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I can't solve a single problem!

  1. Dec 31, 2011 #1
    Ok, not all, but still a considerable amount of "situational" kinematics problems (for example there's a context or "situation" and you answer questions). Also for reference I'm an IB year 1 student.

    Usually what I do with such problems is read the problem once, write down known variables, write down unknown variables, make links between variables (if any) and finally I try to think about which equations may relate better to finding one or more unknown variables.

    Occasionally, this works fine. But then comes along certain problems that I just can't figure no matter what...

    So my question is, how do you guys (who are probably far more advance and this may seem like nothing) deal with kinematics in general???

    P.S.: I noticed there's a Homework Help section....but I'm studying in FRENCH. I was thinking about translating the problems before posting, but man I'm probably sure this won't really help anyone trying to assist me.
     
  2. jcsd
  3. Dec 31, 2011 #2
    In general, my approach is to try to understand what's being asked, and to think about the problem as a whole first.

    I come up with an overall impression of what it is I am looking for, and a rough order of magnitude, and in what units, that would make sense.

    Once I have an idea of what the end result should look like, I THEN look for the tools to get the specifics.

    Otherwise, you can find a variable set that seems to fit, but can end up answering the wrong question. You can also end up with an answer that seems to work mathematically, but not logically..so if you're thinking logically that the answer is going to be roughly X, and your equation yields 2x or 0.5X, etc....you'd notice there's something wrong.
     
  4. Dec 31, 2011 #3
    Drawing a picture/free body diagram usually helps A LOT, at least for me. You didn't say whether you explicitly do this or not but I would strongly suggest you start. The other steps you mentioned are good, keep doing them.
     
  5. Jan 1, 2012 #4

    Feel free to post in French. I can read it.

    Re: Problem solving.

    1. Before writing down a single equation, think about what the problem is asking for (e.g., final speed, angle of incidence, frequency of oscillation). If you're not clear what you're solving for, you're not likely to come up with the correct answer. Is it a vector quantity, or a scalar?

    2. Once you know what you're being asked to find, make some sort of educated guess as to what you *think* the final answer might be (e.g., an order of magnitude, or even something simple like "the object slows down"). The whole point of this step is to develop some intuition.

    3. Once you know what you're being asked to find, evaluate the information you're given. 99% of the time, you're given EVERYTHING you need to know to solve the problem. Sometimes, you're expected to look up a piece of information in a table or recall material from a previous chapter, but you should have everything you need at your disposal. Consider the relationships between all the variables you're given: they should all fit nicely into an equation or two. Take the time to figure out what equation(s) this must be, and try to justify why the equation(s) must be the correct one.

    4. If you're still stuck, consider the general topic of the chapter. If it's conservation of momentum, for example, then it's likely that your problem requires momentum conservation to solve. If you're doing simple harmonic oscillations, then you should expect a final equation of the form m x" = - k x (or something that approximates that equation in the given limit).

    5. If you're still unable to figure out the correct equation(s) to use, do dimensional analysis with the variables you're given. If you're asked to solve for a time, for example, then the variables you have must add or multiply to give something with the units of time. That simple.

    6. Check your final answer against your initial guess. Also check that the answer makes sense (e.g., that the speed you get isn't greater than the speed of light, or that the alcohol in the thermometer didn't expand by several cubic kilometers when its temperature increased by 3 degrees Celsius).
     
    Last edited: Jan 1, 2012
  6. Jan 1, 2012 #5

    Vanadium 50

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    Please don't. The PF Rules, which everyone agreed to when they joined,. say "All posts must be in English--posts in other languages will be deleted"
     
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