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Power Problem

  1. Feb 24, 2008 #1
    [SOLVED] Power Problem

    1. The problem statement, all variables and given/known data
    An initially stationary 2.9 kg object accelerates horizontally and uniformly to a speed of 14 m/s in 4.7 s. (a) In that 4.7 s interval, how much work is done on the object by the force accelerating it? What is the instantaneous power due to that force (b) at the end of the interval and (c) at the end of the first half of the interval?


    2. Relevant equations



    3. The attempt at a solution

    Not really sure whats involved in this question. I need to find Power which is F*velocity but I don't know the force. Force = m*a. What I am given is the mass and speed is the velocity right? So I need to solve for a. Is this correct?
     
  2. jcsd
  3. Feb 24, 2008 #2
    Sounds good to me.
     
  4. Feb 24, 2008 #3
    So finding a via v=v0+at I get a = 2.978723404. So for a it should be m*a = 8.638297872 but this is wrong. What did I do wrong :(?
     
  5. Feb 24, 2008 #4
    a Asks you for WORK. What is the definition of work done by a constant force?
     
    Last edited: Feb 24, 2008
  6. Feb 24, 2008 #5
    Nono I meant a = acceleration. I get 8.638 N for the force.
     
  7. Feb 24, 2008 #6
    I get the following:
    a = 2.979; F = ma = 2.9*2.979 = 8.638 N

    V1 = a*t/2 = 2.979*(4.7/2) = 7.000 m/s
    V2 = 14.000 (given)

    P1 = F*V1 = 8.638*7 = 60.466 W = (c)
    P2 = 8.638*14 = 120.932 W = (b)

    But it was (a) to be in Joules. However when I multiply my F in Newtons by meters (14*4.7) the answer isn't correct :( but (b) and (c) are.
     
    Last edited: Feb 24, 2008
  8. Feb 24, 2008 #7
    W = KE = ½mV² = ½*2.9*14² = 284.2 J
     
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