Thinking based questions,Class 8 , ch 2 -Physical quantities Measurement

Fig (a)

Fig(b)

EXPLANATION-BASED QUESTIONS

Q1. Explain why an iron needle sinks in water, but a ship made of iron floats on water.

An iron needle sinks because its density is greater than water and it displaces very little water.
A ship floats because its shape increases volume and lowers average density, so it displaces enough water to balance its weight.

Q2. It is easier to swim in sea water than in river water. Explain the reason.

Sea water has more density due to dissolved salts, so it provides greater buoyant force, making swimming easier.

Q3. Icebergs floating on sea water are dangerous for ships. Explain the reason.

Most part of an iceberg remains submerged under water and is not visible, so ships may collide with the hidden portion.

Q4. Explain why it is easier to lift a stone under water than in air.

Water exerts an upward buoyant force on the stone, reducing its apparent weight, so it is easier to lift.

Q5. What is a submarine? How can it be made to dive in water and come to the surface?

A submarine is a vessel that can travel underwater.
It dives by filling its tanks with water (increasing density) and rises by pumping out water and filling tanks with air (decreasing density).

Q6. A balloon filled with hydrogen rises in air. Explain the reason.

Hydrogen is less dense than air, so the buoyant force on the balloon is greater than its weight, causing it to rise.

Q7. A cork piece floats on water surface while an iron nail sinks in it. Explain the reason.

Cork has lower density than water, so it floats.
Iron has higher density than water, so it sinks.

Q8. (a) In which liquid does the body experience the greatest buoyant force?
(b) Which liquid has the least density?
(c) Which liquid has the highest density?

(a) Same in each liquid
(b) Liquid A has least density
(c) Liquid C has highest density

Q9. Why does a piece of ice float on water?

Ice has lower density than water, so it floats.

Q10. An egg sinks in fresh water but floats in salt solution. Give reason.

Salt solution has higher density than fresh water, so it provides more buoyant force and the egg floats.

Q11. A ship submerges more as it sails from sea water to river water. Give reason.

River water is less dense than sea water, so the ship must sink more to displace enough water to balance its weight.

Q12. Three different shapes of the same mass are cut from a sheet of iron. Identify the shape with the highest density. Give reason.

All shapes have the same density because density depends on material, not shape or size.