Quick answer: Gravitation combines two things students find hard: the universal law numericals with big powers of ten, and buoyancy. Both become simple with a fixed method.
Universal law
- F = G m₁m₂/d²; G = 6.67 × 10⁻¹¹ N m²/kg²
- Example: force between two 1 kg masses 1 m apart = 6.67 × 10⁻¹¹ N (tiny — that is why we only feel Earth’s gravity).
g and weight
- g = GM/R² = 9.8 m/s² on Earth; on the Moon about 1/6.
- Mass (kg) is constant; weight W = mg (N) changes with g.
- A 60 kg person weighs 588 N on Earth, 98 N on the Moon.
Free fall
Use the equations of motion with a = g (downward positive): v = gt, h = ½gt², v² = 2gh. Upward throw: a = −g, at the top v = 0.
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Example: A ball is thrown up at 20 m/s (g = 10). Max height = 400/20 = 20 m; time to top = 2 s; total time = 4 s.
Thrust and pressure
- Pressure = thrust/area (Pa). Sharp knife → small area → high pressure.
- Liquid pressure increases with depth.
Buoyancy and Archimedes
- Buoyant force = weight of liquid displaced = Vdisplaced × ρliquid × g.
- Object floats if its density < liquid density.
- Relative density = density of substance / density of water (no unit).
- Example: A block of 200 g displaces 150 cm³ of water → buoyant force = 150 × 1 × 10⁻³ × 10 = 1.5 N; weight = 2 N → it sinks.
Common mistakes
- Forgetting to square the distance
- Using g = 10 in one step and 9.8 in another
- Calling weight a scalar
Related: Force and Laws of Motion.
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Frequently asked questions
Are these Gravitation Class 9 Notes notes enough for the exam?
They cover every formula, definition and question type that appears in school and board papers for this chapter. Use them with the NCERT examples, exercise questions and the last five years' papers to be fully prepared.
How should I revise Gravitation Class 9 Notes?
Revise the notes after 1 day, 7 days and 30 days. Solve five questions each time instead of only reading, and keep an error list of the steps you get wrong.
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