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!

Combining Conservation Laws

  1. Nov 7, 2006 #1
    Having a difficult time determining which laws and equations to use?

    A 15.0 kg block is attached to a very light horizontal spring of force constant 350 N/m and is resting on a smooth horizontal table. Suddenly it is struck by a 3.00 kg stone traveling horizontally at 8.00m/s to the right, whereupon the stone rebounds at 2.00 m/s horizontally to the left.

    Find the maximum distance in meters that the block will compress the spring after the collision.
     
  2. jcsd
  3. Nov 7, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Think of it as two problems. Take them one at a time:
    (1) The collision of stone with block--what's conserved there?
    (2) The compression of the spring--what's conserved here?
     
  4. Nov 7, 2006 #3
    Combining Conservation Laws; where'd I go wrong?

    A 15.0 kg block is attached to a very light horizontal spring of force constant 350 N/m and is resting on a smooth horizontal table. Suddenly it is struck by a 3.00 kg stone traveling horizontally at 8.00 m/s to the right, whereupon the stone rebounds at 2.00 m/s horizontally to the left.

    Don't know where I went wrong?
    Here's my setup:

    Collision:

    Change in KE(stone) = (1/2)(m)(vfsquared-visquared)
    = (1/2)(3.00kg)(-2m/s squared - 8 m/s squared)
    = -90

    After the collision:

    Change in KE(stone) = (1/2)(Force constant)xsquared
    -90 = (1/2)(350N/m)xsquared
    x = 0.72m
     
  5. Nov 7, 2006 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Please don't start a second thread on the same problem.

    Start by answering the questions I posed in my earlier response.
     
  6. Nov 7, 2006 #5
    1.) Kinetic energy
    2.) Kinetic energy of block is transferred to the spring, where it's stored as potential energy
     
  7. Nov 7, 2006 #6

    Doc Al

    User Avatar

    Staff: Mentor

    In general, KE is not conserved in a collision. But what is conserved in every collision?

    Right!
     
  8. Nov 7, 2006 #7
    1.) Momentum
     
  9. Nov 7, 2006 #8

    Doc Al

    User Avatar

    Staff: Mentor

    Right. Now use that to find the speed of the block just after the collision.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?