Concepts of Physics

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

Zipping Action due to Cohesive Forces

Objective

To study the cohesive and adhesive forces.

Apparatus

a newspaper, scissor

Procedure

Cut two paper strips, about 2 cm wide along the length of a newspaper and wet them. Hold the wet strips about 3 cm apart, hanging parallel to each other. Ask your friend to join the lower ends of the strips together. Soon the strips will stick all the way to the top, like zipping of a zipper (except near the upper ends).

Discussion

Water molecules have a property of sticking to one another (called cohesion) and also to molecules of other materials (called adhesion). Paper gets wet due to adhesive property of water. When you bring the bottom ends together, due to cohesive property, water molecules at the bottom of two strips cling together. In turn, water molecules just above them come closer and they also cling. This process continues all the way to the top.