Physics

Physics

Problem

A thin square plate of side 5 cm is suspended vertically from a balance so that lower side just dips into water with plate perpendicular to surface. When the plate is clean ( θ=0∘\theta = 0^\circ ), it appears to weigh 0.044N. But when the plate is greasy ( θ=180∘\theta = 180^\circ ), it appears to weigh 0.03N. The surface tension of water is: (A) 3.5×10−23.5 \times 10^{-2} N/m (B) 7.0×10−27.0 \times 10^{-2} N/m (C) 14.0×10−214.0 \times 10^{-2} N/m (D) 1.0N N/m
Intuitive explanation

Surface tension creates an upward force along the perimeter where water meets the plate—the key is that this force depends on the contact angle θ\theta. When the plate is clean, water wets it completely (θ=0°\theta = 0°), so surface tension pulls up maximally; when greasy, water beads up (θ=180°\theta = 180°), so the vertical component reverses and pulls down. The difference in apparent weight between these two states directly reveals the surface tension force, which you can extract using F=γ×perimeter×cos⁡θF = \gamma \times \text{perimeter} \times \cos\theta.

Intuitive explanation

Surface tension creates an upward force along the perimeter where water meets the plate—the key is that this force depends on the contact angle θ\theta. When the plate is clean, water wets it completely (θ=0°\theta = 0°), so surface tension pulls up maximally; when greasy, water beads up (θ=180°\theta = 180°), so the vertical component reverses and pulls down. The difference in apparent weight between these two states directly reveals the surface tension force, which you can extract using F=γ×perimeter×cos⁡θF = \gamma \times \text{perimeter} \times \cos\theta.

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