Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

Newton’s third law

Is Newton's law of gravity consistent with Newton's third law?

The third law requires two things. First, m1's force on m2 should be the same as m2's force on m1. This works out, because the product m1m2 gives the same result if we interchange the labels 1 and 2. Second, the forces should be in opposite directions. This condition is also satisfied, because Newton's law of gravity refers to an attraction: each mass pulls the other toward itself.

Last Update: 2009-06-21