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I have read section from "Kleppner and Kolenkow, An Introduction to Mechanics" about Newton's 3rd law of motion, What does red-lined sentence means ?

Why "somewhere in the universe ... on some other body" ?

Please explain.

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    $\begingroup$ That sentence just says that if there is a force present, then there is a copy of it present somewhere else. Somewhere in the universe that copy must be present. The force of gravity on the Sun because of earth has an equal force acting on the earth because of the Sun. This is somewhere else in the universe - that is all $\endgroup$ – Steeven Sep 29 '17 at 13:04
  • $\begingroup$ Can you be more specific what you don't understand about that sentence? Its meaning seems unambiguous to me. $\endgroup$ – ACuriousMind Sep 29 '17 at 13:57
  • $\begingroup$ @ACuriousMind ♦ Now it's clear for me thanks to @ Steeven. Other answers are also good. $\endgroup$ – Quark Sep 29 '17 at 14:10
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They are laying the groundwork for a discussion of fictitious forces such as centrifugal force. In a non-inertial frame, an object may be found to be accelerating, and we attach a force to that acceleration. But if we look for an equal and opposite force acting on some other body, we won't find one.

A fictitious force is not a consequence of an interaction between two objects, hence there is no real force, and no reaction force. No "some other body".

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He is saying that a force cannot be generated out of nothing.

A force on you must have been created by something and then in turn you must be creating an equal magnitude and opposite direction force on that something.

The "somewhere in the Universe" is a way of saying that it is always true and he has pointed out earlier that the fact that the law is always true leads to the law of conservation of momentum..

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