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Homework Help: Work and Kinetic Energy and elastic collision

  1. Jul 4, 2004 #1
    hey all... have a question:

    a body of 2 kg mass makes an elastic collision with another body at rest and afterwards continues to move in the original direction but with one-fourth of it's original speed. What is the mass of the struck body?

    I started with comparing kinetic energy, then took a look at momentum... but I'm just not sure how I can solve for body #2's mass when I don't have a resultant velocity or anything... please point me in the right direction.

  2. jcsd
  3. Jul 4, 2004 #2


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    But you do have a resultant velocity, as it is elastic. Therefore, speed of approach must equal the relative speed after the collision.

    V = Z - W

    Where V is the initial velocity, Z is the resulting velocity of the struck object, and W is the resulting velocity of the first object.
  4. Jul 5, 2004 #3
    Let V be final velocity and v be initial velocity of the 2 kg mass. Take the basic equation and multiply each side by v:

    V/v = [(m1-m2)/(m1 + m2)]

    We know that the ratio of V/v = 1/4. Therefore

    1/4 = (m1 - m2)/(m1 + m2)

    Cross multiply:

    m1 + m2 = 4m1 - 4m2

    0 = 3m1 - 5m2

    0 = 3 (2 kg) - 5 m2
  5. Jul 5, 2004 #4
    Sorry; I meant divide each side by v
  6. Jul 12, 2004 #5
    thanks... appreciate it more than you know
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