Why does a needle or paperclip float on the surface of the water if it is kept gently?

Due to surface tension of the water on the surface the needle floats. Consider the water surface like an elastic rope. When a small object having less weight is kept on the rope the rope deforms down a little amount. In the same way when the needle is kept on the water surface it floats.

Due to surface tension of the water on the surface the needle floats. Consider the water surface like an elastic rope. When a small object having less weight is kept on the rope the rope deforms down a little amount. In the same way when the needle is kept on the water surface it floats.

What is surface tension?

The force of attraction of the molecules which are in the surface of the liquid per unit length is called surface tension. This force is due to cohesion of the molecules.

There are two kinds of forces.

1. Adhesion force: The force of attraction between different types of molecules. For instance, adhesive (Feviquick- commonly used) for joining two kinds of different items. The force of attraction is between adhesive and another material (e.g. paper - adhesive)
2. Cohesion force: The force of attraction between the same types of molecules. For instance, due to this attraction water tries to be in collective. If you pour water on a glass surface at two places and try to bring it together allowing small gap. The water itself tries to stick together. See the fig below.

The surface tension is due to the cohesion force of the water.

The water molecule A is inside the water. It is attracted by other molecules in all directions equally. So it balances each other.

For example, 5N Left – 5N Right = 0N, 5N Up – 5N Down = 0N.

But the water molecule B is near the surface of water and no other molecules above it. So no other molecules will attract it above the surface. But the attraction is on left, right and bottom sides only.

For example, 5N Left – 5N Right = 0N, 5N Bottom – 0N up = 5N Bottom.

The molecule tries to go down but it can’t. So always there is a force on the surface of the water. This force per unit area is called surface tension.

How does the paperclip or the needle float on the water?

When we keep the needle or the paperclip gently on the surface of the water it pushes the water down. So the water surrounding this area should rise. But when the water tries to raise on sides the inward force of the molecules don’t allow it to rise. So it again pushes the water which is below the needle or the paperclip upward. So equilibrium is maintained and the needle or the paperclip floats on the water. For understand it clearly see the figure below.