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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?
