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Homework Help: Work/kinetic energy problem

  1. Jul 27, 2008 #1
    I have been annoyed by a problem that I can't figure out. The topic is work and kinetic energy (chapter 6 in the sixth edition of physics for scientist and engineers by Paul A. Tipler). Problem 63 pg. 199

    A single horizontal force F acts in the +x direction on a mass m. The intial velocity is zero, the velocity is given as a function of x v=Cx where C is a contant. If the starting point x=0 and the final point is x = x' find the force and the work.

    my solution is w= .5mv^2 = .5m(Cx')^2 Therefore Fx' = .5m(Cx')^2 therefore
    F=.5mx'C^2 which is the correct answer.

    However, first I tried to do this

    f=ma = m (dv/dt) = m[C*dx/dt] = mCx' Why is this wrong???
  2. jcsd
  3. Jul 27, 2008 #2


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    Homework Helper

    x' in the book doesn't refer to dx/dt. It just refers to the final destination of the mass.
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