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Newtons cradle

  1. Jan 22, 2011 #1
    assuming collisions are elastic...

    right 5 equal masses, if you displace the first ball then you observe balls 2-4 stationary and 5 moves off with equal speed to the right, as all the momentum and energy is passed to ball 2, ball 1 stops, ball 2 then passes all its momentum and energy to ball 3 and then stops etc...

    This conserves momentum and k.e.

    However, if balls 2 and 5 are glued together, and you displace ball 1 with velocity 'u', then the other 4 must move of withspped 4/u to conseve momentum as mu = 4m*u/4, but then kinetic energy is not conserved...?

    thankssss !
     
  2. jcsd
  3. Jan 22, 2011 #2

    Doc Al

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    Staff: Mentor

    I assume you mean that balls 2 through 5 are glued together?
    Don't assume that ball 1 stops dead.
     
  4. Jan 22, 2011 #3
    Doc Al is right
    in case I, ball 1 stops as both ball 1.2 has same mass
    but now its different case, ball 1 collides with mass 4m, so it will now recoil!!!with somewhat lesser velocity than u.
     
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