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Homework Help: Question about choosing which Physics equation to use?

  1. Jan 17, 2014 #1
    So, when you are trying to solve a problem (we are currently learning about velocity, acceleration, time, distance, Kinetic Motion equations), how should you approach the problem?

    I've heard list the variables you know, and then figure out which variables you have, which will determine which equation you use.

    However, I get confused when making sure I have all of the variables. For example, how do you tell if there is a final velocity if they don't specifically say it? If they have an "initial velocity" listed, is there always going to be a final velocity hidden somewhere in the problem?
  2. jcsd
  3. Jan 17, 2014 #2

    Simon Bridge

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    There are two main approaches to kinetic problems given in secondary school.

    1. memorization:
    What you said. There should be one equation (out of five) that has one unknown.

    2. use velocity-time graphs
    Sketch the v-t graph for the motion ... you will have to put letters in for some of the values.
    recall that the slope of the graph is the acceleration and the area under it is the displacement - write this out, using the graph to guide you. (this way you don't need to memorize any kinematic equations.)
    now you have two equations with two unknowns: simultaneous equations.

    Most people seem to use method 1, but only remember about 3 equations so they have to use two of them and some algebra.

    To your question: if you are uncertain about the variables ...
    You know if you are missing a variable if there are the wrong number for the equations ... there should be no more than two unknowns. If there are more, then you have to go back over the question.

    Spotting the implied variables comes down to experience with the English language - or whatever language you are taught in.

    i.e. "from rest" means u=0 and "until it stops" means v=0.
    If you have a specific example of where you get puzzled - please show us.
    Last edited: Jan 17, 2014
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