The length of a needle floating on the surface of water is $1.5\,cm$. The force in addition to its weight required to lift the needle from the water surface will be...... $N$ (surface tension of water $= 7.5\,N/cm$).

Two pieces of glass plates placed one upon the other with a little water in between them cannot be separated easily because of:

$A$ thin liquid film formed between a $U$-shaped wire and a light slider supports a weight of $1.5 \times 10^{-2} \; N$ (see figure). The length of the slider is $30 \; cm$ and its weight is negligible. The surface tension of the liquid film is (in $; N m^{-1}$)

Two glass plates of area $10^{-2} \,m^2$ have a $10 \,cm$ thick water film between them. The surface tension of water is $70 \times 10^{-3} \,N/m$. The force required to separate the two glass plates from each other is (in $\,N$)

It is better to wash clothes in hot water than in cold water. Explain.

The free surface of a liquid has a tendency to contract. This property is called ...... .