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Newton's Third Law doesn't make sense

  1. Sep 8, 2014 #1
    Newton's third law states that for every action, there is an equal and opposite reaction. This doesn't make sense to me, because it seems as though everything would cancel out and so nothing would ever happen.
    For example, imagine a person attempting to push a large box; not just a quick push, but constantly pushing it so that the box eventually moves along the floor whilst the person remains in contact and continues to apply the same force. Imagine this person applies 10N to the box. According to the 3rd law, the box will apply 10N back onto the person. This is what doesn't make sense, since the box and the person are applying equal forces upon each other, both of them would not move anywhere, even if it is a relatively light box and friction is negligible. A good analogy would be arm wrestling: if person A applies a force to the left and person B applies an equal force to the right, both their hands would remain in the middle and no one would win. But clearly this is not a problem in everyday experiences - things are able to move if a large enough force is applied. I can easily drag a suitcase along the floor despite it applying the same force in the opposite direction. But my question is, how? If all interacting objects are applying equal and opposite forces to each other, how is anything able to move? To be more specific, how are any two objects able to move together despite them applying opposite forces to each other? Surely there needs to be a resultant force? This is driving me crazy!
    Last edited: Sep 8, 2014
  2. jcsd
  3. Sep 8, 2014 #2


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  4. Sep 8, 2014 #3
  5. Sep 8, 2014 #4
    I can totally agree with you. A lot of things that you are told to just accept in Physics, are purely maddening if you sit and dwell on them for too long!

    In the case of Newton's Third Law, equal and opposite forces SOUND like they would just cancel each other out...but that is not the case because those forces are not the ONLY forces in action. So in the case of your own example, arm wrestling:


    Person A goes the gym every day. Person B plays too much League of Legends and doesn't lift much more on a daily basis then a can of soda. They decide to arm wrestle, because Person A wants to prove that his 21inch biceps serve a purpose. Clearly, Person A is going to win. Why? Because his Muscles are able to overcome MUCH more Force than Person A's can. So despite the fact that he is experiencing the same amount of Force back on his hand/arm that he is dishing out on his opponent, that Force is not enough that his Muscles cannot 'absorb' it.

    A better to way to think about it, is that the reaction force NEVER acts on the same object as the force that causes the reaction. The initial Action, and the opposing Reaction occur on two different objects. In the case of the arm wrestling match, you would draw two separate diagrams with separate (but related) forces. The Force that you would label 'Muscles' would be different for each Person depending on their fitness level, which would then result in a force imbalance, thus causing acceleration.

    Hopefully this helps a tad. :)
  6. Sep 8, 2014 #5

    There is one force more you have to take into account in your problem, this is the force done against the ground by the person. This force revents into both the person and the box. The net force on the person is the one given by the force against the floor minus the 10N "spent" pushing the box as well as friction forces.

    Best regards,
  7. Sep 9, 2014 #6
    You missed an important point: action and reaction act on different bodies.
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