🌍 Gravitation – Class 9 Science
In this chapter, we learn how gravitational force holds the entire universe together, why objects fall, and what causes things to float or sink. Let’s explore!
🌌 Universal Law of Gravitation
Every object in the universe attracts every other object with a force called gravitational force.
Formula: F = G × (m₁ × m₂) / r²
- F = Gravitational force
- G = Gravitational constant
- m₁ and m₂ = Masses of two objects
- r = Distance between their centers
💡 Trivia:
Newton guessed the inverse square law by observing an apple falling and linking it to the Moon’s orbit!
🌠 Gravitational Constant (G)
- G = 6.674 × 10⁻¹¹ Nm²/kg²
- It is a universal constant, same everywhere in the universe
📌 Real-life Insight:
Gravitational force keeps the Earth revolving around the Sun and the Moon around the Earth.
🔁 Kepler’s Laws of Planetary Motion
- Law of Orbits: Planets move in elliptical orbits with the Sun at one focus.
- Law of Areas: A line joining the planet and the Sun sweeps equal areas in equal intervals.
- Law of Periods: T² ∝ R³ (square of time period ∝ cube of distance)
🔽 Free Fall & Acceleration Due to Gravity (g)
- Free fall happens when only gravity acts on a body.
- g = 9.8 m/s² on Earth
- It does not depend on mass of the object.
✅ Equations for freely falling bodies:
- v = u + gt
- s = ut + ½gt²
- v² = u² + 2gs
⚖️ Mass and Weight
- Mass is constant; Weight = mg
- Weight on Moon = 1/6 of weight on Earth
💡 Quote:
“Mass is how much you are made of. Weight is how hard gravity pulls you down.”
💥 Thrust and Pressure
- Thrust: Force acting perpendicularly on a surface
- Pressure = Thrust / Area
- More area → less pressure
📌 Everyday Examples:
- Knives have sharp edges to increase pressure
- Camels have wide feet to reduce pressure on sand
🌊 Pressure in Fluids & Buoyancy
- Fluids exert pressure in all directions
- Buoyancy: Upward force exerted by a fluid
✅ Buoyant Force Depends On:
- Volume of object immersed
- Density of fluid
🔬 Archimedes’ Principle
When a body is immersed in a fluid, it experiences an upward force equal to the weight of the displaced fluid.
📌 Applications:
- Hydrometers
- Ship design
- Submarines
⛵ Principle of Flotation
An object floats if its weight is equal to the buoyant force acting on it.
- Less dense objects float in denser fluids
- Ships float due to shape and displacement of water
📚 Quick Revision
- Gravitational force acts between all masses
- Free fall occurs under gravity alone
- g = 9.8 m/s²
- Weight = mg, varies from place to place
- Pressure = Force/Area
- Buoyancy causes floating
- Archimedes’ Principle explains buoyant force
💬 “Gravity explains the motion of planets, but it cannot explain who pulls us toward dreams.” – Einstein
🔗 Next Steps
👉 Go to the Gravitation Quiz to test yourself!
📘 Coming Next: Work and Energy – Class 9 Science