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Homework Help: Internal vs external force - conservation of momentum in one dimension

  1. Nov 13, 2013 #1
    1. The problem statement, all variables and given/known data

    This problem concerns a collision experiment performed on a frictionless surface with gliders A and B, with masses m(a) and m(b) respectively.

    In a level track, glider B has a spring-loaded plunger attached to it. At time t(i), glider A moves to the right with speed v(0), and glider B is at rest. The plunger is initially compressed, and it is released when the gliders come into contact. At time t(f), glider A moves to the left with speed greater than v(0).

    Is the magnitude of the momentum of the system of the two gliders at time t(f) greater than, less than or equal to the magnitude of the momentum of the system at time t(i)? Explain.

    2. Relevant equations

    conservation of momentum : mivi = mfvf

    3. The attempt at a solution

    At first I thought that this the momentum of the system at t(f) must be greater than at t(i), because the plunger "pushes" A, adding an extra force to glider A, (this would be the impulse, so change in momentum added to the glider?). But would this force be counted as an internal force versus an external force instead, and so momentum is conserved?
  2. jcsd
  3. Nov 13, 2013 #2


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    The system comprises the two gliders which only "push" against each other, nothing else. You can't make a reactionless rocket so I'd say the total system momentum is the same.
  4. Nov 13, 2013 #3
    Thanks! That makes sense.
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