Why do we apply paint on iron articles?

When do we say that work is done?

When do we call a body rigid?

Give reason - Why : Galvanised iron particles are protected from rusting even if the zinc coating is broken.

Why do soaps not work in hard water?

Why do we need Quantum theory?

Why does sound travels faster in iron than in air?

What do we call the gravitational force between the earth and an object?