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What is Bob's maximum power output as he throws the rock?

  1. Jun 18, 2008 #1
    1. The problem statement, all variables and given/known data
    Bob can throw a 500 g rock with a speed of 30 m/s. He moves his hand forward 1.0 m while doing so.
    a. How much work does Bob do on the rock?
    b. How much force, assumed to be constant, does Bob apply to the rock?
    c. What is Bob's maximum power output as he throws the rock?


    2. Relevant equations
    vf^2= vi^2 + 2ad
    F=ma
    W= F.d

    P= F.v

    3. The attempt at a solution

    Find the acceration provided by the applied force.

    vf^2= vi^2 + 2ad
    30^2= 2a (1)
    a= 450 m/s^2

    Then find the force using newton's second law:
    F=ma
    F= 450 (0.5)
    F= 225 N

    Now calculate work using the force applied.

    W= F.d
    W= 225 *1= 225 J

    Power= F.v= 225*30= 6750 W.

    Is this the correct way to do it? my textbook doesn't have solutions to even-numbered problems and I'm not sure if my way of calculating acceleration and then using newton's second law is correct. Any help would be greatly appreciated!
     
  2. jcsd
  3. Jun 18, 2008 #2

    PhanthomJay

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    That looks just fine. You could have done it perhaps a bit quicker using work-energy methods (if you are familiar with this method), but there's always more than one way to attack a problem.
     
  4. Jun 18, 2008 #3
    We apparently covered this method but the only work-energy equation that I know is: Ki+ Ui + Wext= Kf+ Uf+ delta E (thermal). Can you please guide me on how to set it up the work-energy way because I've seen other questions in the textbooks like that but I do them different.Thanks for your help =).
     
  5. Jun 18, 2008 #4

    tiny-tim

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    Hi habibclan! :smile:

    Just put delta E = 0.

    So in this case the equation is KE = work times distance.

    Try it! :smile:
     
  6. Jun 18, 2008 #5
    But I don't know work. I still don't get it :(.
     
  7. Jun 18, 2008 #6

    PhanthomJay

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    As tiny-tim noted, delta E =0. So using your equation, 0 + 0 + Wext = 1/2mv^2 + 0 + 0. You know v=30, and m= .5, solve for Wext (the work done by Bob on the rock). Then since Wext = Fd, solve for F.
     
  8. Jun 18, 2008 #7
    Thanks a lot tiny-tim and PhantomJay! The help is much appreciated!
     
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