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Most Class 9 gravitation material online was written for the old NCERT book. From 2026–27 that book has been replaced and the gravitation chapter is gone, but the ideas have only moved. This post shows where each one now sits, works through the calculations you still need, and points out the mistakes that keep turning up.
Where did the gravitation chapter go?
The CBSE syllabus for 2025–26 listed "Gravitation; Universal Law of Gravitation, Force of Gravitation of the earth (gravity), Acceleration due to Gravity; Mass and Weight; Free fall" and, separately, "Floatation: Thrust and Pressure. Archimedes' Principle; Buoyancy."
The CBSE Class IX syllabus for 2026–27 follows the new NCERT book *Exploration*, which has no chapter by that name. Here is where the pieces are now:
g as the acceleration of a falling object: Chapter 4, Describing Motion Around Us, Example 4.4.
Weight as a force, F = mg: Chapter 6, How Forces Affect Motion, which also notes that g does not depend on the mass of the object.
Gravitational potential energy, mgh, and energy in free fall: Chapter 7, Work, Energy, and Simple Machines.
Mass versus weight, buoyant force and Archimedes' principle: Chapter 5 of the Grade 8 book Curiosity, Exploring Forces.
The universal law, F = Gm₁m₂/r², with G: NCERT Class 11 Physics, Chapter 7, Gravitation.
The CBSE competency list for Classes 9 and 10 still mentions explaining mass and weight "using universal law of gravitation", so check with your teacher what your school will assess. If your board still uses the older book, all of the ideas below are in your syllabus.
What is g, and does a heavier ball fall faster?
NCERT's Example 4.4 drops an object and records its velocity every second: 9.8, 19.6, 29.4 and 39.2 m/s. The velocity grows by 9.8 m/s each second, so the acceleration is constant at 9.8 m/s². This is g. Ignoring air resistance, it is the same for a heavy ball and a light one, which is why Chapter 6 stresses that g does not depend on mass.
Worked example: free fall. A stone is dropped from a 20 m high terrace. Ignore air resistance and take g = 9.8 m/s².
Time to fall: s = ½gt², so t = √(2s/g) = √(40/9.8) ≈ 2.0 s.
Speed on landing: v² = 2gs = 2 × 9.8 × 20 = 392, so v ≈ 19.8 m/s.
At the top of a ball's upward throw, its velocity is zero but its acceleration is still g, downwards.
What is the difference between mass and weight?
Mass is the amount of matter, in kilograms, and is the same everywhere. Weight is the gravitational pull on that mass, in newtons, and depends on where you are.
Worked example: you on the Moon. A student has a mass of 50 kg.
On Earth: W = mg = 50 × 9.8 = 490 N.
On the Moon, where NASA's lunar fact sheet gives surface gravity as 1.62 m/s²: W = 50 × 1.62 = 81 N.
The mass is still 50 kg. The weight is about one-sixth, since 1.62 ÷ 9.8 ≈ 0.17.
Where does g come from? (Class 11 preview)
Newton's universal law says every two masses attract with F = Gm₁m₂/r². The Class 11 NCERT chapter gives G = 6.67 × 10⁻¹¹ N m²/kg². For an object at the surface of a planet of mass M and radius R, this gives g = GM/R².
Using NASA's values for Earth (mass 5.97 × 10²⁴ kg, mean radius 6371 km): g = (6.67 × 10⁻¹¹ × 5.97 × 10²⁴) ÷ (6.371 × 10⁶)² ≈ 9.8 m/s². The same formula with the Moon's values (mass 7.35 × 10²² kg, radius 1737 km) gives about 1.62 m/s². Henry Cavendish measured this kind of pull between lead balls in a laboratory in 1798, in his experiments to determine the density of the Earth.
The pull between everyday objects is tiny. Two 50 kg students 1 m apart attract with F = 6.67 × 10⁻¹¹ × 50 × 50 ÷ 1² ≈ 1.7 × 10⁻⁷ N. When you reach Class 11, our guide to gravitation and satellite problems picks up from here.
Why do some objects float?
Grade 8 *Curiosity* explains that a liquid pushes up on an object with a buoyant force, and states Archimedes' principle: the upward force equals the weight of liquid displaced. If the object's weight is more than the largest buoyant force it can get, it sinks.
Worked example: a wooden block. A block has mass 0.5 kg and volume 600 cm³. Water has a density of 1 g/cm³. Take g = 9.8 m/s².
Weight of the block: 0.5 × 9.8 = 4.9 N.
If pushed fully under, it displaces 600 g of water, so the buoyant force is 0.6 × 9.8 = 5.88 N. That is more than its weight, so it rises.
It floats when the displaced water weighs 4.9 N, that is, when 500 cm³ is under water. So 500/600, or five-sixths, of the block sits below the surface.
For the story behind the principle, read our post on Archimedes and buoyancy.
Which mistakes come up most?
Writing weight in kilograms.
Believing heavier objects fall faster in the absence of air resistance.
Saying acceleration is zero at the top of a throw.
Thinking only floating objects feel a buoyant force. A sinking stone feels one too; it is just smaller than its weight.
Forgetting to square r in the universal law.
To test your predictions on falling and floating, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read, so you can test a prediction before you calculate it.
A practice routine
Redo NCERT Example 4.4 and Chapter 6 Example 6.4 (the barbell, 294 N) without looking.
Calculate your own weight on Earth and on the Moon.
Do two floating problems: one object that floats and one that sinks, with the buoyant force found each time.
If you are aiming for Class 11, calculate g for Earth from G, M and R once by hand.
Start by dropping two different balls from the same height at home and noting whether they land together.
