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Homework Help: Conservation of Momentum, Energy

  1. Jun 14, 2010 #1
    1. The problem statement, all variables and given/known data

    A bullet with mass 20 grams and velocity 100 m/s collides with a wooden block of mass 2 kg. The wooden block is initially at rest, and is connected to a spring with k = 800 N/m. The other end of the spring is attached to an immovable wall. What is the maximum compression of the spring?

    2. Relevant equations

    Spring potential energy = .5kx^2

    3. The attempt at a solution

    I tried doing M(bullet)V(initial) = [M(bullet) + M(block)][Vfinal], and then I used that final velocity in the energy equation .5mv^2 = .5kx^2 to find x. I keep getting the answer to be .49 meters, but this isn't correct.

    What am I doing wrong?
  2. jcsd
  3. Jun 14, 2010 #2


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    Science Advisor

    I think you have the concepts correct and are just doing the numbers wrong. With these inputs, 0.5mv^2~=1J = 0.5 kx^2, so x^2 ~= 1/400 m^2. How are you getting 0.49m?
  4. Jun 14, 2010 #3
    Whoops, I mean I keep getting .049 meters.

    But still, doing what you said isn't giving me the right answer!
  5. Jun 14, 2010 #4
    Okay, I got it, never mind!

    I was doing it correctly, I just didn't put in an exact enough answer!

    It ended up being .0497!
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