Ncert Exercises & Solutions

5.29 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 7th coin (counted from the bottom) due to all the coins on its top,

(b) the force on the 7th coin by the eighth coin,

(a) If F₇ be the force on the 7^th coin (counted from the bottom) experienced due to all coins above it, then
F₇ = weight of three coins above it = 3 mg N (downwards)
(b) F₈₇= force on 7^th coin by 8^th coin, then the 8^th coin has to support the weight of the two coins above it. So, the 8^th coin shall exert the force T₈₇such that F₈₇ = weight of 8^th coin + weight of two coins above the 8^th coin = mg + 2mg = 3mg (N) and it acts downwards.
(c) The sixth coin experiences force equal to the weight of the four coins above it. Hence reaction due to 6^th coin on 7^th coin = 4mg N and it acts vertically upwards.

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