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Why someone cannot raise him/herself up by holding the hair?

  1. Nov 21, 2011 #1
    Can someone please answer this question in terms of the forces acting?
  2. jcsd
  3. Nov 21, 2011 #2
    The force that you apply to lift something is equally applied pushing your body down, the harder you pull your head up, the more force you push your body down... since they are connecting you just wind up stretching your neck
  4. Nov 21, 2011 #3
    Your hand pulls your hair and your hair pulls your hand with the same magnitude of force.In the end the forces on you cancel out.
  5. Nov 22, 2011 #4
    So that is because of the applier of the force and the object to which the force is applied are the same? I understand that the action and reaction forces cancel each other out but this is also true for a force that is applied by another person.

    That is just like the case when someone tries to push a bus when he/she is in the bus. What is the physical explanation for this, is there any law -say- to name that case?
  6. Nov 22, 2011 #5
    Something like that. The problem lies in the wording: "raise yourself up". Raise yourself up relative to what? We have to assume you mean relative to the earth. That being the case, why should a pull you exert directly on your hair be expected to change your altitude with respect to the earth? To change your altitude with respect to the earth you really have to exert a force against the earth, somehow. By stepping up onto a staircase that's sitting on the earth, for example.

    Likewise with the bus. You can't make the bus move relative to the ground by applying forces that act exclusively between you and the bus. If you do that, the only change you'll get will be limited to changes between you and the bus. To make the bus move relative to the ground you have to apply forces that act between the bus and the ground.

    When the bus is stuck and the men come out and push on the bus, it is not really men against bus, but ground against bus, and bus against ground: the men are acting as intermediaries between the bus and the ground. Their arms act against the bus and their feet act against the ground. If they are successful, the bus will move relative to the ground.

    When you pull on your hair, the hair pulls back on your hand. That is what Newton's Third Law is about. The ground, the earth, is not involved in this action-reaction pair of the hair and the hand that pulls it. If you lift someone else by the hair, though, you are the intermediary between them and the ground. Therefore, they move relative to the ground.

    The law you need is Newton's Third Law of motion. You just need to be careful about determining what is really acting against what in each case.
  7. Nov 22, 2011 #6
    If the hair was long enough(!!!!!) to be passed over a pulley attached to the ceiling you could pull DOWN and pull yourself up !!
  8. Nov 22, 2011 #7


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    No, it isn't. One scenario has two forces (one force pair) and the other has four.
  9. Nov 22, 2011 #8
    But then you introduce an external force, from the pulley/ceiling/earth.
  10. Nov 22, 2011 #9


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    When you pull on your own hair, any upwards force on your hair will be opposed by a downwards force at your shoulder. This involves two sets of Newton third law pairs of forces: 1 - your hand exerts an upwards force on your hair, and your hair exerts a downwards force on your hand; 2 - your arm exerts a downwards force on your shoulder, and your shoulder exerts an upwards force on your arm.

    A more general rule is the center of mass of any closed system can't be accelerated without an external force.
  11. Nov 22, 2011 #10
    Nasu has it exactly.
    In the original statement there is no EXTERNAL force acting on the system.
    With the hair (or a rope) over a pulley there is an external force because of the pulley attached to the ceiling.
  12. Nov 23, 2011 #11
    Thank you for your answers. zoobyshoe, i really appreciate.
  13. Nov 23, 2011 #12


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    Newton's First Law of Motion puts it very well.
  14. Nov 23, 2011 #13
    Damn, you're right.

    Reminds me of the movie "Sling Blade". Autistic guy has a reputation of being a "genius" at fixing lawnmowers. Guy brings him his lawnmower, complaining it just quit and won't work anymore. Autistic guy looks it over unscrewing this and that and announces "It's outta gas."
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