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Newton's 3rd Law Pairs

  1. Jun 11, 2015 #1
    1. The problem statement, all variables and given/known data
    Forces involved when a book is resting on top of a table.

    2. Relevant equations
    F = ma

    3. The attempt at a solution
    This is not really a homework question but it is from school. I found a previous thread (https://www.physicsforums.com/threads/misinterpretation-of-newtons-third-law.526181/). After reading this a few times it seems to make perfect sense to me. The weight force happens to be the same as the normal force the book exerts on the table but they are not an interaction pair. However, I was reading through one of my old GCSE revision guides for physics. I'm British and GCSE are the main exams you sit before going to college (ages 15-16). If I have understood the attached post then this revision guide is completely incorrect. I have attached the page from the book which is titled "Newton's 3rd Law" and it says in capital letters at the bottom in a bright red box that the reaction force is equal and opposite to the weight! This is a mainstream revision guide available from amazon. Have a missed something or is this completely wrong? It also says before the statement, learn this very important FACT.

    Attached Files:

  2. jcsd
  3. Jun 11, 2015 #2
    The book exerts a force on the table (equal to the weight of the book) and the table exerts an equal and opposite force on the book. The earth also exerts a force on the book (equal to the weight of the book) and the book exerts an equal and opposite force on the earth.

    The book has weight due to the earth's gravitational pull, but I don't think that is the intended interaction in the diagram.
  4. Jun 11, 2015 #3
    But it says in the red box that "the total reaction force is equal and opposite to the weight" and it is in a section titled "Newton's 3rd Law"?
  5. Jun 11, 2015 #4
    Yes. On a horizontal surface the 'reaction' force of the table on the book to the 'action' force of the book pushing on the table will be equal to its weight. I see why you take issue with the phrasing. How would you prefer this fact phrased instead?
  6. Jun 11, 2015 #5
    Perhaps one thing to notice is that not all action-reaction pairs are shown. There should be an orange arrow pointing upward on earth's center of mass (the reaction force to the book-earth interaction) and a purple arrow pointing down (the 'reaction' force, if you will, of the book pushing on the table). Of course, that might make the drawing a bit confusing.
  7. Jun 11, 2015 #6
    I think I'm fine with the "equal and opposite" force because it is equal and opposite to the weight it is just not an interaction pair. So if this distinction is made clear (i.e. that it is not an interaction pair) then there is no problem. However, this is in a section all about Newton's 3rd Law and this seems totally wrong to me. Regardless of whether the phrasing is correct or incorrect it is definitely not a Newton's 3rd Law pair but they are saying it is! Why else would they put it in a section about Newton's 3rd Law? If it was in a section labelled Newton's 2nd Law then I would understand.
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