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Homework Help: Ballistic Pendulum initial velocity of bullet

  1. Mar 1, 2013 #1
    1. The problem statement, all variables and given/known data
    A ballistic pendulum is used to measure the speed of a bullet fired from a gun. The pendulum consists of a 20.0g block hanging from a string. A bullet, with mass 0.50gg, imbeds itself in the block causing the block to swing up to a height of 5.00cm. Find the speed of the bullet as it leave the gun.

    2. Relevant equations
    W_nc = ΔK + ΔU
    K = 1/2 mv^2
    U = mgh
    W = Fdcos∅
    p = mv

    3. The attempt at a solution
    I wanted someone to double check my work, and if I used correct units or not.

    U = mgh
    U = (20.0g)(9.8)(0.05cm)
    U = 9.8 Joules (this is not gravity it is what mgh came out to)

    K = 1/2 mv^2
    9.8(U of pendulum) = 1/2(0.005)v^2
    v^2 = 9.8/0.0025
    v^2 = 3930
    v = 62.6099
    v = 62.6 m/s
  2. jcsd
  3. Mar 1, 2013 #2
    The bullet EMBEDS itself in the block.
  4. Mar 1, 2013 #3


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    In case you don't understand the significance of voko's comment, you have assumed work is conserved. That is unjustified (and quite wrong here).
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