PHYSICS 321 – Homework Strategy and Tactics

MY PHILOSOPHY:

Homework exists to teach physics. When learning, ones makes mistakes, possibly multiple times. Therefore, homework should not be graded as a ranking exercise. (That is a job for exams.)

Scientists need to be excellent communicators of complex ideas. They must communicate clearly to others through papers and presentations, and to themselves months or years after they last worked on a problem.

YOUR STRATEGY:

In approaching each homework problem, assume I am a well-trained and busy physicist who is not familiar with the problem at all. Your goal is to explain your approach to the problem and guide me through your solution so that I am quickly convinced that your approach and calculations are correct. More whimsically, your goal is to transport me to a state of nirvana in which I am so transfixed by your prowess that I never doubt anything you do. I call the state I wish you to induce “physics bliss”.

Physics bliss is the state you will need to induce in your future employer.

DETAILS (TACTICS):

Young and Freedman, (current) freshman text, introduced the ISEE method. This acronym is perhaps too cute, but their approach is valid.

I = Identify the Problem

S=Setup the solution

E=Execute the solution

E=Evaluate the solution

Here is how I would interpret these steps as an upper-class physics major.

Identify Most problems have a “crux” or a fundamental “insight” required to even begin to solve the problem. In fact, as more experienced physicists, I would replace the word “identify”, with the word “insight”.

What is the key insight required to solve (or simplify) the problem? Are you going to use conservation of energy? Are you going to choose a non-inertial reference frame? Are you going to solve the problem with reference to the center of mass of the system? Is there something even sneakier that you need to do. Once you've decided, explain your approach.

Setup A picture of the system should always be drawn. The picture should be clearly readable (meaning also not too small!). Perhaps forces should be indicated on the picture. Almost certainly your choice of origin should be indicated. The coordinate axes should be labeled and the direction in which they are positive labeled (or you could draw x, y and indicate that you are using a right-handed (or left handed!) coordinate system. The variables you will use should be identified, listed and defined. The key formulae required should be listed in generic form, then specialized to the problem at hand. Often certain terms in a formula may be ignored, or set to zero, or to constants. You should do this, and if it isn’t obvious why it’s OK to do this, you should explain it in a sentence. As we move into the Lagrangian method, a key element to the solution is the correct choice of “generalized” coordinates. You should indicate why you chose the coordinates that you did.

Execute This is the number crunching step. Show ALL your work. Keep it orderly. If I can’t read it EASILY, I will give it back to you.

Use the English language where needed (e.g. “regrouping”, “canceling like terms”, “multiplying both sides by 1”). If your handwriting is fundamentally messy (some of us have this curse), you need to compensate (as I do) by leaving plenty of WHITE SPACE so I can read your formulae more easily. Your final answer should be enclosed in a box. When you have nearly finished the problem, you may find that it is rather illegible. Often the “Insight” step is not really complete until you have worked all the way through, or nearly so. You may need to rewrite the problem for clarity of presentation after having solved it once in a rough way.

Evaluate Check that your answer is reasonable. Often this is done by reducing a complex formula to a simpler case in which the solution is more obvious. Check that your result DOES simplify in a sensible way. Sometimes an order of magnitude estimate is useful. Sometimes an alternate approach is available. (Often a conservation method may be used instead of a force method to get a solution that is the same, or approximately the same).

MATLAB HW:

I will provide sample scripts and plots. You should follow the formats you see. Plots and scripts should have your name. Scripts should be submitted along with plots. Scripts should define your variable names in comments (just like the Setup step for ordinary HW). If a hand-drawn diagram is necessary for clarity, provide one. Discussion (the identify step) can be either in the comments or on separate paper. If the problem asks you to “discuss your results”, do so, on separate paper or in the script file. I will sometimes ask you to upload your script to my computer so I can run it.

GRADING:

You must submit your homework on time for full credit. Late homework is accepted at an initial 10-15% penalty. If bliss is induced immediately, you are done. If there are problems I need you to rework, I will indicate on the paper with “RW” (I will NOT write a detailed essay on your paper telling you what to fix. I expect you to know by referring to this sheet, though I will be happy to explain things verbally to you during office hours.). You may resubmit the work generally until the next homework is due. Once it is correct, you will still get full credit. I am willing to go through several submission cycles if needed, but beyond a certain cut-off point, your maximum grade will reduce to ½ credit rather than full credit. (For example, if a test on the material is coming up or I’m going to review the assignment in class).