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Homework Help: Ballistic pendelum problem

  1. Apr 19, 2008 #1
    [SOLVED] Ballistic pendelum problem

    1. The problem statement, all variables and given/known data
    16 g rifle bullet traveling 180 m/s buries itself in a 4.0 kg pendulum hanging on a 3.5 m long string, which makes the pendulum swing upward in an arc. Determine the horizontal component of the pendulum's maximum displacement

    2. Relevant equations
    arc length s= rtheta

    3. The attempt at a solution

    i dont think the block and bullet make a angle with a vertical
    so work= F*s where S= Rtheta, but i cant find theta,
  2. jcsd
  3. Apr 19, 2008 #2


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    Hi Oomair! :smile:

    Energy isn't conserved in the collision (you know that because the question tells you that the bullet "buries itself" in the pendulum).

    So use conservation of momentum to find the initial speed of the pendulum.

    Then find its maximum height by … ? :smile:
  4. Apr 19, 2008 #3
    ok i found the max height which came out to be .0262 meters, but its wants the horizontal displacement, i dont know how to relate the height to the horizontal displacement,

    since its displacement is like an arc, so S= rtheta, so this is where im stuck, i have tried freebody diagrams and even used integration,
  5. Apr 19, 2008 #4


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    Hi Oomair! :smile:

    Yes, .0262m looks right.

    ok, the rest is just geometry.

    Horizontal displacement just means the sideways component of distance.

    So draw a triangle. The hypotenuse is 3.5. One side is 3.5 minus .0262. So the third side is … ? :smile:
  6. Apr 19, 2008 #5
    thanks, that worked, i didnt know that the hypotenuse could be the lenght of the string, what a wierd problem lol
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