Question

Why does it happen ?
a)Fruits can easily be cut with a sharp knife.
b)The wall of a dam is broad at its base

Answer 1

b)pressure is directly proportional to depth so to avoid any damage to the dam it is made broader at the base as max pressure is experienced by it at its base.
a)Sharp knife have less area contact with the fruit. Hence, if the area occupied is less and force applied is much then the pressure applied by knife on fruit is pretty much high. This applies pressure results to have cut on them easily.

Answer 2

a. the fruits can easily be cut with a sharp knife because in a sharp knife the area of edge of the sharp side is less so small force creates a large pressure .
b. the wall of a dam is made thicker at the base to withstand the greater lateral pressure due to the large quantity of water above it.