Tamil Nadu Class 8 Science: Force and Pressure

Quick Summary

Force and Pressure is part of Term 1 in Class 8 Science. This chapter covers what force is, types of forces, the concept of pressure, atmospheric pressure, and Archimedes' principle — all fundamental ideas in physics.

Key Definitions

• Force: A push or pull that changes or tends to change the state of motion or shape of an object.
• Contact force: A force that acts only when objects are in physical contact (e.g., friction, tension).
• Non-contact force: A force that acts without physical contact (e.g., gravitational, magnetic, electrostatic).
• Pressure: Force applied per unit area. P = F/A.
• Atmospheric pressure: The pressure exerted by the atmosphere on all surfaces at the earth's surface (~101,325 Pa at sea level).
• Buoyancy: The upward force exerted by a fluid on an object submerged in it.
• Archimedes' principle: The buoyant force on a submerged object equals the weight of fluid displaced.

Key Formulae

Types of Force

Contact Forces

• Muscular force — exerted by muscles (e.g., pushing a cart)
• Frictional force — acts opposite to motion between surfaces
• Normal force — perpendicular support from a surface
• Tension — force in a stretched string or rope
• Spring force — force exerted by a compressed or stretched spring

Non-Contact Forces

• Gravitational force — attraction between masses
• Magnetic force — attraction/repulsion between magnets or magnetic materials
• Electrostatic force — force between electrically charged objects

Pressure in Liquids

• Pressure in a liquid increases with depth.
• Pressure at the same depth is equal in all directions.
• Liquid pressure depends on density and depth: P = ρgh

Atmospheric Pressure

• Caused by the weight of air above the earth's surface.
• Decreases as altitude increases.
• Measured using a barometer.
• Value at sea level: ~1.013 × 10⁵ Pa (1 atm).

Archimedes' Principle

• Buoyant force = weight of fluid displaced.
• An object floats if its weight ≤ buoyant force.
• An object sinks if its weight > buoyant force.
• Application: ships, submarines, hydrometers.

Important Questions

One Mark

• State Archimedes' principle.
• What is the SI unit of pressure?
• Give one example of a non-contact force.
• Define buoyancy.
• Write the formula for pressure.

Short Answer (3 marks)

• Distinguish between contact forces and non-contact forces with two examples each.
• Explain why a sharp needle exerts more pressure than a flat thumb, even with the same force applied.
• Describe how a submarine uses the principle of buoyancy to rise and sink.

Long Answer (5 marks)

• State and explain Archimedes' principle with an experiment. Write two applications of this principle.
• Explain atmospheric pressure and its effects. How does it change with altitude? Give two everyday examples.

Common Mistakes

• Confusing mass with weight (mass in kg, weight in N).
• Forgetting that pressure decreases with greater area (same force).
• Mixing up buoyancy with weight of the object itself.

Revision Checklist

• [ ] Textbook reading completed
• [ ] Formulae for pressure and buoyancy written and practised
• [ ] Contact and non-contact forces listed with examples
• [ ] Archimedes' principle experiment understood
• [ ] Important questions answered in notebook