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

Homework Help: Cart Collision Problem

  1. Nov 25, 2006 #1
    A 2.4 kg dynamics cart with a linear elastic spring attached to its front end is moving at 1.5 m/s [W] when it collides head on with a stationary 3.6 kg cart.

    What is the velocity of each cart at minimum separation?

    The answer is 0.6 m/s [W]

    Can anyone show me the steps?
  2. jcsd
  3. Nov 26, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    There is a very easy way to do this. Since there are no external forces, the total momentum the system does not change. What is the speed of the centre of mass? (What is the speed of the observer who sees the two cars colliding with equal and opposite momenta?)

    What is the relationship between the speed of the centre of mass and the speeds relative to the centre of mass of the two cars at minimum separation?

  4. Nov 26, 2006 #3
    Oh I got it, thanks!

    This is what I did..


    m2v2 becomes zero since its stationary




    v'= -0.6 m/s ---> 0.6m/s[W]

    I didn't know it was completely elastic collision, that's why I was stuck.
  5. Nov 26, 2006 #4
    I'm stuck on part c now...

    Calculus the change in total kinetic energy of the system at minimum separation. Here's my work


    1/2m1v1^2 + 1/2m2v2^2 = 1/2m1v1'^2+1/2m2v2'^2

    cross out all the 1/2s and also v2 becomes zero

    m1v1^2 = m1v1'^2 + m2v2'^2

    (2.4)(-1.5)^2 = (2.4)(0.6)^2 + (3.6)(0.6)^2

    5.4 = 0.864 + 1.296

    5.4 = 2.16

    5.4-2.16 = 3.24 J

    And this is where i'm stuck...

    The answer at the back is -1.6 J
  6. Nov 26, 2006 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Don't write this as an equation (since they are not equal!) and don't cross anything out. You have the speeds and the masses, so just calculate the initial and final KEs by plugging into the KE formula. Then compare KEi to KEf. (By "final" I mean at the point of minimum separation.)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook