Question

Ten one- rupee coins are put on top of each other on a table. Each coin has a mass $m$. Which of the following statements is not true?

• A. The force on the $6^{\text{th }}$ coin (counted from the bottom) due to all the coins on its top is equal to $4mg$ (downwards)
• B. The force on $6^{\text{th }}$ coin due to $7^{\text{th }}$ coin is $4mg$ (downwards)
• C. The reaction of the $6^{\text{th }}$ coin on the $7^{\text{th }}$ coin is $4mg$ (upwards)
• D. The total force on the $10^{\text{th }}$ coin in $9mg$ (downwards)

Answer 1

Answer: (D)

Ten coins are placed one over another as shown.
As the $6^{\text{th }}$ coin has $4$ coins on its top, therefore the force experienced by the $6^{\text{th }}$ coin will be $4mg$. As $7^{\text{th }}$ coin has $3$ coin on its top, therefore it will exert a force of $4mg$ on $6^{\text{th }}$ coin.
As every action has equal and opposite reaction therefore $6^{\text{th }}$ coin will give a reaction of $4mg$ to the $7^{\text{th }}$ coin upwards

The $10^{\text{th }}$ coin will experience a reaction force of $1mg$ as it exerts a force of $1mg$.