NCERT Class 9 Science Chapter 9 – Gravitation

NCERT Class 9 Science Chapter 9 explains the universal force of gravitation and its applications in everyday life and planetary motion. NCERT Class 9 Science Chapter 9 builds strong conceptual clarity about gravitational force, free fall, mass, weight and pressure, which are fundamental for higher physics.

The chapter introduces the Universal Law of Gravitation proposed by Sir Isaac Newton (1687), expressed mathematically as:

F = G (m₁m₂ / r²)

where G is the Universal Gravitational Constant (6.67 × 10⁻¹¹ N m² kg⁻²).

NCERT Class 9 Science Chapter 9 also explains acceleration due to gravity (g = 9.8 m/s²), equations of free fall, and the distinction between mass and weight. It further introduces thrust, pressure (1 Pascal = 1 N/m²), buoyancy and Archimedes’ Principle (287–212 BCE).

These concepts form the foundation for advanced topics like circular motion, satellites and fluid mechanics in senior secondary classes.

For structured preparation of NCERT Class 9–12 for UPSC, BPSC and State PCS examinations, strengthen your basics with our complete NCERT Book Notes PDF for Class 9-12, available inside the NCERT foundation course level-2.

Access Complete NCERT Book Notes PDF

1. Introduction

• Every object in the universe attracts every other object due to a force called gravitation.
• The motion of planets around the Sun and the Moon around the Earth is governed by gravitational force.
• The concept was mathematically explained by Sir Isaac Newton (1642–1727).
• Newton was inspired by the falling of an apple in Woolsthorpe, England.
• Gravitation explains phenomena such as falling of objects, tides in oceans and orbital motion of satellites.

2. Universal law of gravitation

• Proposed by Sir Isaac Newton in 1687 in his book Principia.
• Every object attracts every other object with a force proportional to product of their masses.
• Force is inversely proportional to square of distance between them.
• Mathematical expression: F = G (m₁m₂ / r²).
• Here, G is Universal Gravitational Constant.

3. Universal gravitational constant (G)

• Value of G = 6.67 × 10⁻¹¹ N m² kg⁻².
• Determined experimentally by Henry Cavendish (1798) using torsion balance experiment in England.
• G is constant throughout the universe.
• Unit of G is N m² kg⁻².

4. Importance of universal law

• Explains motion of planets around Sun.
• Explains motion of Moon around Earth.
• Explains tides due to gravitational pull of Moon.
• Explains why objects fall towards Earth.

5. Free fall

• When objects fall under gravitational force alone, motion is called free fall.
• Acceleration produced is called acceleration due to gravity (g).
• Value of g near Earth’s surface = 9.8 m/s².
• g is directed towards centre of Earth.
• For free fall, equations of motion apply with a replaced by g.

6. Acceleration due to gravity (g)

• g = GM/R², where M is mass of Earth and R is radius of Earth.
• Mass of Earth ≈ 6 × 10²⁴ kg.
• Radius of Earth ≈ 6.4 × 10⁶ m.
• g decreases with increase in altitude and depth.
• g is zero at centre of Earth.

7. Equations for free fall

• v = u + gt.
• s = ut + ½ gt².
• v² – u² = 2gs.
• If object dropped from rest, u = 0.
• Time to reach ground from height h = √(2h/g).

8. Mass and weight

• Mass is amount of matter in body.
• SI unit of mass is kilogram (kg).
• Mass remains constant everywhere.
• Weight is force with which Earth attracts object.
• Weight = W = mg.
• SI unit of weight is Newton (N).
• Weight changes with location due to variation in g.

9. Difference between mass and weight

• Mass is scalar quantity, weight is vector quantity.
• Mass measured by beam balance, weight measured by spring balance.
• On Moon, mass remains same but weight becomes 1/6th of Earth’s weight.
• g on Moon ≈ 1.63 m/s².

10. Thrust and pressure

• Thrust is force acting perpendicular to surface.
• SI unit of thrust is Newton (N).
• Pressure = Thrust / Area.
• SI unit of pressure is Pascal (Pa).
• 1 Pascal = 1 N/m².
• Smaller area increases pressure for same thrust.

11. Pressure in liquids

• Liquids exert pressure in all directions.
• Pressure increases with depth.
• Formula: Pressure = hρg, where h = depth, ρ = density.
• Unit of density = kg/m³.
• Example: Dams have thicker walls at bottom.

12. Buoyancy

• Liquids exert upward force called buoyant force.
• Buoyant force depends on density of liquid.
• Objects lighter than liquid float.
• Objects heavier than liquid sink.

13. Archimedes’ principle

• Discovered by Archimedes (287–212 BCE) in Syracuse, Greece.
• States that a body immersed in fluid experiences upward force equal to weight of fluid displaced.
• Used to determine relative density.
• Explains floating of ships.

14. Relative density

• Relative density = Density of substance / Density of water at 4°C.
• It has no unit.
• If relative density < 1, substance floats.
• If relative density > 1, substance sinks.

15. Numericals on gravitation

• Force between two 1 kg masses separated by 1 m = 6.67 × 10⁻¹¹ N.
• Weight of 50 kg person on Earth = 490 N.
• Weight on Moon = ≈ 81.5 N.
• Time taken for object to fall from 19.6 m = 2 seconds.
• Pressure exerted by 500 N force on area 2 m² = 250 Pa.

16. Applications of gravitation

• Artificial satellites revolve due to gravitational force.
• Geostationary satellites remain fixed relative to Earth.
• Tides caused due to gravitational pull of Moon and Sun.
• Orbiting bodies maintain balance between gravitational and centripetal force.

17. Conclusion

• Gravitation is universal attractive force.
• Universal law applies to all objects in universe.
• g explains free fall motion.
• Mass differs from weight.
• Archimedes’ principle explains buoyancy and floating.

18. Exam oriented facts

• Universal Gravitational Constant (G) = 6.67 × 10⁻¹¹ N m² kg⁻².
• Acceleration due to gravity (g) = 9.8 m/s² near Earth’s surface.
• Mass of Earth ≈ 6 × 10²⁴ kg.
• Radius of Earth ≈ 6.4 × 10⁶ m.
• Weight on Moon = 1/6th of Earth’s weight.
• Archimedes (287–212 BCE) – Gave principle of buoyancy.
• 1 Pascal = 1 N/m².
• Henry Cavendish (1798) – Measured value of G.
• Free fall – Motion under gravity alone.
• Relative density – Ratio of densities without unit.

Understanding NCERT Class 9 Science Chapter 9 is essential for explaining why objects fall toward Earth and how gravitational forces govern motion in the universe.

This chapter forms the base for advanced topics such as work-energy theorem, circular motion, orbital mechanics and fluid pressure.

For school examinations, students must focus on the Universal Law of Gravitation, value of G and g, difference between mass and weight, equations of free fall and Archimedes’ Principle.

For competitive examinations like JEE and NDA, NCERT Class 9 Science Chapter 9 provides fundamental clarity required for solving numerical problems in gravitation and fluid mechanics.

Continue reading NCERT Class 9 Science Chapter 10 – Work and Energy to understand energy transformation and conservation laws.

FAQs

Q1. What is NCERT Class 9 Science Chapter 9 about?
It explains the universal law of gravitation, acceleration due to gravity, free fall motion, mass and weight, pressure and Archimedes’ Principle.

Q2. What is the Universal Law of Gravitation?
It states that every object attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Q3. What is the value of acceleration due to gravity (g)?
The value of g near Earth’s surface is approximately 9.8 m/s².

Q4. What is Archimedes’ Principle?
It states that a body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced by it.

Q5. Why is NCERT Class 9 Science Chapter 9 important for competitive exams?
It builds the foundation for gravitational calculations, fluid pressure concepts and advanced mechanics topics required in engineering entrance examinations.

---