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Newton's first vs third law

  1. Jul 30, 2011 #1
    Hi there,

    Assume that I push a box with a force of say 50 Newtons and the box moves forward. At the same time, the box pushes me back with the same amount of force. One can argue that hence, the net force here is zero and the box shouldn't move, right?

    Compare it to the situation where the box lays on the floor. The reason the box doesn't bounce up is that the net force (in the vertical axis) is zero, i.e. the force imposed by the floor to the box is countered by the force imposed by the box to the floor.

    Thanks for the help
  2. jcsd
  3. Jul 30, 2011 #2
    when calculating acceleration of the particle we should consider the force applied on the particle only not the force applied by the particle. The force applied by the particle will alter the motion of the object on which the force is applied(in above situation it alters your motion)

    when you are pushing the box the *net force on the box is not zero* as the only force applied on the box is the 50 N applied by you. Therefore it moves as it is not in equiliberium.

    box also applies a force of 50 N on you but not on itself so the force applied by box will move you not the box

    NOW considering the second situation in which the box is on the floor : there are two forces aplied on the box : 1) gravity (downwards)
    2) NORMAL (upwards)

    gravity forces equals Normal force so *net force on the box is zero* so it remains at rest
  4. Jul 30, 2011 #3


    Staff: Mentor

    Hi kevsven, welcome to PF!
    One could argue that but one would be wrong. The net force is the sum of the forces on a single object, therefore you cannot add the force on you with the force on the box to get a net force.

    Again, those are forces on different objects so they have nothing to do with net force.

    The reason that the box is stationary is because the gravitational force on the box is equal to mg pointing down and the contact force on the box is equal to mg pointing up. These two forces are both acting on the box, and the sum of those two forces acting on the box is 0. It has nothing to do with the contact force acting on the floor.
  5. Aug 2, 2011 #4
    when you are concerned about the acceleration of block then you need to concern on all the forces applied on block externally. I think you should not think about the force that block applies back to origin force. Because a body can't apply force on itself.

    Most of all (100 bat ki ek bat)
    draw it's free body you will get the point.
  6. Aug 3, 2011 #5
    Friction has a lot to do with this kind of scenario. 2 ice skaters with same mass pushing against each other with same steady non impulsive force will both move away because of minimal friction. One can imagine the same experiment between astronauts in free fall.
  7. Aug 4, 2011 #6
    newton third laws work on the different things.ex : when you kick the ball, the ball give you the same force as you kick the ball, see ,,, two differents things !! ball and your foot.
    while in your example you say that that laws work on the same stuff, other wrong example is the normal force vs weight force that work on a single thing.
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