GRAVITATION CLASS 9 NCERT NOTES

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CHAPTER NOTES – GRAVITATION (Pointwise, Clear & Complete)

10.1 Gravitation

  • Objects fall towards Earth and planets orbit the Sun → a force must be acting.
  • Newton realized the same force causes:
    • Apple falling
    • Moon orbiting Earth
  • Centripetal force keeps bodies moving in circular paths.
  • Earth’s gravity provides centripetal force for the moon.

10.1.1 Universal Law of Gravitation

  • Every object attracts every other object.
  • Force is:
    • ∝ product of masses (M × m)
    • ∝ 1 / distance² between them (1/d²)
  • Formula:
    F = GMm / d²
  • G = Universal gravitational constant
    = 6.67 × 10⁻¹¹ N m² kg⁻²
  • Force acts along the line joining centers of both bodies.

Importance of Universal Law of Gravitation

It explains—

  1. Why we stay on Earth.
  2. Moon’s motion around Earth.
  3. Planetary motion around the Sun.
  4. Tides (due to moon & sun).

10.2 Free Fall

  • Objects falling under gravity alone are in free fall.
  • They undergo an acceleration g (acceleration due to gravity).
  • g on Earth ≈ 9.8 m/s²

Deriving ‘g’

Using:
F = mg
and
F = GMm/R²
→ g = GM / R²

  • g is maximum at poles, minimum at equator.

10.2.2 Motion Under Gravity

Use equations of motion with a = g:

  • v = u + gt
  • s = ut + ½gt²
  • v² = u² + 2gs
    (Use +g downward, –g upward)

10.3 Mass

  • Measure of inertia.
  • Mass is constant everywhere (earth, moon, space).

10.4 Weight

  • Weight = gravitational force on an object.
  • W = mg
  • Unit: Newton (N)
  • Weight changes from place to place because g changes.

10.4.1 Weight on the Moon

  • Moon’s gravity is 1/6 of Earth’s.
  • Therefore:
    W(moon) = (1/6) × W(earth)

10.5 Thrust and Pressure

  • Thrust = force acting perpendicular to a surface.
  • Pressure = thrust / area
    → P = F/A
  • Unit: Pascal (Pa) = N/m²
  • Same force on smaller area → higher pressure.

10.5.1 Pressure in Fluids

  • Fluids (liquids and gases) also exert pressure.
  • Pressure is transmitted equally in all directions in a fluid.

10.5.2 Buoyancy

  • Fluids exert an upward force on immersed objects → Buoyant force or Upthrust.
  • Buoyant force increases with depth.
  • If buoyant force > weight → object rises.

10.5.3 Floating or Sinking

  • Depends on density.
  • If density of object < density of fluid → floats.
  • If density of object > density of fluid → sinks.

10.6 Archimedes’ Principle

When a body is fully/partially immersed in a fluid, it experiences an upward buoyant force equal to the weight of fluid displaced.

Applications:

  • Designing ships & submarines
  • Hydrometers
  • Lactometers

10.7 Relative Density

Relative Density =
Density of substance / Density of water

  • No unit (ratio).

End-of-Chapter Summary (Ultra-Condensed)

  • Gravity attracts all objects.
  • Universal law: F = GMm/d²
  • Free-fall acceleration g = 9.8 m/s² (near Earth)
  • Mass constant; weight changes (W = mg)
  • Moon’s gravity = 1/6 of Earth
  • Pressure = force/area
  • Buoyant force = weight of displaced fluid
  • Float if density < liquid, sink if >
  • Relative density = ratio of densities

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