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Homework Help: Finding the Force Acting on a particle

  1. Sep 20, 2009 #1
    1. The problem statement, all variables and given/known data
    Given that the velocity of a particle in rectilinear motion varies with the displacement x according to the equation
    x* = bx-3

    where b is a positive constant, find the force acting on the particle as a function of x.

    2. Relevant equations
    Hint: F = mx** = mx* dx*/dx

    Note: All the x's have dots on top of them representing with respect to time. I used * here instead.

    3. The attempt at a solution

    I used the hint and solved for the derivative of bx-3 and got -3bx-4. I put this back into the hint equation and ended up getting
    -3b2m all divided by x7

    Is this correct? I think I am doing something wrong.
  2. jcsd
  3. Sep 21, 2009 #2


    User Avatar
    Science Advisor
    Gold Member

    Remember you are taking a time derivative so you must use the chain rule:

    x'(t) = f(x)
    x"(t) = f'(x) x'
    (where x' = dx/dt)
    (Which the hint equation is giving you.)
    It looks like you are doing it right.
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