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Homework Help: Work done problem

  1. Oct 7, 2009 #1
    1. The problem statement, all variables and given/known data

    A force of 4.7 N acts on a 17 kg body initially at rest. Compute the work done by the force in (a) the first, (b) the second, and (c) the third seconds and (d) the instantaneous power due to the force at the end of the third second.

    2. Relevant equations

    f= ma

    d= vit + 1/2 at^2
    vf= vi + at

    3. The attempt at a solution

    i got parts a and d.. but b and c seem to be the problem
    first, i used f= ma to get the acceleration
    then i plugged it in to d= 1/2at^2 to find d, and i got it
    then i did work = fd and found the work
    that was part a, and it was correct

    for parts b and c.. i tried to use vf = vi + at
    i think the problem is that i was assuming that vi is 0, but i don't know how else to do it

    and for d, i just took the velocity that i found in part c, (even though the answer i got for c was wrong) and did p = fxv to get the power and it was also correct

    please help, this homework is due tonight and i'm pretty confused!
  2. jcsd
  3. Oct 7, 2009 #2
    Have you tried using energies? W = dE/dt = (E_f - E_i)/t so if you assume that potential energy is constant you can use the formula for kinetic energy and measure work as the difference in the kinetic energy from t = 2.0 to t = 3.0 (which is the work done in the 2nd second), etc.
  4. Oct 7, 2009 #3
    i actually just got it, thanks for your help!
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