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Homework Help: I need help with this type of problem.

  1. Nov 27, 2007 #1
    1. The problem statement, all variables and given/known data
    A bullet of mass 10.0 g is fired into a ballistic pendulum of mass 3.00 kg. The pendulum swings back and rises 12.0 cm. Find the original speed of the bullet.

    2. Relevant equations
    p1 + p2 = p1' + p2'
    We're learning momentum right now, and I think that's what I need.

    3. The attempt at a solution
    I'm too confused to even write much down. I've only thought about it. If the string's length isn't given, how can I solve this? If a string was 50000 meters long and someone hit it, wouldn't it swing less than a string 10 meters long? It would travel a different distance along the circumference with differing string lengths, right?I wrote this down and can't go any further: .5 * .01 * v^2 = 9.8 * .12 * 3.01(wrong answer for v) The only way I could write anything down was using energy, but that's not what this chapter is about :(. I don't even mind if you don't solve it all the way; a step in the right direction would be appreciated.

    I should have posted this in introductory physics. Sorry, I was in a rush.
    Last edited: Nov 27, 2007
  2. jcsd
  3. Nov 27, 2007 #2
    I figured it out. SOLVED Thanks for the... forum space? lol

    I'll explain how I did it just in case someone finds this post in search of the answer i sought.

    (m1 + m2) * g * h = 1/2 * (m1 + m2) * v^2

    the potential energy gained by the masses together(since they stuck) equals the kinetic energy of both masses RIGHT when the bullet struck.

    Momentum in this system is conserved so p1 + p2 = p1' + p2'

    m1 * v + m2 * 0 = (m1 + m2) * (the v we found up there)

    solve for v
    Last edited: Nov 27, 2007
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