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Newton's Life


Physics 121 -- Mechanics -- Spring '00

Prof. S. Teitel: stte@pas.rochester.edu, 275-4039, B&L 455B

Frequently Asked Questions

This is an informal guide to common student questions and comments. For a more complete and official discussion of course requirements, policies, and general information, go here.

general | homework | exams


Lectures are boring! There are too many derivations. I don't care about the theory, I want to see how to solve problems.

I will try to work out example problems during lecture. But students often make the assumption that a calculation which winds up with a number at the end is a "problem", while one which winds up with an equation at the end is a "derivation" -- this is a complete misconception of what physics is about. Physics is neither about getting a final number, nor a final equation. It is about the idea that one can formulate the laws of nature in terms of a few basic principles -- and it is about the process of logical and mathematical thinking that lets one manipulate these basic laws to make quantitative predictions about the outcome of more complex situations. Every time I go through a "derivation" in lecture, it is the process of getting the final result, rather than the final result itself, that is what you should pay attention to and try to learn. Plugging numbers into a calculator is NOT what physics is about. Once you are able to see this (and it is not easy), things will become easier, make more sense, and you will do better in the course.

The course is too much math! Why isn't there more physics?

Math is the natural language of physics -- you can't do serious physics without it. It is no surprise that Isaac Newton, who first formulated the laws of Mechanics, was also one of the discoverers of calculus! The histories of mathematics and physics have always been, and continue to be, closely intertwined. The use of mathematics is what gives physics its predictive power.

I'm not doing well in the course. What can I do?

The best preparation for doing well on exams and in the course in general is to make sure you understand the homework problems. Do you start your homework early enough to go to the Help Room if you get stuck, or do you save it for the last night? After an assignment is over do you forget about it, or do you read the solutions in the library to make sure you understand your mistakes? If you work in a group (I recommend doing so) do you let others do all the work, or do you participate -- do you leave a group meeting confident you could now repeat the solutions to the problems on your own? If you take your responsibilities as a student in the course seriously, you are almost certain to pass -- you might even do very well!

Who is that guy whose picture is on the website?

That's Sir Isaac Newton! The greatest physicist of all time!


I had an exam on Thursday and couldn't finish my WebWork by the deadline. Can I get an extension?

No. You have a week to work on WebWork problems. Plan your time accordingly. Do not wait until the last night -- if something unexpected comes up, it is still your responsibility to have your work done on time.

I'm sure I have the correct answer, but WebWork keeps telling me I am wrong!

The WebWork problems have been checked through before they are posted. It is most likely that WebWork is rejecting your answer because you are not doing the calculation correctly. On a rare occasion, however, there may indeed be a bug in WebWork. If you feel that you are correct, email me your answer BEFORE the due time. Explain as best you can how you are doing the problem, instead of only giving the final numerical answer. If you are correct, I will give you credit.

But my roommate used the same method and got the problem correct, while WebWork is still rejecting my answer!

There are several possibilities. Each student's WebWork problem has different numerical values from everyone else's. Perhaps your method is correct but you are making an error with your calculation when plugging in your specific numbers. Perhaps you are making an error in the units of the answer. Perhaps your method is not really correct, or perhaps it is only approximately correct -- it may be that for your roommate's specific numerical values the method accidently gives a close enough answer, while for your specific numerical values it does not. Perhaps your roomate is not telling you the whole story! If you still feel that your solution is correct, see the answer to the preceding question.

I couldn't finish my WebWork because the system crashed (or it wouldn't let me logon) before I finished entering in my answers Thursday night. Can I get an extension?

No. You have one week to complete the WebWork assignment. Do not wait until the last night to begin work. It should not happen, but if it does happen that the system crashes, or does not let you logon, or will not accept your answers, or behaves improperly in any other way, email me your answers BEFORE the due time. If you are correct, I will give you credit. But of course you had to have printed out a hard copy of your assignment before hand, so that you can continue to work on the problems even if the system goes down -- so again, don't wait until the last night to begin work!

I HATE WebWork. I get so frustrated when it rejects my answer without telling me where I went wrong, and without caring about all my effort!

WebWork is only a machine -- don't take it so personally! Computers are not yet sophisticated enough to think about what mistakes you might be making, and then offer useful comments. People are still better at this. If you have tried several times without getting a problem correct, go see a human for help! -- see a classmate, a TA in the Help Room, or see me or send email. This is the purpose of WebWork -- by giving you instant feedback it lets you know what concepts you are not understanding correctly, so that you can go and get help, and then try again. But to take advantage of this, you must start work early -- do not wait until the last night before trying the problems.

My TA took off points for not explaining my answer, even though I got it correct. What's the matter with him? If I got it right, shouldn't I get full credit?

No. In science, engineering, and in life, it is usually not enough to get the right answer -- you have to convince others that you have gotten the right answer. In physics in particular, explaining how you logically arrived at your solution is as important as the answer itself, for demonstrating your understanding of the material. Requiring an explanation is also a safeguard against simple cheating. And for those of you that have not gotten a problem correct, explaining your solution completely so that the TA can follow your approach is the only way for him to be able to give you partial credit, and try to make helpful comments. It is unfair to ask the TA to spend his time guessing the logic behind your unexplained numerical computations.

I go to lecture and read the text. I can follow the example problems. But the homework problems all seem different from these and I do not know where to start. What can I do?

This is the hardest question to answer -- it strikes at the heart of what physics is about. Physics is not about memorizing lots of facts and equations, which one then applies in one or two simple steps to solve problems. Physics is about how to think -- how to take a few basic principles and use them to build the solutions to much more complicated questions. When you solve problems, you should NOT just flip through the text looking for the most similar such example problem, and then take the final equation and try to apply it to the problem you wish to solve. Each problem is different, and so the "final" equation of one need not be relevant to the solution of another, even if the problems are based on the same concept. When reading worked out examples, what you need to focus on is the process of starting from the basics and arriving at the "final" equation that gives the solution -- the actual plugging in of numbers at the end is the most trivial and unimportant step. When you go to solve your homework problems, you should similarly always try to first understand the basic physical concept involved, then start with the simplest most basic physical laws that relate to that concept, and build from these to get the final answer. Always start with the basic most general equations rather than later derived equations that apply only to specific situations. This is hard to do, and probably very different from most other subjects you have taken. But if you can succeed in doing things this way, you will slowly find that physics is making much more sense to you -- and you will find that you only need to remember a few basic results rather than all the hundreds of equations in the text.


I'm late to the exam because I stayed up late studying and overslept. What can I do?
I'm late to the exam because I thought it was in Hoyt, not Lower Strong. What can I do?
or, even worse,
I didn't realize that there was an exam today. What can I do?

It is your responsibility to make it to the exam, in the right place, at the right time. If you fail to do so, you suffer the natural consequences -- there will be no extension of time, no make-up exam, and no substitute work. The only possible exception will be in the case of a documented medical or other serious emergency. In such cases, an ORAL make-up exam will be given.

I forgot to bring my calculator to the exam. What can I do?

In virtually all problems, the main thing is knowing how to set up the problem correctly and work it through to the get the correct final answer in the form of an equation that gives the unknown quantity in terms of the known quantities. You should always try to do as much as possible symbolically, and only plug in the numbers at the end. If you do this correctly, you will get close to full credit on the exam, even if you don't get the right numerical answer.

Why do you make the homework problems so hard when the exam problems are relatively easier?

Think twice before asking this question -- the result might be that the exam problems get more difficult! Seriously, I try (but I don't guarantee to succeed) to make the exam questions somewhat easier than the more challenging of the homework problems. In the pressure of an hour and a half examination I can't expect you to to answer the most challenging questions. For homework, however, where you have more time and the ability to discuss with classmates, it is appropriate to give problems that make you think and struggle more. Such problems are an important part of the learning process.

This page was last updated: Mon, Apr 3, 2000