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Homework Help: Two physics KE/PE questions(and possibly more, studying for a test)

  1. Dec 15, 2009 #1
    1. The problem statement, all variables and given/known data

    a carpenter moves his .5 kg hammer at 3m/s when it contacts a nail. The nail recieves an average force of 75N as it is driven 2cm int the wood board. The carpenter notices that the nail feels warm after he hits it with the hammer. How much heat energy was transferred to the nail.

    2. Relevant equations
    W = fd


    3. The attempt at a solution

    work w =force Fx distsnce D
    w=75nx.02m
    w=1.5j

    Maybe im supposed to do Ke = .5mv^2 and get 2.25J, then 2.25- 1.5J = .75J?

    Except that was wrong?



    Next problem

    How much work is done by accelerating a 1200kg card from rest to a speed of 12m/s while it is movin up a frictionless hill that is 18m high.

    force = mass x acceleration F= 1200x12=14400n
    w=force x distance w=14400x18=259200j
    =259.2kj

    Except my friend is getting 298080J. Not sure whos right here. Thanks everyone!

    Hes doing
    W = mg(18) + .5m(12)^2
     
    Last edited: Dec 15, 2009
  2. jcsd
  3. Dec 15, 2009 #2

    diazona

    User Avatar
    Homework Helper

    When you calculate the work, you do need to take into account any change in potential energy. Try that for the first problem and see if it gets you a better result.
     
  4. Dec 15, 2009 #3
    So for the first one, I tried

    5mv^2 + mgh = W + Q
    W = fd = 75N * .02m
    = 1.5J
    .5(.5kg)(3m/s)^2 = 2.25J
    = KE
    .5kg * 9.8m/s^2 * .02m = PE = .98J
    2.25+ .98 = 1.5 + Q

    So is Q 1.73J?
     
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