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

## Apparatus

a stand, A long thread and a book

## Procedure

• Tie a book to a thread such that some portion of the thread hangs below the book.
• Suspend this arrangement from a support.
• Pull the thread with a jerk from the lower end. The portion of the thread below the book breaks. Why?
• Now tie the thread back. Again pull the thread from below but this time pull it slowly. The thread above the book breaks from the upper support. Why?

## Discussion

A sudden jerk on the thread from below increases the tension in the thread locally. The stress produced by the jerk exceeds the breaking limit and the thread breaks from below. The book and the thread above the book are not affected because the time duration of the jerk is so small that before the disturbance produced due to it reaches the upper end of thread, the lower thread breaks.

When the pull is gradual there is enough time for the disturbance to travel the entire length of the thread. Due to the weight of the book the thread above the book now experiences a greater tension as compared to the thread below the book. Hence the thread breaks from its upper end.

Similarly when we hold a branch of mango tree and give it a jerk we manage to increase the tension locally to the twig with which mangoes are attached to the main branch. This enable us to enjoy the mangoes!