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Newton's Third Law of women standing

  1. Apr 30, 2007 #1
    1. The problem statement, all variables and given/known data
    A woman is standing on a flat section of ground. Her weight is 500N. Newton's third law states that there must be an equal an equal and opposite force to her weight, which is

    A. The earth exerting an upward force of 500 N on the woman.

    B. The woman exerting an upward force of 500 N on the earth

    C. the woman exerting a downward force of 500 N on the earth

    D. the earth exerting a downward force of 500 N on the woman


    2. Relevant equations

    Well, the fact that every force has an equal and opposite force acting on it


    3. The attempt at a solution

    C and D are obviously nonsense.

    I would automatically have gone for 'A', but the answer in the book begs to differ, saying the answer is 'B' .

    I always thought the Earth exerts an equal and opposite force on all the things sitting on it.

    So how can a woman exert an upward force of 500 N on the earth?
     
  2. jcsd
  3. Apr 30, 2007 #2

    PhanthomJay

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    It is important to remember when using Newton 3 that the ''equal and opposite'' force acts on different objects. That is, if X exerts a downward force on Y, then Y must exert an upward force on X. When you answer choice "A", both forces (the downward force of the earth on the woman (her weight), and the upward force of the earth on the woman (the Normal force), acts on the same object (the woman).
     
  4. Apr 30, 2007 #3
    Ahh, i see. Different Objects, cool.

    Thanks, man.
     
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