CBSE (Science) Class 11CBSE
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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 the Force on the 7th Coin (Counted from the Bottom) Due to All the Coins on Its Top, the Force on the 7th Coin by the Eighth Coin, - CBSE (Science) Class 11 - Physics

ConceptNewton's Third Law of Motion

Question

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,

(c) the reaction of the 6th coin on the 7th coin.

Solution 1

(a) Force on the seventh coin is exerted by the weight of the three coins on its top.

Weight of one coin = mg

Weight of three coins = 3mg

Hence, the force exerted on the 7th coin by the three coins on its top is 3mg. This force acts vertically downward.

(b) Force on the seventh coin by the eighth coin is because of the weight of the eighth coin and the other two coins (ninth and tenth) on its top.

Weight of the eighth coin = mg

Weight of the ninth coin = mg

Weight of the tenth coin = mg

Total weight of these three coins = 3mg

Hence, the force exerted on the 7th coin by the eighth coin is 3mg. This force acts vertically downward.

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

Therefore, the total downward force experienced by the 6th coin is 4mg.

As per Newton’s third law of motion, the 6th coin will produce an equal reaction force on the 7th coin, but in the opposite direction. Hence, the reaction force of the 6th coin on the 7th coin is of magnitude 4mg. This force acts in the upward direction.

Solution 2

(a) The force on 7th coin is due to the weight of the three coins lying above it. Therefore F = (3 m) kgf = (3 mg) 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 weight too. Hence force on 7th coin due to 8th coin is sum of the two forces i.e. F = 2 m + m = (3 m) kg f = (3 mg) N The force acts vertically downwards.

(c) The sixth coin is under the weight of four coins above it.

Reaction, R = – F = – 4 m (kg) = – (4 mgf) N Minus sign indicates that the reaction acts vertically upwards, opposite to the weight

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Solution 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 the Force on the 7th Coin (Counted from the Bottom) Due to All the Coins on Its Top, the Force on the 7th Coin by the Eighth Coin, Concept: Newton's Third Law of Motion.
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