Archimedes’ Principle MCQs Quiz | Class 9
Test your understanding of Class IX Science (Code 086) Unit III: Motion, Force and Work. This quiz covers Archimedes’ Principle, specifically the statement and the displacement idea. Solve these 10 MCQs, check your score, and download the PDF answer key for revision.
Understanding Archimedes’ Principle
Archimedes’ Principle is a fundamental concept in fluid mechanics that explains why objects float or sink. It states that when a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.
Key Concepts
- Upthrust (Buoyant Force): The upward force exerted by a fluid on an immersed object. Its magnitude depends on the density of the fluid and the volume of the immersed part of the body.
- Displacement Idea: When an object enters water, it pushes water out of the way (displaces it). The volume of water displaced equals the volume of the submerged part of the object.
- Apparent Weight: An object feels lighter in water because the upthrust counteracts gravity. Apparent Weight = Real Weight – Upthrust.
Why Objects Float or Sink?
| Condition | Result |
|---|---|
| Density of Object < Density of Fluid | Object Floats |
| Density of Object > Density of Fluid | Object Sinks |
| Density of Object = Density of Fluid | Floats just below the surface |
Applications
This principle is used in designing ships and submarines, lactometers (to check milk purity), and hydrometers (to determine liquid density).
Quick Revision List
- SI unit of Upthrust is Newton (N).
- Buoyant force acts vertically upwards.
- Iron ships float because their shape displaces a large amount of water, making the average density less than water.
Extra Practice Questions
- Q: Why does a block of plastic released under water come up to the surface?
A: Because the upthrust acting on it is greater than its weight. - Q: What happens to the buoyant force if the density of the fluid increases?
A: The buoyant force increases. - Q: Does Archimedes principle apply to gases?
A: Yes, it applies to all fluids (liquids and gases). - Q: What is the direction of buoyant force?
A: Vertically upwards. - Q: Why is it easier to swim in sea water than in river water?
A: Sea water is denser, providing more upthrust.
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