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Homework Help: Finding the total momentum of system

  1. Dec 6, 2017 #1
    1. The problem statement, all variables and given/known data

    A car of mass 1500 kg is at rest on a platform of mass 3000 kg, which is also at rest. The platform has frictionless wheels attached to its bottom. The whole system is initially at rest. The car starts to move forward at a speed of 5.0 m/s with respect to the ground. (Consider all measurements with respect to the ground).

    What is the momentum of the car as it starts to move?

    What is the momentum of the car-platform system as the car starts to move?

    2. Relevant equations

    P = mv
    Ptotal = m1v1 + m2v2

    3. The attempt at a solution

    I was able to solve the first question using the equation P = mv, getting a momentum of 7500 Ns for the car. I don't know how to solve for the momentum of the platform-car system, since the speed of the platform with respect to the ground is not given.
  2. jcsd
  3. Dec 6, 2017 #2


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    What forces act on the car-platform system?
  4. Dec 6, 2017 #3
    There's an thing called momentum conservation. The car+platform system started out from rest position. And 1 more hint, P=mv is actually a directional entity, i.e. vector.
  5. Dec 7, 2017 #4
    Force of gravity and normal force which cancel out. Is there an applied force from the car, or are there no unbalanced forces on the platform?
  6. Dec 7, 2017 #5


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    Newton's second law: If the car accelerated, it was subject to a net force.
    Newton's third law: If the car was under a force from something, that other thing was under a force from the car.

    The car's tires are in contact with that "other thing".
  7. Dec 7, 2017 #6
    Since the system is isolated and initially at rest, would it mean that, according to Conservation of Momentum, that the momentum of the system after the car starts to move is 0?
  8. Dec 7, 2017 #7


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    Yes. That's why it said "frictionless wheels". So that no momentum could be transferred to the ground/Earth. If the wheels were not frictionless, then the momentum of the car-platform system would not be conserved.
  9. Dec 7, 2017 #8
    And to find the velocity of the platform when the car started to move, would the equation Ptotal = m1v1 + m2v2 be used (with Ptotal being 0)?
  10. Dec 7, 2017 #9


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