# NCERT Class 11 Physics Solutions: Chapter 6 – Work, Energy, and Power Part 10 (For CBSE, ICSE, IAS, NET, NRA 2022)

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**Question 5.27**:

A helicopter of mass rises with a vertical acceleration of the crew and the passengers weigh given the magnitude and direction of the

(a) Force on the floor by the crew and passengers,

(b) Action of the rotor of the helicopter on the surrounding air,

(c) Force on the helicopter due to the surrounding air.

**Answer**:

(a) Force on the floor by the crew and passengers:

Mass of the helicopter,

Mass of the crew and passengers,

Total mass of the system,

Acceleration of the helicopter,

Using Newton՚s second law of motion, the reaction force R, on the system by the floor can be calculated as:

N

Since the helicopter is accelerating vertically upward, the reaction force will also be directed upward. Therefore, as per Newton՚s third law of motion, the force on the floor by the crew and passengers is , directed downward.

(b) Action of the rotor of the helicopter on the surrounding air:

Using Newton՚s second law of motion, the reactions force , experienced by the helicopter can be calculated as:

The reaction force experienced by the helicopter from the surrounding air is acting upward. Hence, as per Newton՚s third law of motion, the action of the rotor on the surrounding air will be Directed downward.

(c) Force on the helicopter due to the surrounding air:

The force on the helicopter due to the surrounding air is , directed upward.

**Question 5.28**:

A stream of water flowing horizontally with a speed of gushes out of a tube of cross-sectional area , and hits a vertical wall nearby. What is the force exerted on the wall by the impact of water, assuming it does not rebound?

**Answer**:

Speed of the water stream,

Cross-sectional area of the tube,

Volume of water coming out from the pipe per second,

Density of water,

Mass of water flowing out through the pipe per second

The water strikes the wall and does not rebound. Therefore, the force exerted by the water on the wall is given by Newton՚s second law of motion as:

Rate of change of momentum

**Question 5.29**:

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

(a) The force on the coin (counted from the bottom) due to all the coins on its top,

(b) The force on the coin by the eighth coin,

(c) The reaction of the coin on the coin.

**Answer**:

(a) The force on the coin (counted from the bottom) due to all the coins on its top:

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

Weight of one coin

Weight of three coins

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

(b) The force on the coin by the eighth coin:

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

Weight of the ninth coin

Weight of the tenth coin

Total weight of these three coins

Hence, the force exerted on the coin by the eighth coin is . This force acts vertically downward.

(c) The reaction of the Therefore, the total downward force experienced by the 6^{th} coin is 4mg. coin on the coin:

The coin experiences a downward force because of the weight of the four coins on its top.

Therefore, the total downward force experienced by the coin is .

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