a) Define the term surface tension (b) Calculate the force required to lift a needle 4cm long off the surface of water if the surface tension of water is 7....
Question 1 Report
a) Define the term surface tension
(b) Calculate the force required to lift a needle 4cm long off the surface of water if the surface tension of water is 7.3 x 10\(^{-4}\) NM\(^{-1}\)
Answer Details
(a) Surface tension
Surface tension is the force per unit length acting along (tangent to) the surface of a liquid, at right angles to a line drawn in the surface. It arises from the net inward attraction on the surface molecules and makes the surface behave like a stretched elastic skin. Its unit is \(\text{Nm}^{-1}\).
(b) Force to lift the needle
A floating needle is held by the surface film on both sides (two surfaces), each of length \(L\). The force due to surface tension is:
\[ F = \gamma \times 2L \]Data: \(\gamma = 7.3\times10^{-4}\,\text{Nm}^{-1}\), \(L = 4\,\text{cm} = 0.04\,\text{m}\).
\[ F = 7.3\times10^{-4} \times 2 \times 0.04 = 7.3\times10^{-4}\times0.08 = 5.84\times10^{-5}\,\text{N} \]The force required is about \(5.84\times10^{-5}\,\text{N}\).
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