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

Car 1 is pushed at a velocity of 5m/s

  1. Oct 30, 2003 #1
    Car "1" is pushed at a velocity of 5m/s

    Car "1" is pushed at a velocity of 5m/s toward Car "2" (car 2 is not moving).
    When they hit, Cart 1 bounces back a .5m/s and car 2 moves at 5.5m/s.
    Is that possible?

    Why or why not?

    I think that it's not possible, but i'm not sure why. Maybe b/c energy was gained somehow for car 2 to bounce back faster, which is not possible.

    How do i prove that?
     
    Last edited by a moderator: Feb 7, 2013
  2. jcsd
  3. Oct 30, 2003 #2

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Conservation of linear momentum.

    m1v1 + m2v2 = constant

    - Warren
     
  4. Oct 30, 2003 #3
    What is the constant?
     
  5. Oct 30, 2003 #4
    The constant is total momentum. In other words, although v1 and v2 change after the collision, the above formula is equal to what it equalled using the velocities before the collision: total momentum is conserved.
     
  6. Oct 30, 2003 #5
    so would it be (5*5)+(5*-0.5)=5*5.5+0, 22.5=27.5 ???
    Is that the proof that it is not possible?
     
  7. Oct 30, 2003 #6

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Whatever it was at the start of the problem is what it will always be. That's what 'constant' means.

    - Warren
     
  8. Oct 30, 2003 #7

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not quite. Try this:

    Initially, total momentum is (assuming the cars have the same mass):

    m1v1 + m2v2 = m * 5 + m * 0 = 5m

    In the end, if the cars did what the problem said, the total momentum would be

    m1v1 + m2v2 = m * -0.5 + m * 5.5 = 5m

    which is the same answer. The total momentum is the same before and after, so the result is plausible. The actual velocities that would result in an experiment of this sort would depend upon the "elasticity" of the collision, which is something you'll probably learn about later.

    - Warren
     
  9. Oct 31, 2003 #8

    russ_watters

    User Avatar

    Staff: Mentor

    That is possible if car 1 is heavier than car 2.
     
  10. Oct 31, 2003 #9

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Actually, lol, you're right russ. Sure, this proposed collision conserves momentum, but it does not conserve energy (assuming the cars have the same mass).

    *slaps forehead*

    - Warren
     
  11. Oct 31, 2003 #10
    Momentum before must equal moment after

    5. m/s equals 5.5 m/s + - 0.5 m/s

    Nautica
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?