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Homework Help: Physics bullet in wood; I can't figure it out?

  1. Apr 26, 2012 #1
    1. The problem statement, all variables and given/known data
    I can't figure this one out. I can't use any of newtons laws to solve this, kinetic formulas, and w=Em+Ek doesn't seem to be working for energy. I know the answer, but I can't solve it.

    kg = kilogram
    m/s = meters per second
    m = meter

    A 0.002kg bullet is traveling at 87 m/s, after passing through a 4 cm block of wood its speed is 12 m/s.

    a) How much work is lost due to friction (Answer is -7.4 J but I can't solve it mathematically)
    b) Average force of block (Answer is -180 N)

    2. Relevant equations
    a) For work lost due to friction I'm guessing they want to know the air resistance, I have no idea how to do this part

    b) Average force of block:
    wcd = (.5*12^2)-(.5*87^2) = -3 712.5 J

    P = w/t but the only problem is no time is supplied. I'm not quite sure what to do

    3. The attempt at a solution
    a) .5 * 87^2 = .5 * 12^2
  2. jcsd
  3. Apr 26, 2012 #2


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    Gold Member

    Work isn't lost, it's "done." Energy is lost. Do you know the work-energy theorem (I'm not sure if the w=Em+Ek is supposed to be some form of that)?
  4. Apr 26, 2012 #3
    a) KEoriginal must equal the workfriction + KEafter
    b) You must find the a that the block causes on the bullet first. Use v22 = v12 + 2ad to solve for a.
  5. Apr 26, 2012 #4
    Yes, W = Ek2 - Ek1 = 1/2m(v2^2-v1^2)
    1/2 * 0.002 * (87^2-12^2) = 7.4 J. I can't believe I forgot about that. Thank you.

    I can't solve for a because I don't have the displacement or such, unless it is 4 CM?

    87^2 = 12^2 + 2 * a * 0.04

    7425/0.08 = a
    a = 92 812.5

    F = 0.002 * 92 812.5 = 185.62500 N

    Thank you for the help, I really appreciate it
    Last edited: Apr 26, 2012
  6. Apr 26, 2012 #5
    Nicely done, and glad to be of help.
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