Question

(ii) Ten one-rupee coins are put on top of each other on a table. Each coin has a mass $m$. Given the magnitude and direction of
the force on the $7^{th}$ coin by the eight coin.

(iii)Ten one-rupee coins are put on top of each other on a table. Each coin has a mass $m$. Given the magnitude and direction of
"the reaction of the $6^{th}$ coin on the $7^{th}$ coin"

Answer 1

(ii) Step 1: force on $7^{th}$ coin by $8^{th}$ coin

Force on seventh coin by the eighth coin is because of the weight of the eighth coin and other two coins (ninth + tenth) on its top.
Weight of one coin = mg
Total weight of these three coins =$3mg$

Hence, the force exerted on the $7^{th}$ coin by the eight coin is $3mg$ , in vertically downward direction.

(iii) Step 1: Reaction force of $6^{th}$ coin on the $7^{th}$ coin

The $6^{th}$ coin experiences a downward force because of the weight of the four coins $(7^{th},8^{th},9^{th},{10}^{th})$ on its top.

So, the total downwards force experienced by the $6^{th}$ coin is $4mg$

According to Newton’s third law of motion, the $6^{th}$ coin will produce equal force on the $7^{th}$ coin in opposite direction.

So, the reaction force of $6^{th}$ coin on the $7^{th}$ coin is of magnitude $4mg$ and is acting upwards.