Archimedes Principle – Complete Notes, Revision, Important Questions & Downloads
This topic contains Buoyancy and Upthrust and Apparent Weight and Relative Density. NEET tests Archimedes Principle through direct upthrust calculations, apparent-weight reduction in liquids, relative-density formulae, and comparison of loss of weight in different fluids. The OCR derives the principle from hydrostatic pressure on the top and bottom faces of an immersed body, so the net force is not guessed but calculated as the weight of displaced fluid. Once that derivation is clear, the later formulas for apparent weight and relative density become immediate consequences instead of separate rules.
NEET Weightage - Archimedes Principle
Fluid Mechanics| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| NEET 2024 | 0 | 0 | |
| NEET 2023 | 0 | 0 | |
| NEET 2022 | 1 | 4 | |
| NEET 2021 | 0 | 0 | |
| NEET 2020 | 0 | 0 | |
| NEET 2019 | 0 | 0 | |
| Total (2019-2024) | 1 | 4 |
For a fully submerged body, upthrust is Vσg, where σ is the density of the fluid displaced.
Apparent weight is actual weight minus upthrust, so it depends on both body density and fluid density.
Relative density can be obtained directly from weight in air and loss of weight in water, a favourite objective-question format.
Archimedes Principle Strategy for NEET
Start with displaced fluid, not with the body mass Upthrust is controlled by the fluid displaced and the immersed volume, so identify the displaced-fluid density and immersed volume before looking at body density.
Derive the direction from pressure difference The lower face is deeper and therefore feels larger pressure than the upper face, which is why the net force is upward. That derivation keeps the sign correct in mixed questions.
Use apparent weight as a subtraction formula Write Wapp = W - Fup every time. If the body becomes lighter in the fluid, the missing weight is exactly the buoyant force.
Translate weighing data into relative density When the question gives weight in air and weight in water, the loss of weight is the water thrust, so RD = Wair/(Wair - Wwater) follows immediately.
Check whether the body is fully or partly immersed The OCR derivation is shown for a fully submerged block but states that the result also holds for partial immersion. In numericals, the immersed volume is the key quantity.
Archimedes Principle Study Materials
PDF · Cheat Sheet · MCQ Set · PYQArchimedes Principle Subtopics
2-Column TableRapid Revision - Archimedes Principle
Concept → Trap → Example1) Buoyancy and Upthrust
Weight of displaced fluidArchimedes principle states that a body immersed partly or wholly in a fluid is buoyed up by a force equal to the weight of the fluid displaced by it. For a fully submerged body, upthrust is F = Vσg.
- The force acts vertically upward through the centre of buoyancy, the centre of gravity of the displaced fluid.
- Upthrust depends on the nature of the fluid and immersed volume, not on the body's mass alone.
- The trap is to use the density of the body instead of the density of the displaced fluid in the buoyant-force formula.
2) Apparent Weight and Relative Density
Weighing in air and waterApparent weight equals actual weight minus upthrust, so Wapp = W(1 - σ/ρ) for a body of density ρ immersed in a fluid of density σ. Relative density of a body is RD = W1/(W1 - W2), where W1 is weight in air and W2 is weight in water.
- Loss of weight in water equals the buoyant force in water.
- Comparing losses of weight in two liquids gives the ratio of their densities.
- Students often use W2/W1 for relative density, which reverses the actual weighing relation.
US Curriculum Gaps - Archimedes Principle
What U.S. Students Usually MissIntroductory buoyancy units often stop at the statement of the principle
NEET expects the derivation from pressure difference between the top and bottom surfaces and then uses that derivation in quick quantitative questions on upthrust, immersed volume, and apparent weight.
- Upthrust comes from deeper pressure at the lower face.
- Displaced-fluid density, not body density, appears in the buoyant-force formula.
Relative-density formulas from weighing data are not always emphasised
The OCR turns Archimedes Principle into a measurement tool: weight in air, weight in water, and loss of weight all become exam-ready variables. That compressed experimental form is heavily favoured in NEET-style practice.
- RD uses weight in air divided by loss of weight in water.
- Comparing losses of weight in two liquids gives a density ratio.
Concept IQ Check - Archimedes Principle
4 NEET-style MCQs with AnswersPractice Questions - Archimedes Principle
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Archimedes Principle FAQs
Notes · Downloads · Revision · Important QuestionsWhy is the buoyant force upward?
What is the centre of buoyancy?
Does upthrust depend on the body's mass?
Why is apparent weight smaller in a fluid than in air?
Why is a body lighter in sea water than in fresh water?
How is relative density measured using Archimedes Principle?
Can Archimedes Principle be used for partially immersed bodies?
What is the most common numerical mistake in this topic?
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