First slide

Surface tension

Question

An iron needle slowly placed on the surface of water floats on it because

Moderate

A

when inside in water it will displace water more than its weight
B

the density of the material of needle is less than that of water
C

of surface tension
D

of its shape

Solution

Due to the concave shape of the surface film, total surface tension force on the needle = sl+sl =2sl. Also water displaced by the needle produces a buoyant force B on the needle. Resultant of buoyant force and surface tension force, balances the weight of the needle. Since buoyant force B is very small, weight of the needle is balanced chiefly by surface tension force.

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