Chapter 9 · NCERT Class 9 Science

Gravitation

The same pull that drops an apple keeps the Moon circling the Earth. One law, F = G·m₁·m₂ ÷ d², runs the whole show — from falling stones to the tides. Tap each idea to see how it works.

🌍 3 topics⏱ ~25 min📝 12-question quiz

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The six ideas of gravitation

Gravity is one force, but it shows up under different names. Tap each term to see what it means, its unit, and how it connects to the others.

Explore · Gravitationtap a term

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The three big ideas

Every mass attracts every other mass. The force is F = G·m₁·m₂ ÷ d² — bigger masses pull harder; the force weakens as the square of the distance grows.
G is the universal gravitational constant — the same everywhere in the universe.
The same force makes an apple fall to the ground and keeps the Moon in orbit around the Earth.

Acceleration due to gravity near the Earth is g ≈ 9.8 m/s².
In free fall only gravity acts, so all objects fall at the same rate — a feather and a stone land together when there is no air to slow the feather.

Mass — the amount of matter in an object. Constant everywhere. Unit: kilogram (kg).
Weight — the force of gravity on the object: W = m·g. It changes with g. Unit: newton (N).
On the Moon g is about 1/6 of Earth’s, so weight is about one-sixth — but the mass is unchanged.

Worked example. A 10 kg box rests on the Earth where g = 9.8 m/s². How much does it weigh?

W = m·g = 10 × 9.8 = 98 N.

Common mistake: treating mass and weight as the same thing. Mass (kg) stays constant; weight (N) depends on g — so you weigh less on the Moon even though your mass is unchanged.

Where you'll meet it

Gravity at work

Satellites & orbits

A satellite does not fall to Earth because Earth’s gravity bends its path while its forward motion carries it sideways. Gravity + motion = a stable orbit — the same balance that holds the Moon.

Ocean tides

The Moon’s gravity tugs hardest on the water nearest it, raising a bulge of ocean. As the Earth turns, coastlines pass through these bulges, giving the daily rise and fall of high and low tides.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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