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Physics Exam Advice

December 2002

Physics Exam Advice

December, that magical time of year when people stop and reflect on... whether they're going to pass Physics! Having marked Physics exams at three Universities I've found a few general principles that seem to help. If you'd like to do better in Physics than what you're doing right now, here are some points to consider which may help in other subjects as well:

  1. Go for the part marks: too many students think its all or nothing. they don't hand in imperfect assignments, and don't write down enough to give them part marks on exam questions. Believe me, most teachers/professors are looking for every opportunity to give you marks - and they don't watch you come into class the next day thinking "there's Joe, boy was that a dim-witted answer to number 5". We've seen it all, brilliantly good and incredibly bad. If you're not convinced, consider which will look worse 4 out of ten, or zero out of 10. If we look down on anybody, it's those who hand in nothing. You're not going to get negative marks for writing down something wrong, nor will marks be deducted from another question. The worst a bad answer will give you is zero. So, your choice is a worst case of zero versus a guaranteed zero on the question --- what to do should be a no-brainer. Consider a couple of scenarios:
    1) Jeff gets 5 out of 10 questions completely correct and puts nothing down for the other 5 -- 50%, D minus.
    2) Kim gets 5 out of 10 questions completely correct and puts down few equations she thinks are applicable, a couple of free-body diagrams, and some notes about how she thinks she should proceed - much of which is wrong, but some of which is right... 60%, C minus (correlation between percentage and grade varies).

    Run some scenarios of your own, figuring that you might get 1 or 2 marks out of 10 for going beyond what you are sure of and writing in some stuff that you think shows some correct understanding of how to tackle the problem. Your 6 out of 10 on a question could become a 7 or 8 if you go out on a limb on the part you don't know. You will probably find differences like D becomes C, or B+ becomes A-.

  2. Make a crib sheet: perhaps you're even allowed a crib sheet. If so, don't just copy your friend's to save time. If your professor provides one, make your own even though you won't take it to the exam. The magic of a crib sheet is not that it helps you with equations you couldn't remember, it is that making it forces you to organize the course material in your mind. By the time you've made your own crib sheet you probably won't need to look at it.

    Some of my graduate courses would spend around 50 pages spanning several lectures to derive a single result. I would look at this stuff and think there was no way I could learn the details of the derivation, but then I would make the crib sheet. Typically I could get the entire term's material down to about 30 pages on the first pass. I would chop, organize, paraphrase, until I had about 30 pages that would suffice for notes if it was all I was allowed to bring to the exam. On the next pass I might condense this down to 10 pages, then to 3 then to 1. By the time I had gone over the course layout enough that I had a single page which I thought would get me started sufficiently to do everything else if asked... I didn't even need that sheet any more.

  3. Come in rested: this is easier said than done, but a Physics exam requires being sharp. The odds are that if you are up late the night before cramming, the extra information you cram in will be more than offset by the sluggish pace you answer the questions due to being over tired. Get a good night's sleep, and the day of the exam get into a good frame of mind: listen to music, go jogging, pray... whatever helps you become mentally set.

  4. Practice Solving Problems: the vast majority of most Physics exams consists of problem solving. You need to be good and fast at solving the type of problems you will be asked. Doing all of your assigned questions during the term should be the bare minimum you consider - redo them when preparing for the exam. Do any supplemental questions given, look for other questions of the same type in your text. Get another text from the library and use the questions from the same sections (most introductory Physics textbooks are laid out pretty much the same). If it helps, try my page Practice Physics Problems. On the left you may see an ad for books with titles like "3000 Solved Physics Problems" (the ads are random, so it may require some reloading if you wish to see these ones). Books like this can be very helpful Remember to do lots of these three things: practice, practice and practice.

  5. Learn to Recognize Problem Types: when you do an assignment you typically get a question from a section of the book - so you flip to that section to check out the equations and examples. On an exam, you have to figure out for yourself whether a ball hitting the ground problem is: collision, gravity, trajectory, energy conservation... or a mixture of these. In fact, it is the temptation to mix problem types on an exam that I think accidentally makes many exams more difficult than intended. After a student uses energy conservation to find how fast a ball hits the ground, why not get them to do an inelastic collision to find how fast it rebounds and then do a trivial 1D trajectory to figure out how long it will be til it bounces again? This train is easy to follow if you are practiced at figuring out for yourself what type of problem you are dealing with, and recognize that in multistep problems the type can keep changing. To look for examples in your text, this is often what gets labeled as the challenge problems - they are a challenge because you need to go beyond what you could immediately find in the same chapter.

  6. Take the Time to Write in the Units: "Dimensional Analysis" is one of the easiest and yet most powerful tools for checking your work. Far too many Physics Exams only have units written on the final answer. Consider the trivial example of finding the speed of a car which travels 10000 cm in 5 seconds. Your work should read something like:
    v = 10000 cm/ 5 s =2000 cm/s
    then, assuming you are expected to give the answer in m/s, your next line would be a unit conversion:
    v = 2000 cm/s * 0.01 m/cm = 20 m/s

    Notice that writing in the units will make it less likely that you make the mistake of missing the cm -> m unit conversion that needs to be done. If you have the formula wrong you would also discover that... if you get s/m it means you put each number in the wrong part of the ratio. If you get m2/s it means you got an exponent wrong. Practice writing units in all you calculations when you do assignments and it will become effortless quickly. It will then guard you against many needlessly lost marks on exams.

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