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Homework Help: Rifle Bullet Pendulum (Solved just need help on 1 aspect)

  1. Oct 7, 2011 #1
    1. The problem statement, all variables and given/known data
    A 36 g rifle bullet traveling 310 m/s buries itself in a 5.2 kg pendulum hanging on a 2m long string, which makes the pendulum swing upward in an arc. Determine the maximum verticle displacement of the pendulum (height)(g=9.8m/s^2)

    2. Relevant equations
    Pi = Pf
    Law of conservation

    3. The attempt at a solution
    well i solved it out and im pretty sure im right, i found the velocity to be 2.131 m/s then plugged it into the law of conservation equation where the masses cancel out and you end up with 1/2(2.131)^2 = gh

    h = .2318

    but my question is because it says the pendulum hangs on a 2m long string, how does that impact the problem, do i subtract this height from the string? or vice versa? or add it?. I mean i know a pendulum is like a string or rod or w/e with a weight attached but the way its worded is like that theres an extra 2m string or something? does that impact the MAXIMUM vertical displacement at all? i'm just a little confused, thank you if u can help!
  2. jcsd
  3. Oct 7, 2011 #2


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    Science Advisor

    No, the "height" is the "height". The only thing the "2m long string" tells you is that the block will go upward on an arc of a circle with radius 2m (otherwise the block-bullet combination would just move laterally without ever stopping!). But the height is still given by "conservation of energy" (after the collision- since the bullet is imbedded in the block, that collision is a "perfectly inelastic" collision- energy is NOT conserved).
  4. Oct 7, 2011 #3
    ah ok thank you sir!
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