Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Pull your friend to know Newton's Third Law

Objective

To understand Newton's third law

Introduction

The statement of Newton's third law looks very simple but somehow it turns out to be one of the least understood concept. Some of the misconceptions abut Newton's third law are addressed in this demo.

Procedure

Ask two students, one reasonably healthier (say A) than the other (say B) to come on the stage. Ask them to hold each other by their right hands and try to pull each other. Normally the healthier will win. Now tell the class to assume that the A pulled B by 100 newton of force. By what force does the weaker student pull the healthier one? Is it less than 100 newton, more than 100 newton or equal to 100 newton. The right answer is equal to 100 newton though most of the students feel that it is less than 100 newton.

Discussion

Newton's third law states that the force exerted by A on B is equal to that exerted by B on A. Then why did A win? Because A pushed the ground stronger and hence ground exerted larger frictional force on A than on B.