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 us downhill," Sonali says — "what could it be?"
Key idea
What Is a Force?
Moving a box by pushing, pulling, or lifting all involve applying a force to it.
A push or pull applied on an object. Its SI unit is the newton (N).
Key idea
Effects of Force
1
Start motion
Make a resting object move.
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2
Change speed
Speed something up or slow it down.
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3
Change direction
Turn an object's path.
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4
Change shape
Squash, stretch, or bend an object.
Key idea
Force Is an Interaction
Two objects, always. A force always involves at least two objects interacting — a hand pushing a table means both the hand and the table experience a force. The force disappears the moment the interaction stops.
Key idea
Contact Forces
1
Muscular force
The force from your muscles — walking, lifting, pushing — and even internally, like chewing or your heartbeat.
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2
Friction
Opposes motion between two surfaces in contact. Rougher surfaces mean more friction — and liquids and gases resist motion too, which is why planes and ships are streamlined.
Key idea
Magnetic and Electrostatic Force
1
Magnetic force
A magnet acts on another magnet or magnetic material without touching it. Like poles repel, unlike poles attract.
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2
Electrostatic force
A charged object acts on another charged or uncharged object. Rubbing certain materials builds up static charge — like charges repel, unlike charges attract.
Key idea
Gravitational Force
The Earth attracts every object toward itselfthis is gravitational force.
Only ever attractiveUnlike magnetic or electrostatic force, gravity can never push, only pull — it's why a thrown ball always falls back down.
Key idea
Weight vs Mass
Location | Weight of a 1 kg mass
Key idea
Why Things Float or Sink
1
Buoyant force (upthrust)
A liquid pushes an immersed object upward, while gravity pulls it down. If gravity wins, it sinks; if the two balance, it floats.
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2
Archimedes' Principle
An object immersed in a liquid experiences an upward force equal to the weight of the liquid it displaces.
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.
Questions for your notebook
Write these down, then discuss
1Match each type of force to its example: Muscular force, Magnetic force, Frictional force, Gravitational force, Electrostatic force.
2True or False: (i) A force is always needed to change an object's speed. (ii) Friction increases a rolling ball's speed. (iii) There's no force between two nearby charged objects.
3Two balloons, each rubbed with a woollen cloth, are brought near each other. What happens, and why?
Project as-is — students copy the questions, then the class discusses answers together.