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A book weighing 5 N rests on top of a table?

  1. Sep 14, 2007 #1
    This just crossed my mind. The downward force of magnitude from gravity is longer than the upward force of magnitude from the table. Which explains why the downward force of magnitude, the 5 N, comes from gravity. Am I right?
  2. jcsd
  3. Sep 14, 2007 #2
    I don't think that is true, because that would imply that the book is not in static equilibrium and it would fall through the table. The upward is due to the normal force, which is a result of molecular interactions. Think about it, newtons second law is F=ma the book is not accelerating, and has a velocity of zero, so there is no net forces acting on the object therefore all forces are equal.
  4. Sep 14, 2007 #3
    So if there's a downward force of magnitude (5 N) exerting on the book, then what is the cause of it? Ok, if it is gravity, then the table would just collapse. Is the downward force from the table pushing back up on the gravitational force?
  5. Sep 14, 2007 #4
    It works like this:

    You have a book resting on a table. It's not falling through it or anything weird like that. It's just sitting there. According to Newton, there must be no net force acting on this book, otherwise it would be accelerating according to F=ma - in other words, it would be moving somewhere.

    So since the book is stationary, we know there's no net force acting on the book. But that doesn't mean there aren't any forces acting on it at all. We know that gravity is acting on the book. For your book, it's applying a constant 5N of force in the downward direction. But since the book isn't moving, there must be some force balancing the force of gravity. This is known as the normal force, and it's applied by the table in the upward direction, counteracting exactly the force of gravity. Thus the net force on the book is zero and it doesn't move anywhere.

    The table doesn't collapse because it's strong enough to withstand the force of gravity on the book. Put a heavy enough mass on it, though - say a car or a really fat person :tongue: - and the table will indeed collapse. And there's no 'downward' force from the table on the book, only the upward normal force.
  6. Sep 14, 2007 #5
    LOL, ok, I understand. Thank you. :)
  7. Oct 5, 2007 #6
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