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Homework Help: Newton's 3rd Law confusion?

  1. Aug 13, 2012 #1
    If a car is pushing a bus (and the two both move in the direction of the car), I understand that the amount of force with which the car pushes against the bus is equal to the force the bus pushes back on the car.

    What I don't understand is why the car moves. Is it going forward with the same force that it applies on the bus (and therefore receives)?
  2. jcsd
  3. Aug 13, 2012 #2
    If I understand correctly then:
    - You are correct, the car pushes on the bus with equal and opposite force
    - The key point is that this is not the only force on the car. The car also pushes on the earth and
    the earth on the car.
    - This force must be bigger than the force of the bus on the car if the (bus+car) system are
    to keep moving forward i.e, we have some net force in the forward direction
    - Note also that the acceleration of the total (bus+car) system is now much smaller than it would for the car alone because F=ma and
    1. The net force is smaller than the force the cars engine can supply
    2. This smaller force must also move the mass of the bus as well as the car

    Hopefully this cleared things up for you a little bit :)
  4. Aug 13, 2012 #3


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    You ponder whether it is going forward [a motion] with the same force that it applies to the bus [an action] ???
  5. Aug 13, 2012 #4


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    You mention that the car pushes forward on the bus, and the bus pushes back on the car. If that was the only forces acting, then the bus will accelerate forward and the car will accelerate back - until the forces stop.
    Since the forces are contact forces, they can only continue while the car and bus are i contact.
    The instant either car or bus begins to move, contact ceases and so all acceleration ceases.

    BUT, both car and bus moved forward [in the direction the car was pushing on the bus.

    That means there must have been at least one other force acting [actually 2 more, since all forces come in action-reaction pairs].
    The car must have been pushing back on the road, so that the road pushes forward on the car. That force between car and road must be bigger than the force between car and bus, since we are told the car moves forward.
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