Archimedes' principle
- Olympiad · NSEJS
Archimedes' principle says that a body in a fluid is pushed up by a force equal to the weight of the fluid it pushes aside.
A fluid is a liquid or a gas. The upward push is called buoyancy, or upthrust. When you lower a bucket into a well, it feels lighter under water. The water is pushing it up.
The size of the push depends on the weight of the fluid the body moves out of the way. It does not depend on what the body is made of. A body sinks if its weight is more than the largest upthrust it can get. It floats if the upthrust can match its weight before it goes fully under.
A small example
A stone weighs 2.5 N in air. Its volume is 100 cm³. What does it weigh when fully under water? Take g = 10 m/s².
- Under water, the stone pushes aside 100 cm³ of water.
- 100 cm³ of water has a mass of 100 g, which is 0.1 kg.
- Its weight is 0.1 × 10 = 1 N. So the upthrust is 1 N.
- The stone's weight in water is 2.5 - 1 = 1.5 N.
The loss in weight, 1 N, equals the weight of water pushed aside. Comparing the two weights also gives relative density: 2.5 ÷ 1 = 2.5. So the stone is 2.5 times as dense as water.
Where it fits
This goes beyond the Class 10 board syllabus. HBCSE's Junior Science syllabus lists it under floatation, in the unit on motion, force and work. That syllabus is for NSEJS, which the Indian Association of Physics Teachers (IAPT) conducts.
Sources
Facts last checked against these sources on 30 September 2026.
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