First slide

Surface tension

Question

A cube with a mass m = 20 g wettable by water floats on the surface of water. Each side of the cube has length l = 3 cm. The angle of contact between water and glass is zero degree and the surface tension of the water is 7.5 × $10^{- 2}$ N/m. The distance between the lower face of the cube and the surface of the water is

Moderate

A

20/22 cm
B

2.222 cm
C

2.322 cm
D

2.434 cm

Solution

$F_{Uptthrust} = F_{Surface tension} + Weight$
$l^{2} xrg = 4 lT + mg {(. 03)}^{2} \times 10^{3} \times 10 = . 009 + . 2 9 x = . 209 x = . 02322 m = 2.322 cm$

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