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Ballistic Pedulum - Finding Kinetic Energy Lost

  1. Nov 6, 2003 #1
    Here's a problem that I've been having trouble on for awhile and seem to be stuck. I was just wondering if someone could guide me or show me how to do this problem.

    The Ballistic pendulum is a device usd to measure the speed of a fast-moving projectile such as a bullet. The bullet is fired into a large block of wood suspended from some light wires. The bullet is stopped by the block, and the entire system swings through the vertical distance, h. The mass of the bullet (m1=0.068kg), the mass of the pendulum (m2=0.256kg), h=6.2cm.

    Vo=(.324kg/.068kg) x (2 x 9.8 m/s^2 x .062m)^1/2 = 5.25m/s

    KEinitial= 1/2(.068kg x 5.25m/s)^2 = 0.937J

    KEfinal= 1/2(.324kg) x (2 x 9.8 m/s^2 x .062m) = .197J

    I got that far. I was wondering how do I find the kinetic energy lost from the info?
  2. jcsd
  3. Nov 6, 2003 #2
    The kinetic energy lost should be equal to the change in kinectic energy.
  4. Nov 6, 2003 #3


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    The kinetic energy of the bullet is transfered to a POTENTIAL energy change of the block. So the total energy of the system can be found in MgH. M is the mass of the block, g the acceleration due to gravity and H the height change of the block.

    So you have

    KE = mv2/2 = MgH

    m is the mass of the bullet.
  5. Nov 6, 2003 #4


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    This thread should probably be moved to the homework section.

    You need to split this problem into two parts:

    The collision between the bullet and the block, and the movement of the pendulum. You'll have to assume that the colision is instananeous.

    The collision of the bullet with the pendulum is perfectly inelastic since they're stuck together afterwards.

    If you use Integrals' approach you'll get a bullet that moves a bit slow.
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