1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    S_Happens

    User Avatar
    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Physics bullet in wood; I can't figure it out?
Loading...