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Homework Help: Validation: Ballistic Pendulum

  1. Oct 26, 2005 #1
    At one time, the velocity of a rifle bullet was measured using a ballistic pendulum made up of a wooden block suspended from a string.

    The block has a mass of 20kg. A 50g rifle bullet is fired into the block, penetrating it by 10cm and causing it to swing upwards, increasing the height by 20cm.

    a) Calculate the velocity of the bullet as it enters the wooden block. Express your answer in Km/h

    Ep = 20*9.8*0.2 = 39.2 J
    Ex = 0.5mv^2
    39.2 = 0.5*20*v^2
    v = 2m/s

    m1v1 + m2v2 = (m1 + m2)*v'
    0.05*v1 = (20.05)*2
    v1 = 802m/s = 2887.2 km/h

    b) Calculate the frictional force between the bullet and the wooden block

    Wf = Ff * d
    39.2 J = Ff * 0.1
    Ff = 392 N

    I'm not sure i was allowed to use the m1v1 + m2v2 = (m1 + m2)*v' in a).
    Can anyone confirm this?
  2. jcsd
  3. Oct 26, 2005 #2
  4. Oct 26, 2005 #3

    Chi Meson

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    Science Advisor
    Homework Helper

    That is exactly what you do in part a. It is a collision; conservation of momentum governs the velocity outcomes of collisions. Work-energy theorem provides answers to the "force-distance" problems, and impulse-momentum theorem works for the "force-time" problems.
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