Concepts

Class 9
Chapter 10 Class 9 - Gravitation

What is buoyancy?

When an object is placed in liquid, it exerts an upward force on object

which makes the object appear to be lighter

This upward force is called buoyancy

Example 1`

When a bucket of water is immersed in water, it appears lighter

But when we take it out of water, it appears heavier

This is because of upward force exerted on bucket by water

This force is called buoyancy

Example 2

When we swim in a pool, we feel lighter than when we come out

This is because of upward force exerted on our body by water

This force is called buoyancy

What causes buoyancy?

We know as per Newton's third law of Motion

Every action has an equal and opposite reaction

When an object is immersed in water

it is pulled downwards due to gravitational pull of earth

Water exerts upward force on the object

This makes object rise up

This upward force is called buoyancy or upthrust

What is meaning of upthrust

We know that

Thrust is force applied to an object in a perpendicular direction

Similarly

Upthrust is a force applied to an object in upward direction

Questions

Q 2 Page 141 - What do you mean by buoyancy?

Q 3 Page 141 - Why does an object float or sink when placed on the surface of water?

Q 1 Page 142 - You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?

Q 2 Page 142 - You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?

NCERT Question 19 - In what direction does the buoyant force on an object immersed in a liquid act?

NCERT Question 20 - Why does a block of plastic released under water come up to the surface of water?