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Newton's third law

  1. Oct 27, 2015 #1
    1. The problem statement, all variables and given/known data
    I need to explain why I can't lift myself by pulling my shoes' laces.

    2. Relevant equations
    Newton's third law

    3. The attempt at a solution

    My explanation is that by pulling on your laces, there is always going to be a force that goes against your force, so this is why you can't pull yourself up. (So we're using NEwton's third as an explanation) Is this correct ?
     
  2. jcsd
  3. Oct 27, 2015 #2
    No this is not correct. Let me rephrase your answer in a way that might help you see this.

    You pull up on your laces, your laces pull down on you. One of these two forces (they're an action/reaction pair) is on your shoe laces. The other is on you. Since the forces act on different objects, they can't cancel each other out. For two forces to be able to cancel each other they would need to act on the same object.
     
  4. Oct 27, 2015 #3

    andrewkirk

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    I think they may be looking for a little more detail than that. Your hands exert an upwards force on the shoelaces. What is the opposite force to that? What exerts it, and upon what? How does that opposite force end up eventually countering the lift? There's an old song about Ezekiel and bones that may help with this.
     
  5. Oct 27, 2015 #4
    Sorry, I didn't phrase correctly what I meant. But is what you wrote the explanation for why I cna't pull myself ?
     
  6. Oct 27, 2015 #5
    No, of course it's not. I merely explained why your answer was wrong. You said that two equal-but-opposite forces produce a net force of zero. An action/reaction pair of of forces can never produce a net force of zero because they act on different objects.
     
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