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Homework Help: Ballistic pendulum change in kinetic energy problem

  1. Dec 16, 2009 #1
    1. The problem statement, all variables and given/known data

    a bullet of mass M is fired into a block of wood of mass m, the bullet sticks in the wood and causes it to swing upwards by height h

    calculate the change in kinetic energy from the bullet to the bullet+wood

    2. Relevant equations

    kinetic energy = 1/2MV^2
    V = SQRT (2gh)
    Inital speed = m+M / m Sqrt(2gh)

    so initial kinetic energy is 1/2 M m+M / m Sqrt(2gh)
    final kinetic energy is 1/2 M + m SQRT (2GH)

    3. The attempt at a solution

    i know i need to devide final EK by initial EK but i am not sure if i have all the equations correct as when i try and do this i cant get to an answer

    can anyone help>
  2. jcsd
  3. Dec 16, 2009 #2


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    Homework Helper

    Since mass of the bullet is M, your expression for initial velocity is wrong. Check it.
    They expect to find (initial KE - final KE)
  4. Dec 16, 2009 #3
    i have had another look and im not sure if i should cancel the m's as they dont apply yet this would give me

    Ini Vel = M SQRT(2gh)

    is this better ?
  5. Dec 16, 2009 #4


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    Vi = (m+M)/M*sqrt(2gh)
  6. Dec 16, 2009 #5
    ok then so where am i going wrong ? i have been thru my text book and asked my teacher but i just cant get it right, i think the problem is with the formula for inital KE but however i go thru it i get the same result,

    this needs to be in tomorrow so i am having a panic now

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