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Third Law of Motion :Confusion

  1. Jul 24, 2012 #1
    When we push a body say for example,a crate on ground.Then in response to our force the crate also exerts a force on our hands.Then my question is why does the crate move forward and not we that move backward?
  2. jcsd
  3. Jul 24, 2012 #2
    Because your feet are in contact with the floor and there is a force of friction on your feet from the floor exactly equal and opposite to the force on you from the crate, because friction acts in exactly the way it needs to to prevent your feet from moving.

    There is a limit to how strong friction can be though, and it won't necessarily be the crate that moves. The crate is in contact with the floor too and if it can sustain a higher friction force than your feet (because it is heavy enough or the materials have more grip) then you will move backwards before the crate moves forwards. Try pushing as hard as you can against something heavy with good grip, a car with the brakes on for instance, with your feet up close to the car and say wearing socks to keep friction low, I think you'll find that it's you that slides backwards.
  4. Jul 24, 2012 #3

    Doc Al

    User Avatar

    Staff: Mentor

    The interaction between you and the crate involves equal and opposite forces that you exert on each other.
    Whether an object accelerates depends on the net force acting on it. Presumably there are other forces acting on you, such as friction from the ground to hold you steady. But if you and the crate were on frictionless ice, then you would both move as a result of your push: You'd move backward and the crate would move forward.
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