NCERT Curiosity · Class 8 · Science
Exploring Forces
Pedaling into the wind feels hard. Coasting downhill, you speed up without pedaling at all — something is pulling you down.
Contact Forces Magnetic & Electrostatic Gravity Buoyancy
Key idea
What a Force Can Do
A force is a push or pull applied on an object — its SI unit is the newton (N).

Start motion

Make a resting object move.

Change speed

Speed up or slow down.

Change direction

Turn an object's path.

Change shape

Squash, stretch, or bend it.

Key idea
A force always involves two objects interacting.
A hand pushing a table means both the hand and the table experience a force. It disappears the moment the interaction stops.
Key idea
Contact Forces
Muscular force

From your muscles — walking, lifting, even chewing or your heartbeat.

Friction

Opposes motion between surfaces — rougher surfaces mean more friction.

Key idea
Magnetic and Electrostatic Force
Magnetic force

Acts without touching — like poles repel, unlike poles attract.

Electrostatic force

Rubbing builds up static charge — like charges repel, unlike charges attract.

Key idea
Gravity can never push, only pull.
The Earth attracts every object toward itself — unlike magnetic or electrostatic force, gravity is always attractive. That's why a thrown ball always falls back down.
Key idea
Weight of a 1 kg Mass, Around the Solar System

Earth

10 N

Moon

1.6 N

Mars

3.8 N

Venus

9 N

Jupiter

25.4 N

Key idea
An immersed object feels an upward push equal to the liquid it displaces.
That's Archimedes' Principle. If gravity wins, the object sinks; if the two forces balance, it floats.
Chapter · key terms to remember
Key Terms

Force

A push or pull on an object; SI unit newton (N).

Contact force

Acts only when objects are touching.

Friction

Opposes motion between touching surfaces.

Chapter · key terms to remember
More Key Terms

Non-contact force

Acts even without touching.

Gravitational force

Earth's pull on objects; always attractive.

Weight

Force of Earth's pull on an object.

Chapter · key terms to remember
More Key Terms

Mass

Amount of matter in an object; constant everywhere.

Buoyant force (Upthrust)

Upward force a liquid exerts on an object.

Opening

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