinetic energy of the object?

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

Problem 12 (25 points) ANSWER IN BOOK 1

A car of mass M is rounding a horizontal circular curve of radius R. The coefficient of static friction between the car tires in the road is m_s.

a. What is the maximum speed with which the car can go around the curve?

You drive the same car of mass M with speed v, trying to go around a circular banked curve of radius R and bank angle q. You discover that the road is iced over and there is no friction between your tires and the road.

b. If you want to go around the curve, what must be your speed?

c. What happens if your speed is below the speed you calculated in b)?

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

Problem 13 (25 points) ANSWER IN BOOK 2

The Stanford Linear Accelerator Center (SLAC), located at Stanford University in Palo Alto, California, accelerates electrons through a vacuum tube of length L.

experimentsmap

Electrons of mass m, which are initially at rest, are subjected to a continuous force F along the entire length of the tube and reach speeds very close to the speed of light

a. Calculate the final energy of the electrons.

b. Calculate the final momentum of the electrons.

c. Calculate the final speed of the electrons.

d. Calculate the time required to travel the distance L.

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

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