1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Law of conservation of energy problem

  1. Apr 6, 2009 #1
    1. The problem statement, all variables and given/known data
    A .5 kg block is dropped from 15m. A 5 gram bullet hits the block at 233 m/s. At what height must the block be hit to completely stop the blocks freefall momentarily.


    2. Relevant equations
    p=mv
    1/2mv^2=mgh


    3. The attempt at a solution
    bullets ke
    1/2(.005)(233)^2=.5(9.8)(h)
    When the bullets kenetic energy is equal to the blocks potential it will momentarily pause in mid air. Atleast i thought.

    bullet's ke = 135.72 joules= blocks pe = 27.698m
    The block would have to be 27 meters above the ground to have as much potential energy as the bullet described.

    Then i was thinking of solving it using momentum but im not sure how that would work out.
     
  2. jcsd
  3. Apr 6, 2009 #2

    rl.bhat

    User Avatar
    Homework Helper

    Equating momentums of the block and the bullet, you can find the velocity of the block.
    Then by using kinematics you can find h.
     
  4. Apr 6, 2009 #3

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi leapoldstotch! :smile:
    Energy is not conserved in a collision unless the question tells you it is …

    in this case, the bullet will embed in the block, and have the same speed as it, so this is a perfectly inelastic collision and energy is obviously not conserved.

    You can only use conservation of momentum (for the collision itself).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Law of conservation of energy problem
Loading...