Floatation (Concept) MCQs Quiz | Class 9
This quiz for Class IX Science (086), Unit III: Motion, Force and Work, tests your understanding of Floatation. It covers key concepts such as why objects float or sink and the role of buoyancy. Submit your answers to see your score and download the PDF solution.
Overview of Floatation
Floatation is a phenomenon where an object remains suspended on the surface of a fluid or within it. This concept is governed by the forces acting on the object: the gravitational force pulling it downwards and the buoyant force pushing it upwards. Understanding floatation involves mastering the concepts of density, pressure, and buoyancy.
Key Concepts
- Buoyancy: The upward force exerted by a fluid on an object immersed in it. This is also known as upthrust.
- Thrust and Pressure: Thrust is the force acting perpendicularly on a surface, while pressure is thrust per unit area.
- Density: Mass per unit volume. The density of an object relative to the fluid determines whether it floats or sinks.
Why Objects Float or Sink
Whether an object floats or sinks depends on the relationship between the density of the object and the density of the fluid.
| Condition | Result | Force Relationship |
|---|---|---|
| Density of Object < Density of Fluid | Floats | Buoyant Force > Weight |
| Density of Object = Density of Fluid | Suspended (just floats) | Buoyant Force = Weight |
| Density of Object > Density of Fluid | Sinks | Buoyant Force < Weight |
Quick Revision Notes
- The magnitude of the buoyant force depends on the volume of the immersed part of the object and the density of the fluid.
- An iron nail sinks because its density is greater than water, whereas a ship made of iron floats because its shape displaces a large volume of water, creating enough upthrust to support its weight.
- Relative density is the ratio of the density of a substance to the density of water. It has no unit.
Extra Practice Questions
1. Define 1 Pascal.
2. Why is it easier to swim in sea water than in river water?
3. What is the direction of buoyant force?
4. If the relative density of silver is 10.8, what is the density of silver in SI units?
5. Explain why a block of plastic released underwater comes up to the surface.
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