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Simple FBD problem: person, chair, earth

  1. Feb 19, 2007 #1
    1. The problem statement, all variables and given/known data
    A person is sitting on a chair. Draw force diagrams for the person, the chair, and the earth.

    2. Relevant equations
    Identify the third law pairs too, por favor.

    3. The attempt at a solution
    attached GIF is my attempt. what forces do i draw on the earth

    thanks for the help

    Attached Files:

    • pic.GIF
      File size:
      2.5 KB
  2. jcsd
  3. Feb 20, 2007 #2
    reply :zzz:
  4. Feb 20, 2007 #3


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    Gold Member

    In order for the chair to remain on the earth, the net force on the chair (with respect to the earth) must be zero. The weight of the total system is acting downwards on the earth and the earth is exerting a equal but opposite force.
  5. Feb 20, 2007 #4


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    Homework Helper

    The earth attracts the person and the chair (albeit separately!) and vice versa according to Newton's law of Universal gravitation. Since the chair presses into the earth (due to its attraction by the earth and the weight of the person sitting on it) it responds with a force of similar magnitude acting in the opposite direction on the chair (real forces always comes in pairs).
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