Chapter Exercises Solutions

Questions and Answers

1. Choose the correct statement

(i) Water Level in Vessels (Fig 6.21):

• Observation: Three connected vessels of different shapes ($P, Q, R$).
• Principle: Liquids seek their own level. Pressure at the bottom depends only on height, not shape.
• Answer: (d) equal in all three vessels.

(ii) Rubber Sucker on Smooth vs. Rough Surface:

• Concept: Suckers work because a vacuum is created, and atmospheric pressure holds them. On a rough surface, air leaks in through gaps, breaking the vacuum.
• Answer: (c) M will stick but N will not stick.

(iii) Water Tank Height:

• Goal: More pressure on ground floor.
• Relationship: $P \propto Height$.
• Answer: (a) increase the height ‘H’ at which the tank is placed.

(iv) Pressure and Force in Vessels A and B:

• Setup: Both have same water level (Height $h$). A is narrow, B is wide.
• Pressure ($P$): Depends only on $h$. So $P_A = P_B$.
• Force ($F$): $F = P \times Area$. Since Area of B > Area of A, then $F_B > F_A$.
• Answer: (b) $P_A = P_B, F_A < F_B$.

2. True or False

(i) Air flows from higher pressure to lower pressure. [True] (ii) Liquids exert pressure only at the bottom of a container. [False] (They exert pressure in all directions). (iii) Weather is stormy at the eye of a cyclone. [False] (The eye is calm). (iv) During a thunderstorm, it is safer to be in a car. [True] (The metal body acts as a Faraday cage, protecting inside).

3. Boy on Sand (Lying vs Standing)

Question: In which case does the boy sink more? Answer: He sinks more when standing vertically (Fig 6.23b).

• Reason: Pressure $= Force / Area$. His weight (Force) is constant. When standing, his feet have a small area. When lying, his whole body has a large area. Smaller area = Higher Pressure = More sinking.

4. Elephant Pressure Calculation

• Weight ($F$): $20000 \text{ N}$
• Area of 1 foot: $0.25 \text{ m}^2$
• Total Area (4 feet): $4 \times 0.25 = 1 \text{ m}^2$
• Pressure ($P$): $F/A = 20000 / 1 = \mathbf{20000 \text{ Pa}}$.

5. Boat Pressure Calculation

Formula: $P = \text{Total Weight} / \text{Area}$.

Boat A:

• Weight: $5 \text{ persons} \times 700 \text{ N} = 3500 \text{ N}$.
• Area: $7 \text{ m}^2$.
• Pressure: $3500 / 7 = \mathbf{500 \text{ Pa}}$.

Boat B:

• Weight: $3 \text{ persons} \times 700 \text{ N} = 2100 \text{ N}$.
• Area: $3.5 \text{ m}^2$.
• Pressure: $2100 / 3.5 = \mathbf{600 \text{ Pa}}$.

Conclusion: Boat B experiences more pressure (by $100 \text{ Pa}$).

6. Conductive Air and Lightning

Answer: If air were a good conductor, charges would flow continuously to the ground as they formed. There would be no massive build-up of static charge in the clouds, and therefore, no giant lightning bolts would occur.

7. Bottle and Balloons (Fig 6.24)

• Scenario: Water filled to a height in a bottle with two identical balloons attached at the same height.
• Answer: Both balloons will bulge, and they will bulge equally.
• Reason: Liquid pressure at a given depth is equal in all directions. Since the holes are at the same level, the pressure is identical.

9. Sea Coast Trees (Land A or B?)

• Clue: Trees are bending from Side A towards Side B. Wind blows from High Pressure to Low Pressure.
• Context: Summer Afternoon.
  • Land heats up fast $\rightarrow$ Air rises $\rightarrow$ Low Pressure on Land.
  • Sea is cooler $\rightarrow$ High Pressure on Sea.
  • Wind blows Sea to Land (Sea Breeze).
• Conclusion: Wind is blowing towards the land. If trees bend towards B, the wind is coming from A. So A is the Sea, and B is the Land.

13. Holes in Banners

Answer: High-speed winds exert immense pressure. If a banner blocks the wind completely, this high pressure can tear it or knock it down. Holes allow the wind to pass through, reducing the air resistance and pressure on the banner, keeping it safe.