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Action and reaction

  1. Sep 25, 2009 #1
    In a generic case of a mass point and an extended body to interact with gravitational force, the total force on the point admits a reaction to be construed as, on the extended body ?

    The total force on the extended body do not arise from the interaction between two points, but between a point and many points so, where is it applied? (whether it makes sense to speak of application ponit)

    The third law gives us no information about its point of application but it tells us the direction, to and intensity so as to be understood? (regardless of the theorem dl motion of the center of mass, which is another topic)
     
  2. jcsd
  3. Sep 25, 2009 #2
    The graviatational force acts between any two point masses, so for the total force on the extended body, it is actually a summation of the individual forces on each point mass on the extended body. As for point of application goes, gravitation is a field force, not a contact force, so there is no point of application per se. Thus all points on the extended body feel this force. For simplicity, we assume that the point of application of this force is at the center of mass, but that holds true only in certain cases.
     
  4. Sep 25, 2009 #3
    But the law of action/reaction indicates a precise direction and versus of the total force on the body. It seems there was also a point of application...or not? Which is the meaning of this?
     
  5. Sep 25, 2009 #4

    Andy Resnick

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    Newton's 3rd law is a statement about *contact forces*. When two bodies interact via contact (collisions, pulling via a rope, standing on the ground, etc), while we may not yet know the *nature* or the *cause* of the interaction, we can state that whatever force body 1 applies to body 2 is equal and opposite to the force body 2 exerts on body 1.

    For rigid bodies or point masses this is boring. The real power of this concept arises when the bodies can *deform*.
     
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