Question

Why does a needle gently placed on the surface of water float on it instead of sinking?

Answer 1

Surface tension.

1. Surface tension is defined as force per unit length on an imaginary line drawn on the surface of liquid.
2. Due to surface tension the liquid surface behaves like a stretched elastic membrane.
3. Its unit is N/m
4. Hence, when a needle gently placed on the surface of water, it floats due to surface tension as the liquid surface behaves as a stretched membrane