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Homework Help: Question on work and forces

  1. Jun 16, 2010 #1
    1. The problem statement, all variables and given/known data

    A technician uses a hammer to force a nail into a wooden beam.
    The hammer has a mass of 0.40kg.
    It has a speed of 5.0 m/s just before it hits the nail.
    After raising the hammer to a height of 0.75m, how much work does the technician do in hitting the nail with the hammer?


    2. Relevant equations

    I also had to work out two parts before hand -
    1. The kinetic energy just before it hits the nail - which was 5J using [KE=0.5mv^2]
    2. The GPE at a height of 0.75m - which was 3J using [GPE=mgh]

    3. The attempt at a solution
    I got this last part wrong however, I tried using the formula [W=Fs], so 0.75 * 4 = 3. And got it wrong.

    Thanks in advance
     
  2. jcsd
  3. Jun 16, 2010 #2

    ehild

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    Homework Helper

    The hammer got its kinetic energy both from the man and from the gravitational pull of Earth. The work of gravity is equal to the gravitational potential energy that you calculated as 3 J (using g=10 m/s^2) The man exerts some downward force to the hammer, so it will be faster when hitting the nail as it were if the man just dropped it to the nail.

    So: KE (of the hammer) = 3 J + W (work of the man). You need to give W.

    ehild
     
  4. Jun 16, 2010 #3
    So 5-3 = 2, Answer 2J

    Many thanks, I was confused about that
     
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