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Homework Help: Conservation of momentum in an inelastic collision with friction

  1. Feb 5, 2012 #1
    1. The problem statement, all variables and given/known data
    a ball falls in a truck loaded with sand with velocity v having horizontal and vertical component. The angle under which it enters the truck is α, the mass of the ball is m of the track loaded with sand is M. What happens with the momentum after the collision?

    3. The attempt at a solution
    This should be considered as completely inelastic collision.
    In vertical direction the momentum before the collision is the momentum of the ball mcosv.
    After the collision the track is not moving in horizontal direction, but the momentum should be conserved. The ball sinks in the sand, so is this the explanation?
    In horizontal direction the momentum before the collision is msinv. What is the momentum after? There is friction during the collision. It should be (m+M)v', but what about the friction?
  2. jcsd
  3. Feb 5, 2012 #2


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    What happens to the temperature of the sand once the ball makes contact?
  4. Feb 5, 2012 #3
    It does not effect the momentum.
  5. Feb 5, 2012 #4


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    really? what does temperature tell you about the speed of particles in a substance?
  6. Feb 5, 2012 #5


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    If there is no external horizontal force on the system (the truck, sand, and the ball), then momentum is conserved in the horizontal direction. So it really depends on whether you are allowed to neglect friction, which would be a horizontal force if it is present.

    In introductory physics, if there is no mention of friction then it is usually neglected. So it looks like conservation of momentum (in the horizontal direction) applies here. And you know that momentum before the collision is m·sinα·v

    p.s. Don't worry about the temperature, that is unnecessary for solving this problem.
  7. Feb 5, 2012 #6
    Actually I am university student.
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