Question

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

• A. 22.5 Newton
• B. 2.25 Newton
• C. 0.25 Newton
• D. 225 Newton

Answer 1

The correct option is A 22.5 Newton

The total force required to lift the needle would be its weight and the force required to overcome the force due to surface tension.

Thus, in addition to weight only the force due to surface tension would be required.

The total force, F = $T\times 2L$ (because 2 surfaces are involved)

Thus, total force, F = $7.5\times 2\times 1.5$

F = 22.5 N