NCERT Curiosity · Class 9 · Science
How Forces Affect Motion
A canoe moves forward when the paddle pushes water backwards — and faster still, the harder that push.
Newton's Laws Friction Weight Action & Reaction
Key idea
A force always needs a direction along with its magnitude.
Force can set an object moving, change its speed or direction, or change its shape — measured in newtons (N).
Key idea
Balanced and Unbalanced Forces
Balanced

Equal and opposite — they cancel out, so motion doesn't change.

Unbalanced

A non-zero net force results, and the object accelerates in that direction.

Key idea
A box moves only once your push exceeds friction.
Once moving, friction alone brings it to rest if you stop pushing — why a rolling ball or coasting bicycle eventually stops.
Key idea
An object keeps its motion unless a net force acts on it.
Newton called this tendency inertia — first argued by Galileo, who reasoned a moving object needs no force to keep moving forever, if nothing opposes it.
Key idea
F = ma
Acceleration is proportional to the net force, and inversely proportional to mass. 1 newton gives a 1 kg object an acceleration of 1 m/s².
Key idea
F = mg
A 30 kg barbell needs an upward force of 30 × 9.8 = 294 N from the weightlifter, just to hold it steady.
Key idea
Slowing Down Gently Reduces Force
A longer stop

Pulling hands back while catching a ball stretches the stopping time, lowering the force.

Airbags, and coconuts

Airbags cushion a stop; a coconut on hard ground stops instantly, needing enough force to crack its shell.

Key idea
Every force has an equal, opposite reaction force.
The paddle pushes water backwards, and the water pushes the paddle and canoe forward — two forces on two different objects, so they never cancel out.
Key idea
A system of connected objects accelerates like one combined mass.
Two boxes connected by a string, pulled by force F, accelerate as a = F ÷ (m₁ + m₂) — internal tension stays invisible from the outside.
Chapter · key terms to remember
Key Terms

Force

A push or pull, measured in newtons (N).

Balanced / Unbalanced forces

Cancel out / leave a non-zero net force.

Friction

A force opposing relative motion between surfaces.

Chapter · key terms to remember
More Key Terms

Newton's First Law

No net force means no change in motion (inertia).

Newton's Second Law

F = ma — force, mass, and acceleration linked.

Weight

The gravitational force on an object, F = mg.

Chapter · key terms to remember
More Key Terms

Newton's Third Law

Every force has an equal, opposite reaction force.

Tension

The internal pulling force transmitted through a string.

Opening

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