Buoyancy MCQs Quiz | Class 9
Practice Class IX Science (Code 086) Unit III: Motion, Force and Work MCQs on Buoyancy. This quiz covers key concepts including Upthrust and factors affecting buoyant force such as fluid density and object volume. Solve these 10 multiple-choice questions to test your understanding, view your score instantly, and download the detailed answer key PDF for revision.
Understanding Buoyancy and Upthrust
Buoyancy is a fundamental concept in Unit III: Motion, Force and Work for Class 9 Science. It explains why objects feel lighter when submerged in a fluid and why ships float while iron nails sink.
1. What is Buoyant Force (Upthrust)?
When an object is immersed partially or wholly in a fluid (liquid or gas), the fluid exerts an upward force on the object. This upward force is known as upthrust or buoyant force. Its S.I. unit is the Newton (N).
2. Factors Affecting Buoyant Force
The magnitude of the buoyant force depends primarily on two factors:
- Volume of the object immersed: The larger the volume of the object submerged in the fluid, the greater the upward force. The force is maximum when the object is completely immersed.
- Density of the fluid: Denser fluids exert a greater buoyant force. For example, it is easier to swim in sea water than in fresh water because sea water is denser (due to dissolved salts) and provides more upthrust.
3. Conditions for Floating and Sinking
Whether an object floats or sinks depends on the relationship between its weight (W) acting downwards and the buoyant force (U) acting upwards:
| Condition | Result | Density Comparison |
|---|---|---|
| Weight > Upthrust | Object Sinks | Density of Object > Density of Fluid |
| Weight = Upthrust | Object Floats (fully submerged) | Density of Object = Density of Fluid |
| Weight < Upthrust | Object Floats (partially submerged) | Density of Object < Density of Fluid |
Quick Revision Points
- Archimedes’ Principle: The buoyant force is equal to the weight of the fluid displaced by the object.
- Apparent Weight: An object appears to lose weight when submerged. Apparent Weight = True Weight – Buoyant Force.
- Temperature Effect: As temperature increases, fluid density generally decreases, slightly reducing the buoyant force.
Extra Practice Questions
- Why does a mug full of water feel heavier when lifted out of the water surface?
- How does the density of a liquid affect the level at which a hydrometer floats?
- Explain why a steel ship floats but a steel ball sinks.
- If two balls of same volume but different materials (wood and iron) are fully submerged in water, which one experiences greater upthrust? (Hint: Same volume means same upthrust).
- What happens to the buoyant force if the density of the liquid is doubled?
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