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

# Rolling and Kinetic Frictions

## Objective

To study that rolling friction is smaller than kinetic friction.

## Introduction

When two surfaces press against each other, the molecules at actual contact get bonded. While sliding, these bonds are broken and new bonds are formed. All this results in kinetic friction. When a body rolls on a surface, there is no sliding, the portion in contact lifts up and the next portion comes into contact. This results in much smaller friction.

## Apparatus

a book, two pencils

## Procedure

Put a heavy book on a table. Try pushing it forward by applying a force on the top. It takes a good amount of effort to move the book. Now keep two pencils in parallel positions between the book and the table. Now apply a force on the top surface of the book in the direction perpendicular to the pencils. The book moves in much less effort showing that rolling friction is much smaller than the kinetic friction.

## Discussion

The mechanism of friction at the atomic level is not well understood. The knowledge about the behaviour of friction is largely based on experimental observations.