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Homework Help: Finding the momentum of the each balls

  1. Aug 13, 2007 #1
    1. The problem statement, all variables and given/known data
    There are two questions.

    1) A baseball, m=0.045 kg, moves in +y-direction with v = 5 m/s. a tennisball is moving in the -x direction with speed of 2m/s. Wat is the magnitude and direction of total momentum in the system?

    2) A 0.01 kg rifle bullet is fired with v = 500 m/s into a ballistic pendulum of mass 5 kg suspended from a cord 0.6 m long. Find the height of the pendulum, the initial kinetic energy of the bullet and the kinetic energy of the bullet and pendulum.

    3. The attempt at a solution

    1) I have no idea, beside finding the momentum of the each balls.

    2) I tried using p_bullet + p_pendulum = (m_bullet + m_pendulum)u, but I get u = 0,998 m/s, which i believe is incorrect.
  2. jcsd
  3. Aug 13, 2007 #2
    1. Look up the definition of the total momentum of a system (of particles).

    2. Why do you believe it is incorrect?
  4. Aug 13, 2007 #3
    1) great, will do!

    2) It seems very small. Must I use that KE_bullet = KE_bullet_and_pendulum?
  5. Aug 13, 2007 #4
    If I do what I wrote in about KE, I get that v_(pendulum and bullet) = 22,34 m/s. But when using p_1 = p_2, I get v = 0,9980 m/s. How is this?
    Last edited: Aug 13, 2007
  6. Aug 13, 2007 #5
    Think potential energy.

    EDIT: For the KE of bullet+pendulum: You have figured out the velocity of the two together. Now given the velocity and mass, find KE.
    Last edited: Aug 13, 2007
  7. Aug 13, 2007 #6
    So KE_bullet = PE_(bullet and pendulum), and from there i find the height.

    I'm sorry, but I don't get the last thing you wrote.
  8. Aug 13, 2007 #7
    Forget about the KE of the bullet...some of it is lost when it hits the block. But mechanical energy is conserved after the bullet embeds itself in the block.

    Just said in a round-about way that KE = 0.5mv2 :wink: You know the mass of the bullet+pendulum, and you have figured out its (initial) velocity. (Although, the question doesn't mention it, I'm assuming that it is initial KE that is to be found)
  9. Aug 13, 2007 #8
    Great, thanks!
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