Question

Ten one rupee coins are put on top of each other on a table. Each coin has mass m. the reaction of the $\;{6^{th}}$ coin (counted from bottom) on the $\;{7^{th}}$ coin is:
(A) 4mg
(B) 6mg
(C) 7mg
(D) 3mg

Answer 1

We will answer this question by using Newton’s laws, mainly we will use Newton’s third law. We will first look at what Newton's first law states. We will define Newton's ${3^{rd}}$ law and then at last we will apply this law to solve the given question.

Complete answer:
Newton’s third law states that when 2 bodies interact, they apply forces on one another that are equal in magnitude and opposite in direction. It is also called the law of action and reaction.
In the above mentioned question, the coins are placed one above the other. Hence they each apply forces on each other. The top most coin which is not in contact with the table but is in contact only with the remaining coins exerts force on the coin below it. Here the reaction of the sixth coin (counted from bottom) on the $\;{7^{th}}$ has been asked. Before this lets consider the ${2^{nd}}$ from the top, the forces acting on this coin are due to 2 coins: the one on the top of it and another below it.
Similarly the $\;{6^{th}}$ coin counted from bottom has force due to the $\;{7^{th}}$ and $\;{5^{th}}$ coin. But the reaction force of the $\;{6^{th}}$ coin on the 7th will be equal to the force on the $\;{6^{th}}$ coin due to the $\;{7^{th}}$ coin which will be equal to 4mg because 4 coins are present above the $\;{6^{th}}$ coin.
Hence the correct answer is option A.

Note:
In the above question reaction force has been asked. If action forces would be asked the answer would be negative (-4mg). Also remember the force on the $\;{6^{th}}$ coin will not be only because of the coin above it, will be because of all the coins above it $\left( {{7^{th}},{\text{ }}{8^{th}},{\text{ }}{9^{th}}and{\text{ }}{{10}^{th}}} \right).$