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Homework Help: Elastic collision problem, I need help to find the mistake

  1. Sep 12, 2015 #1
    1. The problem statement, all variables and given/known data

    Two objects with equal masses undergo an elastic collision in 2 dimensions. Object 1 has an initial velocity u1 and object 2 is initially at rest. After the collision, object 1 is observed to have a velocity equal to 3/5 of it's initial velocity. What is the velocity of object 2 after the collision, v2, in units of u1?

    3. The attempt at a solution


    m1 = m2 = m

    initial speeds,
    u1 = u1

    u2 = 0

    final speeds,

    v1 = (3/5)*u1

    v2 = ?

    Apply conservation of momentum

    m1*u1 + m2*u2 = m1*v1 + m2*v2

    m*u1 + 0 = m*(3/5)*u1 + m*v2

    u1 = (3/5)*u1 + v2

    v2 = u1*(1- 3/5)

    = (2/5)*u1 or 0.4*u1 <<<<<<<-----Answer (unfortunately it's wrong but I don't know why!)
  2. jcsd
  3. Sep 13, 2015 #2


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    Homework Helper

    It is said that the collision is elastic and is two-dimensional- It should mean that the velocity of m1 is not parallel with the initial velocity. Check the original text if the final speed is 3/5 of the initial speed?
  4. Sep 13, 2015 #3
    yes it's 3/5 of the initial speed
  5. Sep 13, 2015 #4


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    Homework Helper

    Assume that both objects have both x and y components and use energy conservation, too.
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