Gravitation
Physics · गुरुत्वाकर्षण
📋Quick Overview
Gravitation is the force of attraction between any two objects in the universe. Newton discovered this when he saw an apple fall from a tree (1687). Every object attracts every other object. The Earth pulls us down — that's why we stay on the ground. The Sun pulls the Earth — that's why Earth orbits the Sun.
📖Newton's Law of Universal Gravitation
F = G × m₁ × m₂ / r². Every object attracts every other object with a force that is: (1) Directly proportional to the product of their masses (m₁ × m₂), (2) Inversely proportional to the square of distance between them (r²). G = Gravitational constant = 6.674 × 10⁻¹¹ N·m²/kg². G was first measured by Henry Cavendish using a torsion balance.
If distance doubles, gravitational force becomes 1/4th (because r²). If distance halves, force becomes 4 times. This is the inverse square law.
📖Weight vs Mass
Weight on Moon = 1/6th of weight on Earth (because Moon's gravity is 1/6th of Earth). If you weigh 60 kg on Earth, your weight on Moon = 10 kg-force. But your MASS stays 60 kg everywhere!
📝Key Concepts
- •Acceleration due to gravity (g) = 9.8 m/s² on Earth's surface. For calculations, we often use g = 10 m/s²
- •g does NOT depend on mass of falling object — a feather and a rock fall at same speed in vacuum (Galileo proved this from Tower of Pisa)
- •g is maximum at poles and minimum at equator (Earth is not perfectly round)
- •g decreases as we go UP (above Earth) or DOWN (inside Earth)
- •Free fall = when only gravity acts on an object. In free fall, object is weightless!
- •Escape velocity = minimum velocity to escape Earth's gravity = 11.2 km/s
- •Orbital velocity of satellite near Earth = 7.9 km/s
📝Kepler's Laws of Planetary Motion
- •1st Law (Law of Orbits): Planets move in ELLIPTICAL orbits with the Sun at one focus
- •2nd Law (Law of Areas): A planet sweeps equal areas in equal times — moves FASTER near Sun, SLOWER far from Sun
- •3rd Law (Law of Periods): T² ∝ r³ — square of time period is proportional to cube of orbital radius