Volume 18: Pages 174-176, 2005

Newton's Third Law for Rotary Motion Derived

Kern E. Kenyon

4632 North Lane, Del Mar, California 92014‐4134 U.S.

An elementary derivation is given for the rotary analogue of Newton's third law: if object A exerts a torque on object B about a given axis, then B exerts an equal but opposite torque on A about the same axis. The derivation, which appears to be missing from the textbooks, uses Newton's third law of linear motion, the definition of torque, and the vector algebra operation of the cross‐product. An example of using Newton's third law in rotary form to solve a problem in fluid dynamics is presented. When a laminar flow adjacent to a curved solid boundary accelerates tangent to the curve, then the fluid can exert a torque on the boundary.

Keywords: Newton's third law, rotary motion, fluid torque

Received: April 27, 2004; Published online: December 15, 2008