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This has to be wrong

  1. Apr 30, 2004 #1


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    Here is the question, it is very simple.

    A toy car accelerates by means of a rocket-type engine for 20 m. If the acceleration during the burn is [tex]5 m/s^2[/tex] and the burn lasts for 3 s, determine the velocity of the car at the end of the burn.

    It seems like the obvious one, and we use.

    [tex]v=v_i + at[/tex]

    ...and we get 15. This works for uniform acceleration, which it says it is.

    They are using...

    [tex]s = v_f t - 1/2 a t^2[/tex]

    They got that by differentiating from [tex]v_i[/tex], instead of the usual [tex]v_f[/tex]

    In the end, they get 14.2 m/s.
    Last edited: Apr 30, 2004
  2. jcsd
  3. Apr 30, 2004 #2


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    Er... you can't just switch around the "variable" in the problem. In

    [tex]v=v_i + at[/tex]

    [tex]v_i[/tex] is a constant of integration that corresponds to the initial condition, where as v is THE variable, i.e. v=v(t).

    This means that v = ds/dt. You cannot equate v_i as ds/dt because v_i IS A CONSTANT with respect to time. So that last equation is meaningless. This explains why you have a discrepancy in your answer.

  4. Apr 30, 2004 #3


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    Hmm... you learn something everyday!

    So, I guess I'm right about that. Also, this is high school physics, too. Mistakes are everywhere. Graphs are all messed up.

    Anyways, I didn't realize that the variable cannot be a constant.
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