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Urgent help on a simple instantaneous power problem

  1. Nov 9, 2009 #1
    1. The problem statement, all variables and given/known data
    The position of a particle on the x axis is given by x = 2t^2, due to an applied force of F = e^(2t) parallel to the x axis.

    a. Write an expression for the power supplied by this force as a function of time.
    b. Compute the instantaneous power when the particle is at x = 8.0


    2. Relevant equations
    P = dW/dt
    P = Fv


    3. The attempt at a solution

    So I've talked to some of my classmates about this and I did P = dW/dt while some of them did P = Fv. I'm having trouble comprehending which one to use and why the two methods give different answers.

    I said W = Fd = 2t^2*e^(2t) and differentiated that using the product rule, resulting in:

    P(t) = 4te^(2t) (t + 1)

    Meanwhile, using P = Fv, I just get 4te^(2t). Can someone explain why this is? Part b won't be hard for me once I figure out which way is correct in part a
     
  2. jcsd
  3. Nov 9, 2009 #2

    berkeman

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    Staff: Mentor

    When power is changing over time, I think P=dW/dt would be harder to work with. The formulation P(t)=F(t)*v(t) would seem to lend itself better to calculating instaneous power...
     
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