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Can gravitational force be reactive?

  1. Nov 13, 2006 #1
    If Earth attracts the Moon with gravitational force, then according to 3,rd Newton's law the Moon attracts the Earth with the same but opposite force. But what if someone argues that the Moon attracts the Earth anyway due to its own gravitation, can we really say that it's the reaction force?
    Another example: You push the car. Here the action force is electromagnetic. The car pushes you back and it's the reaction force, electromagnetic. So it seems that action and reaction must be the forces of the same nature. But what if someone uses the same argument here, and explains the third Newton law like this:
    When you push the car, the action force is electromagnetic. Because of the inertia of the car, it oposes the push and you feel the reaction force. Thus, here the reaction force is the force of inertia and is therefore of different nature than the action force.
    So two questions:
    1 Is this explanation of the 3.rd Newtons law better than the first one?
    2 Can gravitational force really be force of reaction?
    Last edited: Nov 13, 2006
  2. jcsd
  3. Nov 13, 2006 #2
    Lookup some numbers (google calculator). Calculate the force of the moon's gravity attracting the earth. Then calculate the force of the earth's gravity attracting the moon. Learn by exploration. It's not a matter of whether it is the reaction force. The reaction law just asserts that there will be such a force.
    Last edited: Nov 13, 2006
  4. Nov 13, 2006 #3
    In what sence electromangetic are u talkin in refrence to friction which is produced due to electromagtic intraion b/w two surface.
    Now newtons 3rd law is not a general term it always exists in pairs as you know. Gravitaion is again not a general term it also exsists in pair consider this in clasical physic a planet is there in an insolated space and think that there is no other planet or star think it is completely isolated. what could you think what will be its stabiltiy and gravitation
  5. Nov 13, 2006 #4


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    I have not heard of a 'force of inertia' as such. In the example you have given, both force pairs are electromagnetic in nature.
  6. Nov 13, 2006 #5
    I'd like to know if action and reaction must be forces of the same nature according to Newton.
    When the object falls to Earth, we say it falls (accelerates) due to gravitational force. If you push an object it accelerates due to which force?
  7. Nov 14, 2006 #6

    Doc Al

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    OK. And the 3rd law pair to the force of the Earth's gravitational force on the object is the object's gravitational force on the Earth.
    You exert a contact force on the object, which is fundamentally electromagnetic; the object, per Newton, exerts an equal but opposite contact force on you, also electromagnetic.
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