You are here
๐ฌ Think about it
Drop a coin in water and it sinks; drop a big wooden block and it floats. Gravity pulls on both, so why doesn't everything sink? And why does a mug feel lighter underwater? There's an upward push hiding in the water. Let's discover it.
What is buoyant force (upthrust)?
- When you push a closed bottle into water, you feel an upward push.
- The bottle bounces back to the surface when released.
- The upward force a liquid applies on an object is the upthrust or buoyant force.
- All liquids apply such a force.
Why do some objects float and others sink?
- Gravity pulls the object down; the buoyant force pushes it up.
- If gravity is more than the buoyant force, the object sinks.
- If the two forces are equal, the object floats.
- Buoyant force depends on factors such as the density of the liquid.
Float or Sink?
Two forces
Gravity pulls down; buoyant force pushes up
↓
Gravity wins
If gravity is greater, the object sinks
↓
If the forces are equal, the object floats
In this Activity, we will push a closed bottle into water and feel the upward buoyant force.
Materials needed
An empty bottle with its lid closed tightly, and a bucket full of water.
Procedure
1. Push the bottle into the water. Do you feel an upward push?
2. Release the bottle. Does it bounce up to the surface?
2. Release the bottle. Does it bounce up to the surface?
Observation
You feel an upward push, and the bottle bounces back to the surface.
Explanation
The water applies an upward force on the bottle. This upward force from a liquid is the upthrust or buoyant force.
โ Summary
- Water pushes the bottle up
- Bottle bounces back
- This is buoyant force
โก A step further
- Archimedes, a famous Greek scientist, discovered Archimedes' Principle.
- An object in a liquid feels an upward force equal to the weight of the liquid it displaces.
- If the displaced liquid weighs less than the object, the object sinks.
- If the displaced liquid weighs the same as the object, the object floats.
โก Ever heard of ...
- Some rocks can float on water — one is Pumice.
- It forms when gas-filled lava cools quickly during a volcanic eruption.
- Trapped gas bubbles make it light and porous.
- Being less dense than water, it floats.
โ
Test Yourself
-
What is the buoyant force?
View Answer
The upward force a liquid applies on an object placed in it. -
When does an object sink?
View Answer
When the gravitational force on it is more than the buoyant force. -
When does an object float?
View Answer
When the gravitational force and the buoyant force are equal. -
What does Archimedes' Principle state?
View Answer
An immersed object feels an upward force equal to the weight of the liquid it displaces. -
Why does pumice float?
View Answer
It is light and porous, less dense than water, because of trapped gas bubbles.
Important Definitions
- Buoyant force (upthrust) — The upward force applied by a liquid on an object placed in it.
- Archimedes' Principle — An immersed object feels an upward force equal to the weight of the liquid it displaces.
๐ NCERT Question 4 — When you drop a coin in water
A coin dropped in water sinks, but a bigger wooden block floats. Explain why.
View Answer →
๐ NCERT Question 5 — If a ball is thrown upwards
A ball thrown up slows, stops, and falls back. Name the forces and their directions during upward motion, downward motion, and at the top.
View Answer →
๐ NCERT Question 6 — A ball released from point P
A ball released from P stops at A on a horizontal surface. Suggest a way to make it stop (i) before A and (ii) after A.
View Answer →
๐ NCERT Question 10 — Three objects 1, 2, and
Three same-size objects dip to different depths in water. Order their weights w1, w2, w3 from the given options.
View Answer →
