Gravitation — Study Notes

Overview

Gravitation is one of the fundamental forces of nature and a high-yield topic in RRB Group D General Science. This chapter explains why objects fall to the ground, how planets orbit the Sun, and how satellites stay in orbit. Questions typically test Newton's law of universal gravitation, the distinction between mass and weight, concepts of free fall and acceleration due to gravity, and satellite motion. Expect 2–3 direct questions in the exam covering formula-based calculations, conceptual understanding of g-values at different locations, and practical applications like satellite launches. Mastery requires understanding the inverse-square law, memorizing key formulas, and applying them to numerical problems involving weight, gravitational force, and orbital motion.

Key Concepts

• **Universal Law of Gravitation**: Every object in the universe attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres. This is the foundation of planetary motion and satellite dynamics.

• **Mass vs Weight**: Mass is the quantity of matter in an object (measured in kg, constant everywhere). Weight is the gravitational force acting on that mass (measured in N, varies with location). Weight = mass × acceleration due to gravity.

• **Acceleration due to Gravity (g)**: On Earth's surface, g ≈ 9.8 m/s² (often approximated as 10 m/s² for quick calculations). This value decreases with altitude and varies slightly with latitude due to Earth's rotation and non-spherical shape.

• **Free Fall**: When an object falls under gravity alone with no air resistance, it is in free fall. All objects in free fall near Earth's surface accelerate at the same rate (g), regardless of their mass—a feather and a hammer fall at the same rate in vacuum.

• **Variation of g**: The value of g decreases as we go above the Earth's surface (on mountains, in aeroplanes) or below the surface (in mines). At the centre of Earth, g = 0.

• **Gravitational Potential Energy**: The energy possessed by an object due to its position in a gravitational field. Near Earth's surface: PE = mgh, where h is height above reference level.

• **Satellites**: Objects that revolve around a planet in a fixed orbit. Artificial satellites stay in orbit because their tangential velocity balances the gravitational pull, creating a continuous free fall around Earth.

• **Escape Velocity**: The minimum velocity needed for an object to break free from a planet's gravitational field without further propulsion. For Earth, escape velocity ≈ 11.2 km/s.