Ten one-rupee coins are put on top of each other on a table. Each coin has a mass m . Give the magnitude and direction of
(a) the force on the `7^(th)` coin (counted from the bottom due to all the coins on its top.
(b) the force on the `7^(th)` coin by the eight coin.
(c) the reaction of the `6^(th)` coin one the `7^(th)` coin.

Explanation

(a) The force on 7th coin is due to weight of the three coins lying above it.
Therefore,
F = (3m)kgf = (3mg) N
where g is acceleration due to gravity . This force acts vertically downwards .
(b) The eighth coin is already under the weight of two coins above it and it has its own height too. Hence force on 7th coin due to 8th coin is sum of the two forces
i.e.
F = 2m + m = (3m) kg f = (3mg)N. The force acts vertically downwards .
(c ) The sixth coin is under the weight of four coins above it .
Reaction , R = - F = - 4m(kg) = -(4mgf)N Minus sign indicates that the reaction acts vertically upwards , opposite to the weight .