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Homework Help: Simple work-energy problem

  1. Oct 13, 2004 #1
    A force of 5N acts on a 2 kg object that starts from rest. When the force has been acting for 2 s, the rate at which it is doing work is:

    Here's how I tried to solve it.

    Solve for acceleration, which I assumed was constant.

    Code (Text):

    a = F/m = 5N / 2kg = 2.5 m/s/s

    Then use the displacement equation to find the distance traveled.

    Code (Text):

    x = xo + (vo)(t) + (1/2)(a)(t^2)
    x = 0 + 0 + (1/2)(2.5 m/s/s)(2s)^2
    x = 5m

    Work = F * x = 5N * 5m = 25 J

    Power = Work / t = 25J / 2s = 12.5 W
    However, the only possible answer choices are:
    a) 5W b) 10W c) 25W d) 50W e) 75W

    Where am I screwing up?
     
  2. jcsd
  3. Oct 13, 2004 #2
    [tex]P=\frac{dW}{dt}[/tex]
    [tex]P=\frac{d(Fs)}{dt}[/tex]
    [tex]P=F\frac{ds}{dt}[/tex]
    [tex]P=F\frac{d(ut+\frac{1}{2}at^2)}{dt}[/tex]
    [tex]P=F*(u+at)[/tex]
    [tex]P=5*(2.5*2)[/tex]
    [tex]P=25\ W[/tex]
    You formula for power can't be used in this case because P is not constant for P depends on how long the force acts on the object.
     
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