Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook