General Science Notes Chapter 3 Force and Pressure Class 8 Maharashtra Board

Notes For All Chapters – General Science Class 8

What is Force?

• Force is a push or pull applied to an object.
• It changes the speed, direction, or shape of an object.
• The SI unit of force is Newton (N).

Effects of Force:

1. Can make an object move (A football moves when kicked).
2. Can stop a moving object (A bicycle stops when brakes are applied).
3. Can change the speed of an object (A car speeds up when we press the accelerator).
4. Can change the direction of motion (A cricket ball changes direction after hitting the bat).
5. Can change the shape of an object (Clay can be shaped into a pot).

Types of Forces

Forces are divided into two types:

1. Contact Force (Needs direct contact)

• Force that acts when two objects touch each other.

2. Non-Contact Force (Acts from a distance)

• Force that acts without touching the object.

Friction – A Type of Contact Force

• Friction is the force that opposes motion between two surfaces.
• It acts in the opposite direction of movement.

Examples of Friction:

✔ Helps us to walk without slipping.

✔ Allows vehicles to stop when brakes are applied.

✔ Helps in writing with a pen or pencil.

Disadvantages of Friction:

❌ Causes wear and tear of machines.

❌ Produces heat in moving parts of machines.

❌ Reduces efficiency in vehicles.

Balanced and Unbalanced Forces

• Balanced Forces: When two equal forces act in opposite directions, there is no movement (e.g., pushing a wall).
• Unbalanced Forces: When one force is stronger than the other, the object moves in the direction of the stronger force (e.g., kicking a football).

Example of Balanced and Unbalanced Forces:

• Tug of War: If both teams pull with equal force, the rope does not move (balanced force). If one team pulls harder, the rope moves in that direction (unbalanced force).

Inertia and Newton’s First Law of Motion

• Inertia is the tendency of an object to stay in its current state (rest or motion).
• Newton’s First Law of Motion states:”An object at rest stays at rest, and an object in motion stays in motion, unless acted upon by an external force.”

Examples of Inertia:

1. Inertia of Rest: A book on a table stays there unless pushed.
2. Inertia of Motion: A moving car stops slowly after brakes are applied.
3. Inertia of Direction: When a fast-moving car turns suddenly, passengers lean sideways.

What is Pressure?

• Pressure is the force applied per unit area.
• Formula: Pressure=Force / Area
• SI Unit of Pressure: Pascal (Pa) or N/m².

How Area Affects Pressure?

• Smaller area = Higher pressure (e.g., sharp knife cuts better).
• Larger area = Lower pressure (e.g., wide camel feet prevent sinking in sand).

Buoyant Force and Archimedes’ Principle

What is Buoyant Force?

• Buoyant force is the upward force exerted by a liquid on an object.
• It helps objects float in water.

Archimedes’ Principle:

“When an object is placed in a liquid, it experiences an upward force equal to the weight of the displaced liquid.”

Examples:

✔ A boat floats because it displaces enough water to balance its weight.

✔ A stone sinks because its weight is more than the buoyant force.

Atmospheric Pressure

The air around the Earth exerts pressure called atmospheric pressure.

Atmospheric pressure decreases as we go higher (e.g., on mountains).

Examples of Atmospheric Pressure:

• Drinking juice with a straw.
• A suction cup sticking to a surface.

Applications of Force and Pressure in Daily Life