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Homework Help: Very heavy particle collides very light particle

  1. Apr 3, 2013 #1
    A heavy particle of mass m1, moving with speed u, makes a head-on collision with a light particle of mass m2, which is initially at rest. The collision is perfectly elastic, and m2 is very much less than m1, Describe the motion of the particles after the collision.

    Heavy particle's speed is practically unchanged; light particle moves with speed 2u, in same direction as the incident heavy particle.

    My question:
    Why must the speed of the light particle be 2u?

    Lets say I take a 10kg particle colliding with a 0.1kg particle.

    using Principle of Conservation of Momentum,
    10u = 10Va + 0.1Vb

    ....then I take Va to be very small.

    No matter how I see, I still cannot see why it's 2u :grumpy:
  2. jcsd
  3. Apr 3, 2013 #2


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    Why do you take the velocity of the big particle after the collision very small???

    Find the expressions for the velocities after the collision, using also conservation of energy and take the limit m2/m1 --->0.

  4. Apr 4, 2013 #3
    Because "heavy particle's speed is practically unchanged".

    I substituted V1 and V2 into the energy equation and end up in a long-winded equation without being able to get the answer.

    Attached Files:

  5. Apr 4, 2013 #4


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    If it is unchanged and remains u, how does it become very small?
    You have two equations with two unknowns - aren't you able to solve? Express V1 in terms of V2 and substitute into the energy equation. Solve for V2.

  6. Apr 4, 2013 #5
    I got it, thank you.
  7. Oct 26, 2015 #6
    Hi ehild and coconut62,

    I work on this since yesterday, yet i can not solve this question.
    Would you mind showing me how to solve it?

  8. Oct 28, 2015 #7
    The procedure for this type of collision is to write the equations for conservation of energy and momentum
    M u = M v + m V
    M u^2 = M v^2 + m V^2
    Now you can write
    M (u - v) = m V
    M (u^2 - v^2) = m V^2
    Now you can divide equations and continue from there.
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