ten one-rupee coins are put on top of each other on a table
(a) Force on the seventh coin is exerted by the weight of the three coins on its top.
Weight of one coin = mg
Weight of three coins = 3mg
Hence, the force exerted on the 7th coin by the three coins on its top is 3mg.
This force acts vertically downward.
(b) Force on the seventh coin by the eighth coin is because of the weight of the eighth coin and the other two coins (ninth and tenth) on its top.