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Homework Help: Making sense of Newton's Third Law

  1. Sep 27, 2012 #1
    This is going to be a stupid question, for which I apologize. I am probably over-thinking things but my reasoning is flawed somewhere because when I think about Newton's Third Law and it's application to real-world examples, I can't understand how objects move at all, whether accelerating or not.

    For example, me pushing a book on a table. I push the book with some force, and the book pushes back on me with a force of equal magnitude and opposite direction. So what is getting the book to move if the magnitude of my force on the book isn't greater than the magnitude of the force of the book back on me? I am missing something here, obviously.

    An even stupider example that I can't figure out: I drop a ball off a building. The gravitational force from the earth pulls the ball towards it, and the ball pulls the earth towards it with a force of equal magnitude and opposite direction... yet it is the ball is moving.

    Or a hockey stick moving a puck, the stick applies force to the puck, puck applies force of same magnitude in opposite direction, yet the puck moves.

    Please explain (even if it's so elementary it's ridiculous) what I am missing in my thinking.
    Last edited: Sep 27, 2012
  2. jcsd
  3. Sep 27, 2012 #2
    The book requires a force so that it will move.
    So you have to give this amount of force so that it will move.
    How big is the force?

    How you know the amount the force needed?

    So the book "tells" you it needed certain amount of force so that it can accelerate.
    You then will "give" exactly that amount.

    The word "tells" means the book exert force to you.
    In response, you push with equal force.
    Last edited: Sep 27, 2012
  4. Sep 27, 2012 #3
    Assume that I have met the necessary force requirements to make the book move. I am still having trouble making things clear. No matter the force that I apply to the book, whether it be 1 N or 10000000 N, the book will also apply that same force to my hand, in the opposite direction. So what gets the book to move if the force applied by hand on the book is the same magnitude as the force applied by the book on my hand? It seems like they should cancel each other out if they are acting in opposite directions, but obviously the book is moving, which where I'm stuck.
  5. Sep 27, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Note that the two forces act on different objects, thus they don't cancel.
  6. Sep 27, 2012 #5
    That's what I was missing. Thanks.
  7. Sep 27, 2012 #6
    Your question is why the book moved?

    Another senario.
    Put the where the one end is touching a wall.
    Now you push the book against the the wall.
    Will the book moves?
    Why it doesn't move?

    As you said above the book will move because equal forces.

    In physics we have to learn about object not moving(static), constant velocity and acceleration.

    Another example is Artwood's Machine.

    M is heavier than m.
    M will accelerate down. M will pull m and makes it accelerate upward. m too will pull M too. What is the consequence?

    M will pull m. What is the consequence?
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