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Homework Help: Find the force to stop a ball at a given speed

  1. Sep 2, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the force to stop a 5kg ball at 20m/s instantly. Dont take in account resistance.

    2. Relevant equations
    kinetic energy = 0.5*mass*velocity^2
    Energy = force * distance
    force = energy / distance

    3. The attempt at a solution
    0.5 * 5kg * 20m/s^2 = 1000J
    1000J / 0 = ??????

    As seen it does not work if you want to find the force to counter it exactly...
    What am I doing wrong?

  2. jcsd
  3. Sep 2, 2014 #2


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    Science Advisor
    Homework Helper
    Gold Member

    You are not doing it wrong. It is unphysical to ask to stop an object instantly. In the limit as the time goes to zero, the force goes to infinity (using non relativistic formula or even relativistic ones) so it is simply ill-defined.
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