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Action/reaction force in relation to Newton's 3 law

  1. Aug 25, 2006 #1
    How do i differentiate b/t an action/reaction force described in Newton's Third Law acting on two different bodies, with the normal force and the weight force acting on the same body.
    Scenario 1:
    a bin rest on a table: the weight force of the bin exerts on the table due to gravity, and the normal force provided by the table acting on the bin.
    (Both arrows representing the force are shown on the bin). Thus the bin is stationary.
    Scenario 2:
    Similar to scenario 1 but this time the normal foce arrow is shown on the bin and the force bin exerts on table is shown on the table.
    And this is the action/ reaction force described by Newton. Not like the first scenario.

    Very confussing! How do i know when the force is on the same body or on different bodies. B/c both scenarios are similar.
    Any suggestions???
  2. jcsd
  3. Aug 25, 2006 #2


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    Homework Helper

    The reasoning goes somewhat like this ...

    We know the bin is in equilibrium. Therefore resultant force that it experiences is nil. The table therefore provides a normal force equal to the weight of the bin, but in the opposite direction of its weight. This normal force coming from the table is a result of the bin pushing down onto it. How hard and in which direction is the bin pushing down onto it? The table is experiencing the weight of the bin pushing down onto it. The action-reaction pair is the weight of the bin pushing down onto the table (the action force) and the reaction of the table to this force is the normal force with which the table pushes back on the bin (the same magnitude as the action force that it experiences, but in the opposite direction).

    The table is actually experiencing more forces, its own weight and the floor pushing upward on it. These forces also need to balance. Which means that the reaction force of the floor (the normal force) need to balance the weight of the table and the bin on it. So the table serves to transmit the weight of the bin to the floor, which fortunately for us keeps everything resting on it up!
    Last edited: Aug 25, 2006
  4. Aug 25, 2006 #3

    Doc Al

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    Staff: Mentor

    So-called "action/reaction" or 3rd-law pairs can be identified by expressing Newton's 3rd law like this: If body A exerts a force ("action") on body B, then body B exerts and equal and opposite force ("reaction") on body A.

    Always identify the two objects involved in any interaction. In both scenarios, the bin is one object. What other objects are involved? It depends on the force:
    The bin's weight is the gravitational force exerted by the earth on the bin; thus the reaction to that force is the gravitational force exerted by the bin on the earth.

    The normal force is the force exerted by the table on the bin; thus the reaction to that force is the force exerted by the bin on the table.​

    The normal force and the weight can never be action/reaction pairs, since they act on the same object.
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