ASTR 121

# Problem Solving Hints

• Write up Neat Homeworks. Take pride in your homework writeups and do the best job that you can on them. Take the time to solve the homework problems roughly on scratch paper, and then copy them over neatly, filling in additional details on your final copy.

• Show Your Work. Often it is useful to give written descriptions of what you are doing, and why you are doing it. This is especially useful at the beginning of a problem where it will force you to think about the problem physically and formulate your approach mathematically. Descriptions will also maximize the chances that the TA can follow what you have done in a problem (especially if you go off on a wild tangent!) and will help her to give you constructive comments on your work. Give enough detail, and show enough mathematical steps, that students less advanced than you could understand your solution!

• Use Common Sense. Usually you will have some physical insight into how the solution to a problem should look. Is the number too big? Too small? Compare your solution to a problem to what you expect from physical insight. Trust your instincts! If an answer surprises you, go back through your work and look for an error. Test your calculations. If you cannot find an error, make a note of your concerns near your final solution and the TA will comment on them.

• Check Units. Any mathematical expression that you write must be dimensionally correct. Check your equations occasionally as you go through a derivation. It takes just a second to do so, and you can quickly catch many common errors. All distances must have units of length (meters, miles, etc.) all times must have units of time (seconds, years, etc.) and so on. Remember this general rule: the argument of all functions (e.g. trigonometric functions, exponentials, logs, hyperbolic functions, etc.) must be dimensionless. Taking the cosine of something with units of mass or length makes no physical sense.

• Check Limits. Check all of your final answers and important intermediate results to see if they behave correctly in as many different limits as you can think of. Sometimes you will know how a general expression should behave if a particular variable is set to zero, infinity, or some other value. Make sure that your general expression actually displays the expected behavior!

• Take Advantage of Symmetries. Symmetries are fundamental in physics (and astronomy!). Problems can have symmetry about a point (spherical symmetry), a line (cylindrical or axial symmetry), or a plane (mirror symmetry). You can use symmetries in two ways: 1) to check your final answer to a problem or, with a little more effort, 2) to simplify the derivation of that final answer. As an example, time-independent central forces (like gravity) have spherical symmetry because the force depends only on the distance from the origin. In this case, spherical symmetry means that once we find one solution (e.g. a particular ellipse for gravity), all other possible orientations of this solution in space are also solutions.

• Get Help from Others. Work on the homework problems on your own first and get as far as you can on them. This is the best way to improve your problem solving skill and prepare for in class tests. But by all means get help from other people (other students, the TA, or me) when you are stuck! By trying the problems first, you will be able to ask more intelligent questions and better understand the ideas of other students and/or the hints that the TA and I might give.

• Go over Homework Solution Sets. When you get homeworks back, go over the solution sets and your corrected homework together. Use the solution set to see how to get past points where you were stuck, and make sure that you could easily do a similar problem if given the chance, say on a midterm. Even if you get a particular problem correct, there is always much to learn by following through someone else's solution. The TA spends a lot of time writing up solution sets so that you can all improve you problem solving abilities. Take advantage of the opportunity!