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Homework Help: Newton's third law a book and table

  1. Oct 16, 2015 #1
    1. The problem statement, all variables and given/known data
    Newton's third law of motion is often summarised as 'Every action (force) has an equal and opposite reaction'.
    A book rests on a table. If the weight of the book is the 'action' force, what is the 'reaction' force?

    A the pull of the book on the Earth
    B the pull of the Earth on the book
    C the push of the book on the table
    D the push of the table on the book
    2. Relevant equations

    3. The attempt at a solution

    My answer to this is (D)..since two bodies which is the table and the book must act equal and opposite force. But it turns out the true answer is (A)..Could anyone explain to me why?
  2. jcsd
  3. Oct 16, 2015 #2


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    The problem is asking about Newtons 3rd law. Your answer relates to his 1st law. The 1st law is about balanced forces on the same object. The 3rd is about equal and opposite forces acting on different objects. The problem definition of the 3rd law is confusing when stated that way. Rethink your answer.
  4. Oct 16, 2015 #3
    Newton's 3rd law: The force that A exerts on B is equal and opposite (in direction) to the force that B exerts on A.

    Your problem: The force that the earth exerts on the book (i.e., its weight) is equal and opposite (in direction) to the force the book exerts on the earth.

    Substitute the words "the earth" for A and "the book" for B.

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