1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar

    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...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook