EXAM I, PHYSICS 4304



FINAL EXAM, PHYSICS 4304, December 12, 2007

Dr. Charles W. Myles

INSTRUCTIONS: Please read ALL of these before doing anything else!!!

1. PLEASE write on one side of the paper only!! It wastes paper, but it makes my grading easier!

2. PLEASE don’t write on the exam sheets, there won’t be room! If you don’t have paper, I’ll give you some.

3. PLEASE show ALL work, writing down at least the essential steps in the problem solution. Partial credit will be liberal, provided that essential work is shown. Organized work, in a logical, easy to follow order will receive more credit than disorganized work.

4. The setup (PHYSICS) of a problem counts more heavily than the detailed mathematics of working it out.

5. PLEASE write neatly. Before handing in your solutions, PLEASE: a) number the pages & put them in numerical order, b) put the problem solutions in numerical order, and c) clearly mark your final answers. If I can’t read or find your answer, you can't expect me to give it the credit it deserves.

6. NOTE!! The words “Discuss” & “explain” mean to answer in terms of PHYSICS by using a few complete, grammatically correct English sentences. They don’t mean to write (only) equations! The word “Sketch” means to make at least a rough drawing of the item you are asked to sketch.

NOTE: I HAVE 10 EXAMS TO GRADE!!! PLEASE HELP ME GRADE THEM EFFICIENTLY BY FOLLOWING THESE SIMPLE INSTRUCTIONS!!! FAILURE TO FOLLOW THEM MAY RESULT IN A LOWER GRADE!! THANK YOU!!

Work four (4) of the 5 problems. Each is equally weighted & worth 25 points for 100 points

If you have read these instructions, please sign the line below, & I will add five (5) points to your final exam grade to partially compensate for extra hours spent on evening exams. To get this credit, remember to turn this page in with your exam solutions! Note that, those who have not signed this line will clearly not have read the instructions and they will not receive this extra 10 points! Have a good Christmas Break!

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NOTE: SOME PORTIONS OF PROBLEMS 1, 2, & 3 CAN BE DONE WITHOUT THE USE OF LAGRANGE'S OR HAMILTON'S METHODS. HOWEVER, SINCE THIS IS AN EXAM WHICH IS PARTIALLY OVER THESE METHODS, NO CREDIT WILL BE GIVEN FOR SOLUTIONS TO PROBLEMS 1, 2, & 3 WHICH DON’T USE THESE METHODS!!!

NOTE!!!! Work any four (4) of the five problems!!!!

1. See figure. Two masses, m1 & m2, are connected by a massless, inextensible string of length ℓ, which is put over the massless, frictionless pulley at the top of a wedge, as shown. m1 & m2, are allowed to slide under the influence of gravity on the 2 frictionless inclined planes. As suggested in the figure, use the generalized coordinates (ℓ1,θ1,ℓ2,θ2) to solve this.

a. Write expressions for the kinetic energy, the potential energy, & the Lagrangian. How many degrees of freedom are there? (5 points)

b. What is the constraint? Write the equation of constraint. What is the physical significance of the constraint force? (4 points)

c. Use Lagrange’s method to derive the equations of motion. (Use the form without Lagrange

multipliers. First, explicitly use the constraint in the Lagrangian of part a.) (5 points)

d. Derive expressions for the generalized momenta & write an expression for the Hamiltonian. This is messy, but doable. Go as far as you can! (6 points)

e. Derive the equations of motion using Hamilton's equations. Show that these are equivalent to the results of c. (5 points)

NOTE!!!! Work any four (4) of the five problems!!!!

2. See figure. A pendulum is constructed by attaching a mass m to a massless, inextensible string of length ℓ. As shown, the upper end of the string is connected to the uppermost point on a stationary vertical disk of radius R (R < ℓ/π). At any time, a portion of the string of length u isn’t touching the disk & a portion of length s is touching it. Obviously, u & s are time dependent. Of course, Earth’s gravity acts on the mass. As suggested in the figure, use the generalized coordinates (u,s,θ) suggested in the figure to solve this problem.

a. Write expressions for the kinetic energy, the potential energy, & the

Lagrangian. How many degrees of freedom are there? (5 points)

b. What is the constraint? Write the equation of constraint. What is the

physical significance of the constraint force? (4 points)

c. Use Lagrange’s equations to find the equations of motion for this system. (Use the form without Lagrange multipliers. First, explicitly use the constraint in the Lagrangian of part a.) (5 points)

d. Derive expressions for the generalized momenta & write an expression for the Hamiltonian. This is

messy, but doable. Go as far as you can! (6 points)

e. Derive the equations of motion using Hamilton's equations. Show that these are equivalent to the

results of c. (5 points)

3. In Cartesian coordinates, the potential energy for a mass m moving in the xy plane is V = k[x2 + y2](.

It’s kinetic energy is T = (m{(vx)2 + (vy)2 + a[(vx)y + (vy)x]}. a & k are constants, vx,vy are the x & y

velocity components.

a. Write an expression for the Lagrangian for this system and derive the equations of motion using Lagrange's equations. (6 points)

b. Derive expressions for the generalized momenta. (Caution! These are NOT simply the mass times a velocity component!). (6 points)

c. Write an expression for the Hamiltonian for the system. This is messy, but doable. Go as far as you can on this! (7 points)

d. Derive the equations of motion using Hamilton's equations. (6 points)

4. Note: Each of the following problems deals with a particle of mass μ moving in a central force field. Parts d is independent of parts a, b & c!

a. The particle orbit is given by r(θ) = a(1 + cosθ) where a is a positive constant. Find the force F(r)

& the corresponding potential energy U(r). NOTE! F(r) & U(r) should be functions of r ONLY(!),

NOT functions of both r & θ! (6 points) (HINT: The easiest way to solve for F(r) is to use the

differential equation for the orbit, rather than the integral form.)

b. For the potential of part a, assume an angular momentum ℓ & find the effective potential V(r). Make a qualitative SKETCH of V(r) vs. r for different ℓ. Qualitatively DISCUSS (using WORDS in

complete, grammatically correct English sentences!) the particle orbit for different energies E. (6 points)

c. Calculate r(t) & θ(t) for the particle orbit in part a. See below for the needed integrals! Don’t forget

the integration constants! (7 points)

d. The central force field is given by F(r) = -Kr2 where K > 0. For what energy E and angular

momentum ℓ will the orbit be a circle of radius r0 about the origin? What is the period of this circular

motion? (6 points)

The following integrals might be useful. Constants of integration are not shown:

∫dθ cos(θ) = sin(θ) ∫dθ cos2(θ) = (½)θ + (¼)sin(2θ)

NOTE!!!! Work any four (4) of the five problems!!!!

5. This problem requires that you know some details about elliptic orbits. I want NUMBERS with proper UNITS!! Halley’s Comet, mass m = 5 (1015 kg, is in an elliptic orbit about the sun. This orbit has very high eccentricity ε = 0.97. The semi-major axis of the orbit is a = 2.7 ( 1012 m (measured from the center of the sun). Sun mass M = 2 (1030 kg. Gravitation constant G = 6.67 (10-11 N m2/kg2. Calculate:

a. The maximum & minimum distances of the comet from the sun (aphelion & perihelion), rmax & rmin, & the semi-minor axis, b, of the orbit (5 points)

b. The total angular momentum ℓ, of the comet. (4 points)

c. The total mechanical energy, E, of the comet. (4 points)

d. The period of the orbit. (3 points)

e. The speed, vmax of the comet when it is at perihelion (rmin). Its speed, vmin when it is at aphelion (rmax). (Hint: You can use a conservation law to help you find these.) (4 points)

f. The speed, v of the comet when it is a distance r = 5 (1011 m from the center of the sun. (5 points)

6. BONUS!! There are several major differences (philosophical & calculational) between the Hamiltonian Method & the Lagrangian Method of obtaining the equations of motion for a system. For 5 EXTRA POINTS, in a couple of complete sentences, tell me what one of these differences is. In our discussion of Ch. 7, I repeatedly emphasized these differences (in lecture & on the Web Page). If you were paying attention at all during this time, you should be able to answer this!

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