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Homework Help: (Conservation of Linear Momentum) Find u1 speed

  1. Oct 16, 2011 #1
    1. A particle of mass m1 = 2 kg moving at speed u1 makes a one-dimensional completely inelastic collision with a particle of mass m2 = 3 kg, intially at rest.

    If 60 J of kinetic energy are lost, find u1?

    2. Conservation of linear momentum:
    m1u1 + m2u2 = m1v1 + m2v2

    3. Using the conservation of linear momentun formula...
    2u1 + 3(0) = (m1 + m2)V
    2u1 = 5V

    I'm just not sure where the 60 J of ke lost fits in. The loss in kinetic energy of 60 J means
    Kfinal - Kinitial

    which is
    [.5(m1 + m2)v^2] - [((.5)m1u^2) + ((.5)m2(0)]

    But there still remains the unknown variable of u, meaning I went in a circle.
    Last edited: Oct 16, 2011
  2. jcsd
  3. Oct 16, 2011 #2
    You have two equations and two unknowns. There are many ways to solve this. One is way is substituting your answer for v in terms of u1 from the momentum equation into your energy equation. Then solve for u1.
  4. Oct 16, 2011 #3
    Thanks! Once again, i'm left feeling dumb after such a simple solution.
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