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

Energy Problem Help

  1. Mar 14, 2007 #1
    1. The problem statement, all variables and given/known data
    A 0.05 kg bullet traveling at 200 m/s embeds into a 1.0 kg stationary wood block suspended by two light strings. Find the change in vertical height of the block.

    2. Relevant equations
    Wnc = 0 ?
    mgho + .5mvo^2 = mghf + .5mfv^2

    Wnc != 0 ?
    .5mvf^2 + mghf - .5mvo^2 - mgho

    3. The attempt at a solution
    I really have no idea where to start. I'm not entirely sure if the work of the non conservative forces is equal to zero or not. I'm not even sure what I would put as the initial and final velocities ( 200 and 0 respectively??).

    If it's Wnc = 0, then I did this:

    .5 ( 1.05 ) ( 200^2) = (1.05) (9.8) hf
    2100 = 10.29 hf
    2100/10.29 = 204.1m = hf

    But that seems like a rather high number
    Last edited: Mar 14, 2007
  2. jcsd
  3. Mar 14, 2007 #2


    User Avatar
    Homework Helper
    Gold Member

    This problem can be solved in 3 steps....

    1. Find the kinetic energy of the bullet before it hits the block
    2. Use conservation of momentum to find the speed of the block after the collision.(Remember that mechanical energy is not conserved in an inelastic collision.)
    3. Use conservation of mechanical energy and solve for the height of the block.
  4. Mar 14, 2007 #3
    Thank you very much! I finally got the answer, 4.63m!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook