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: Center of Mass Problem

  1. Dec 28, 2012 #1
    A car of mass m moving East at speed v (v << c no need to use relativity) collides with another car of mass 2 m moving North with speed 3v.

    a) What are the x and y components of the center of mass velocity of the system just before the collision?
    b) If the cars stick together during the collision what is their common speed after the collision?
    c) Determine the ratio of the total kinetic energy after the collision to the total kinetic energy
    before the collision?

    d) Explain why kinetic energy is not conserved but momentum is conserved during the collision

    I got the first two parts but I'm not sure how to find the ratio of kinetic energy! Please help!
  2. jcsd
  3. Dec 28, 2012 #2


    User Avatar
    Science Advisor

    You are given the mass and speeds of the cars before the collision so you can easily find the kinetic energy. You say that you found the speed after the collision so you should be able to find the kinetic energy afterwards. So just divide one by the other.
  4. Dec 28, 2012 #3
    Thanks! That was easier than I expected. What about part d? Why isn't kinetic conserved? This concept never really made much sense to me.
  5. Dec 28, 2012 #4


    User Avatar
    Gold Member

    What's the definition of kinetic energy? Does it have to do with motion? What happens if the motion stops?
  6. Dec 28, 2012 #5
    Oh! It's because velocity changes, right?
  7. Dec 28, 2012 #6


    User Avatar
    Science Advisor
    Homework Helper

    Velocity changes in an elastic collision too but kinetic energy isn't lost. What happens when two cars collide besides a change in velocity?
  8. Dec 28, 2012 #7
    Their mass becomes one?
  9. Dec 28, 2012 #8


    User Avatar
    Science Advisor
    Homework Helper

    No, I'm talking more about loud noises and fenders bending. Could that have anything to do with energy? You've already shown kinetic energy isn't conserved, I hope. The question is where could the missing energy have gone.
    Last edited: Dec 28, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook