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: Coefficient of Restitution? PLEASE HELP

  1. Oct 21, 2007 #1
    Coefficient of Restitution? PLEASE HELP!!!

    1. The problem statement, all variables and given/known data

    Two balls of equal mass moving with the speed of 3 m/s, collide head on. Find the speed of each after impact if the coefficient of restitution is 1/3.

    2. Relevant equations

    e=(v2-v1)/(u1-u2)
    v2= velocity after impact of object 2
    v1= velocity after impact of object 1
    u2= velocity before impact of object 2
    u1= velocity before impact of object 1

    3. The attempt at a solution

    I put the numbers into the equation and it yield 0 over something.
    The answer is 1 m/s for both ball, but with that into the equation, it would yield 0/0.
    My book doesn't say anything on how to do this problem. Maybe it is as simple as multiplying the velocities by 1/3 to get the answer.
    Could someone please help me out?
     
    Last edited: Oct 21, 2007
  2. jcsd
  3. Oct 21, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    direction counts!

    Since the balls approach each other, one has a velocity of +3 m/s while the other has a velocity of -3 m/s.
     
  4. Oct 21, 2007 #3
    yes that is true, but i just copied the problem exactly from the book.
     
  5. Oct 21, 2007 #4

    Doc Al

    User Avatar

    Staff: Mentor

    There's nothing wrong with the problem statement, just with your attempted solution.
    That just means you input the wrong numbers. Realize that u and v are velocities, not just speeds.
     
  6. Aug 8, 2008 #5
    Re: Coefficient of Restitution? PLEASE HELP!!!

    Considering
    u1 = 3 m/s
    u2 = -3 m/s
    e = 1/3
    e = (v2-v1)/(u1-u2)
    → v2-v1 = 2 m/s
    u1+u2=v1+v2
    ...(conservation of linear momentum)
    → v1+v2 = 0
    → v1 = -v2
    Therefore, the speed of each ball after impact is 1 m/s.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook