ICSE Class 8 Physics
Long Answer Questions
1. Explain why a spanner (wrench) has a long handle.
The turning effect of a force is called moment of force.
Moment of Force = Force × Perpendicular Distance
A spanner has a long handle so that the distance between the force and the pivot becomes larger. A larger distance produces a greater moment of force. Therefore, a small force is enough to loosen or tighten a nut. This makes the work easier.
2. Why is the steering wheel of a vehicle made large?
A large steering wheel increases the distance of the applied force from the centre of rotation. Due to the larger radius, a greater moment of force is produced. Therefore, the driver can turn the vehicle easily with less effort.
3. Why is it easier to open a door by pushing it near its free end?
The hinges of a door act as the pivot. When force is applied near the free end, the perpendicular distance from the pivot is maximum. Hence a greater moment of force is produced, making the door open easily.
4. Why is a hand flour grinder provided with a handle near the rim?
The handle is fixed near the rim to increase the distance from the centre of rotation. This produces a larger moment of force. Therefore, less effort is needed to rotate the grinder.
5. Explain why wooden sleepers are placed below railway tracks.
Wooden sleepers increase the area of contact between the rails and the ground. Since pressure is inversely proportional to area, the pressure on the ground decreases. This prevents the tracks from sinking into the soil and provides stability.
6. Why do tall buildings have wide foundations?
A tall building exerts a large force on the ground due to its weight. A wide foundation increases the area of contact. As pressure equals thrust divided by area, the pressure on the ground decreases. This prevents the building from sinking and increases stability.
7. Describe an experiment to show that a liquid exerts pressure at the bottom of a container.
Take a tin can with a small hole at its bottom and cover the hole with a rubber sheet.
Pour water into the can.
Observation: The rubber sheet bulges downward.
Conclusion: The water pushes the rubber sheet downward, showing that a liquid exerts pressure at the bottom of a container.
8. Describe an experiment to show that a liquid exerts pressure sideways.
Take a plastic bottle and make a small hole on its side. Fill the bottle with water.
Observation: Water comes out through the side hole in the form of a jet.
Conclusion: Water exerts pressure on the sides of the container. Therefore, liquids exert pressure sideways.
9. Describe an experiment to show that liquid pressure increases with depth.
Make three holes at different heights in the side of a bottle and fill it with water.
Observation: Water from the lowest hole comes out with the greatest force and travels the farthest distance.
Conclusion: Liquid pressure increases with depth.
10. Explain the crushing tin can experiment.
Take a small tin can containing a little water. Heat it until steam is formed. Close the can tightly and cool it quickly.
Observation: The can gets crushed.
Reason: When steam condenses, the pressure inside the can decreases. The atmospheric pressure outside becomes much greater than the pressure inside and crushes the can.
Conclusion: Air exerts pressure in all directions.
11. Why do camels have broad feet?
Camels have broad feet which provide a larger area of contact with the sand. A larger area reduces pressure on the sand. Therefore, camels do not sink easily while walking in deserts.
12. Why is a dam broader at the bottom than at the top?
Liquid pressure increases with depth. The bottom of the dam experiences greater water pressure than the top. Therefore, the lower part of the dam is made thicker and stronger to withstand the greater pressure.
