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Homework Help: Back to Newton's Laws

  1. May 26, 2005 #1
    I am doing a bit of review and have discovered something I havent thought about before. We all know Newton's third law (Fab)=-(Fba). So, how can there be acceleration? Say that Fab is 10N, then -Fba is -10N....10N - 10N = 0N. In an action reaction pair, if this net force is 0N....how can there be acceleration?
  2. jcsd
  3. May 26, 2005 #2


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    Action and reaction forces NEVER act on the same object. They can never cancel.
  4. May 26, 2005 #3
    Yes...they must act on different objects, same magnitude opposite direction. Say for example I push a block with 50N of force...that block pushes back on me with 50N as well...why does the block accelerate?
  5. May 26, 2005 #4


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    Why doesn't a fly make a train crash if they hit and the fly is accelerating and the train is not, after all F=ma.

  6. May 26, 2005 #5
    Okay, so go back to the first law of inertia then. The fly doesnt make the train crash since it is of neglegable mass. Bit it DID still exert a force on the train. Now...im sorry if this seems foolish but theres just something that isnt clicking here. I push a block and exert 50N on that block, it in turn exerts 50N on me. Why does the block accelerate?
  7. May 26, 2005 #6


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    Because by Newton's second law, the acceleration of the block is the result of forces acting on the block, not forces the block is exerting on something else. If the 50N is the only force acting on the block, it must accelerate.

    What happens to you? If the 50N the block is exerting on you is the only force acting on you, then you must accelerate in the opposite direction to the block. If the block is really massive (like the whole earth for example), it will hardly move, but you will. If the block has little mass it will accelerate so quickly that the force will only last for a small fraction of a second, and you will not change your velocity very much, while the block experiences a much larger velocity change.

    This is the origin of the conservation of momentum principle. If you and the block start at rest, and exert force upon one another, each of you will accelerate for the same time interval and acquire velocities in opposite directions. Your speed times your mass will equal the block's speed times its mass at all times during the acceleration and thereafter.
  8. May 26, 2005 #7
    That's much better...thank you. I was having a blonde moment there, but thanks for explaining it :smile:
  9. May 26, 2005 #8


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    You're missing the point. The fact that the block pushes back on your does not change the fact that it's experiencing a 50N force itself.

    Draw a free-body diagram showing all the forces on the block: your pushing, and friction. If your pushing exceeds the friction force, the net force is non-zero, and the block accelerates.

    - Warren
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