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← 05ps-12 Newton's Third Law

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Showing Revision 1 created 10/27/2012 by Amara Bot.

  1. Before we jump into the next question, let's talk about Newton's Third Law.
  2. The easiest way to remember this one is that forces occur in equal and opposite pairs.
  3. Now, that's easy to say but a little harder to understand what it means.
  4. The easiest way to get a feel for what this means is to go push on a wall.
  5. And I mean it. Really push on that wall. What do you feel?
  6. Well as was you exert a force to the left on the wall.
  7. I'll call that the force of you on the wall.
  8. You can feel in your hands that force pushing back. Call that the force of the wall on you.
  9. And the amazing thing is every single force in physics occurs in pairs like this.
  10. Whenever there's a force of A on B, there's an equal but opposite direction force of B on A.
  11. Now, one of the really amazing implications of this has to do with objects in free fall.
  12. If I look at this object as it's falling to the ground, I can draw a force diagram for you,
  13. and if we're ignoring air resistance, the only force is this downwards force of mg.
  14. But that's a lone force. There's no partner to it.
  15. I thought forces always occurred in equal and opposite pairs.
  16. Well, as a matter of fact, they do. In fact, this force is the force of gravity.
  17. We could rephrase it as the force of earth on the ball, F earth-on-ball.
  18. There's actually a corresponding upwards force of the ball pulling on the earth and this is truly amazing.
  19. Right now, you are tugging on the earth with the force exactly equal to your weight.
  20. Pretty amazing, and that is what this next question deals with.