GRAVITY

PRERNA FOR IAS

GRAVITY

1. What is Gravity?

Gravity is a fundamental force of attraction that exists between all objects having mass. According to Newton, every object in the universe attracts every other object, and the strength of this attraction depends on their masses and the distance between them. Larger masses produce stronger gravitational forces, while greater distances weaken the attraction. Einstein later explained gravity as the curvature of spacetime caused by massive objects. In this view, planets move around the Sun because they follow curved paths in warped spacetime. Gravity is responsible for keeping planets in orbit, causing objects to fall toward Earth, and shaping the structure of the universe.

2. Newton’s Universal Law of Gravitation

Newton’s Universal Law of Gravitation states that every particle in the universe attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. The mathematical expression is F = G(M₁M₂)/r², where F is the gravitational force, G is the universal gravitational constant, M₁ and M₂ are the masses, and r is the distance between their centers. The force acts along the line joining the two masses and is always attractive. This law successfully explains planetary motion, satellite orbits, and many gravitational phenomena.

3. The Gravitational Constant (G): Value and Units

The universal gravitational constant, denoted by G, is a fundamental constant used in Newton’s Law of Gravitation. Its value is approximately 6.674 × 10⁻¹¹ N·m²/kg². It represents the strength of gravitational attraction between two masses. The value of G is extremely small, showing that gravity is the weakest of the four fundamental forces. Despite its weakness, gravity dominates at astronomical scales because it acts over very large distances and is always attractive. The SI unit of G is Newton meter squared per kilogram squared. It is the same everywhere in the universe and does not vary with location.

4. Cavendish Experiment (Torsion Balance)

The Cavendish Experiment, conducted by Henry Cavendish in 1798, was designed to measure the value of the gravitational constant G. The experiment used a torsion balance consisting of small masses attached to a rod suspended by a thin fiber. Large masses placed nearby attracted the smaller masses, causing the rod to twist slightly. By measuring this tiny angular displacement, Cavendish calculated the gravitational force between the masses and determined the value of G. This experiment was one of the most precise measurements of its time and provided strong experimental support for Newton’s Law of Gravitation.

5. Why is G Tiny?

The gravitational constant G has a very small numerical value, making gravity the weakest of the four fundamental forces of nature. It is approximately 10⁴⁰ times weaker than the electromagnetic force between charged particles. This is why gravitational attraction between everyday objects is usually unnoticeable. However, gravity dominates on cosmic scales because it acts over infinite distances and is always attractive, unlike electric forces that can cancel each other. The small value of G explains why enormous masses, such as planets, stars, and galaxies, are needed to produce significant gravitational effects. Despite being weak, gravity governs the structure of the universe.

6. G vs g – Difference Between Them

Students often confuse G and g, but they represent different quantities. G is the universal gravitational constant with a fixed value of 6.674 × 10⁻¹¹ N·m²/kg², and it remains the same everywhere in the universe. g is the acceleration due to gravity and has a value of approximately 9.8 m/s² near Earth’s surface. Unlike G, the value of g changes with location, altitude, depth, and planetary conditions. G is used in calculating gravitational forces, while g describes the acceleration experienced by falling objects. Understanding this distinction is important for solving gravitational problems correctly.

7. Key Points of Gravity

Gravity is a universal force that acts between all objects possessing mass. It is always attractive and never repulsive. Gravity is a long-range force, meaning its influence extends across vast distances in space. The force becomes stronger when masses increase and weaker when the distance between objects increases. Gravity is responsible for the motion of planets, moons, satellites, stars, and galaxies. It also causes objects to fall toward Earth and gives them weight. The principle of superposition applies to gravity, meaning the total gravitational effect is the sum of individual gravitational forces acting on an object from multiple sources.