Notices
Results 1 to 3 of 3

Thread: Conservation of momentum in perfectly elastic collisions

  1. #1 Conservation of momentum in perfectly elastic collisions 
    New Member
    Join Date
    Jun 2012
    Posts
    1
    In perfectly elastic collisions, we know that when an object with mass m1 and speed u collides with another object with mass m2 which is stationary(where m1 is much larger than m2), m1 will continue moving at u while m2 will move with a speed of 2u.
    We can see that the total momentum before collision in the system is m1*u+ m2(0)=m1*u
    However, the total momentum after collision is m1*u + m2(2u)
    They are totally inequivalent and there's no external participated in the collision!!
    How does it obey the conservation of momentum???


    Reply With Quote  
     

  2.  
     

  3. #2  
    pmb
    pmb is offline
    Suspended
    Join Date
    Dec 2006
    Posts
    482
    Quote Originally Posted by litterboy View Post
    In perfectly elastic collisions, we know that when an object with mass m1 and speed u collides with another object with mass m2 which is stationary(where m1 is much larger than m2), m1 will continue moving at u while m2 will move with a speed of 2u.
    In general, that's not true. The final speed depends on the masses of both particles. If the particles have about the same mass then both will move at speeds less than u. If m1 >> m2 then what you said is approximately correct.

    Note: By definition, a collision is elastic if kinetic energy is conserved.

    If you use conservation of momentum and kinetic energy then you can find the speeds of the particles after the collision.


    Reply With Quote  
     

  4. #3  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,795
    Quote Originally Posted by litterboy View Post
    In perfectly elastic collisions, we know that when an object with mass m1 and speed u collides with another object with mass m2 which is stationary(where m1 is much larger than m2), m1 will continue moving at u while m2 will move with a speed of 2u.
    We can see that the total momentum before collision in the system is m1*u+ m2(0)=m1*u
    However, the total momentum after collision is m1*u + m2(2u)
    They are totally inequivalent and there's no external participated in the collision!!
    How does it obey the conservation of momentum???
    If m1 is much larger than m2, then m2(2u) is negligible compared to m1u. Call it zero, and momentum is conserved. Remember, this is just an approximation.
    Reply With Quote  
     

Similar Threads

  1. Replies: 3
    Last Post: August 4th, 2010, 12:45 PM
  2. Replies: 7
    Last Post: October 8th, 2009, 12:24 AM
  3. Elastic and Inelastic Collisions
    By Demen Tolden in forum Physics
    Replies: 11
    Last Post: October 30th, 2008, 03:52 PM
  4. Perfectly elastic relativistic collision
    By Photon in forum Physics
    Replies: 4
    Last Post: August 4th, 2006, 09:49 PM
  5. Conservation of Momentum
    By weatherkid11 in forum Physics
    Replies: 6
    Last Post: November 8th, 2005, 04:24 PM
Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •