A straight thread 5.0 cm long lies on a water surface. The surface tension of water is 0.076 N/m. Find the force exerted by the surface film on the thread along one of its sides.

1.9 mN
2.7 mN
3.4 mN
3.8 mN

Correct answer: 3.8 mN

Solution

The surface film pulls along the line of contact with force F = T * L. With T = 0.076 N/m and L = 0.05 m, F = 0.076 * 0.05 = 0.0038 N = 3.8 mN for one side.

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