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Why does the Stone not pull the Horse

  1. May 23, 2008 #1
    Newton’s Third Law. “To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always directed to contrary parts.
    “Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may say so) will be equally drawn back towards the stone; for the distended rope, by the same endeavor to relax to relax or unbend itself, will draw the horse as much towards the stone as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other.…” -Isaac Newton

    Comment by George Gamow: “Why then, one can ask, is the horse pulling the stone, and not the stone pulling the horse? The answer is, of course, that the difference lies in the friction against the ground. The four horseshoes cling more strongly to the ground than does the stone, and if it were not so, the stone would remain in place and the horse’s hoofs would slide [which doesn’t really answer the question, “why does the stone not pull the horse?”].”

    Newton’s law expresses the conservation of momentum between two interacting systems. The momentum lost by one is obtained by the other. An element of momentum, through interaction, is relocated without loss.

    Why odes the stone not pull the horse? With all due respect, I don't think that Newton correctly says what he intends to say, nor that George Gamow answers it. Some fresh minds that can look at this without predisposition would be greatly appreciated!
     
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  3. May 23, 2008 #2

    Doc Al

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    No mystery here. Newton is using "pull" in the sense of "exert a force"--it's certainly true that the stone and horse pull equally on each other. (Indeed, this is Newton's 3rd law.)

    Gamow is answering a different question: Why does the horse go forward, dragging the stone along instead of the reverse. This is an issue for Newton's 2nd law, not the 3rd. (I assure you, Gamow understood Newton's laws.)
     
  4. May 24, 2008 #3

    CompuChip

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    I don't really see how introducing friction doesn't answer the question. Suppose, for the sake of argument, that the stone and the horse would have the same mass. If you put the whole system on ice, instead of grass, then indeed the stone would start sliding towards the horse, and the horse would slide towards the stone with the same velocity, and they would meet in the middle (where, if the collision were perfect, they would both come to rest).
    Maybe it would be instructive if you drew a force diagram (or e.g. a free body diagram for the horse).
     
  5. May 24, 2008 #4

    DaveC426913

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    Please explain your logic as to why would this happen.

    Horse is not eating the rope is it?
     
  6. May 26, 2008 #5
    ... and put a person on the horse's back, facing backwards. The person takes off their shoe and throws it. Newton's 3 law: person pushes shoe & shoe pushes person. The result - shoe moves off in one direction, the horse-person-rope-rock moves off more slowly in the other direction (assuming the icy surface is frictionless).

    To go back to the original scenario: swap the thrown shoe for the pushed ground and the same principles hold - the horse-rope-rock moves in the opposite direction to the ground, its just that the mass of the ground is so vast that its acceleration and final velocity are negligible compared to that of the horse-rope-rock.
     
  7. May 26, 2008 #6
    thank you for your assurances
     
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