Question

The minimum force require to seprate a light glass plate of perimeter 5m from a water surface is ( surface tension of water 70 into 10 raise to poer minus 3 N /m)

Answer 1

The minimum force require to separate a light glass plate of perimeter 5m from a water surface is (surface tension of water 70 into 10 raise to power minus 3 N/m)

Solution

The capillary force that acts on light glass plate is

$$F_{cap} = P\sigma \cos \gamma,$$

where $P$ - perimeter of plate, $\sigma$ - surface tension, $\gamma$ - contact angle.

The contact angle of water on clean glass is very small, and is commonly taken as $0{}^{\circ}$.

The minimum force require to separate a light glass plate is equal and opposite to capillary force:

$$F = F_{cap} = 5m * 70 * 10^{-3} \frac{N}{m} * \cos 0{}^{\circ} = 0.35 \, N.$$

Answer: 0.35 N.