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Homework Help: SAT Question concerning Newton's Third Law

  1. Aug 9, 2007 #1
    I didn't understand the explanation, and think perhaps the book is wrong.

    A person is standing on a horizontal floor fells two forces: the downward pull of gravity and the upward supporting force from the floor. These two forces are:

    I put A) have equal magnitudes and form an action/reaction pair.

    according to the book, they B) have equal mags and do not form an action/reaction pair.

    The book explains that b/c the two forces are felt by the same object (the person), it cannot be an action/reaction pair, but doesn't the push of the person lead to the reaction of the push on the person from the earth?
  2. jcsd
  3. Aug 9, 2007 #2


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    Yes, but action-reaction pairs, per newton 3, refer to forces acting on different objects. What you have described is the equilibrium of forces acting on the same object, which is newton 1. The upward force of the floor on the person is not the same as the person's weight in the general case of accelerating bodies (that's newton 2), you know, like if you're in an elevator accelerating up, the floor force on you is greater than than your weight, per newton 2, but the force of you on the floor is always the same in magnitude as the force of the floor on you, per Newton 3.
    Last edited: Aug 9, 2007
  4. Aug 9, 2007 #3

    Chi Meson

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    There is an unfortunate double-use of the term "reaction force." As the gravitational force on the person pulls him down, the floor "gets in the way." As the two surfaces get close to each other, they begin to repel with a mutual electostatic force. The surface of the person's feet repels the surface of the floor and vice versa. These two forces are action/reaction pairs.

    Whenever you find a reaction pair, all you need to do is switch the places of the nouns.
    action: person pulled by earth
    reaction: earth pulled by person
    action: foot repels floor
    reaction: floor repels foot
    Last edited: Aug 9, 2007
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