Gravitation

5 min

Universal Law, Gravity, and Planetary Motion

Universal Law of Gravitation

The Universal Law of Gravitation states that every mass attracts another with a force. This force depends on the product of masses and distance squared.

Exam Importance

Weight on Moon is exactly 1/6th of your weight on Earth.
Escape velocity for Earth is fixed at 11.2 km/s for exams.
Kepler's Laws are frequently asked in Railway and SSC exams.

Quick Memory Hooks

G is Grand (Universal Constant), g is ground (varies with place).
Mass is Main (never changes), Weight is Wavering (changes everywhere).
11.2: One-One-Two, Earth says 'TATA' to you (Escape velocity).

Mass vs Weight Comparison

Feature	Mass	Weight
Definition	Quantity of matter	Force of gravity
SI Unit	Kilogram (kg)	Newton (N)
Variation	Constant everywhere	Changes with g

Visual Guide to Gravity

F = GmM/r^2

(Force Formula)

Force Formula

Force weakens as distance increases.

g = 9.8 m/s^2

(Earth Gravity)

Earth Gravity

Objects fall at this acceleration.

11.2 km/s

(Escape Speed)

Escape Speed

Speed needed to leave Earth.

Ellipse

(Kepler's Orbit)

Kepler's Orbit

Planets move in oval paths.

Common Exam Traps

Mass of an object changes on the MoonMass is always constant

Only weight changes because gravity (g) changes.

g is highest at the center of Earthg is zero at the center of Earth

Gravity decreases as you go deep into the Earth.

Escape velocity depends on object massIt is independent of object mass

A leaf and a truck need the same 11.2 km/s to escape Earth.

Quick Practice

Q1.If a person's weight on Earth is 600 N, what will be their weight on the Moon?

Q2.What is the escape velocity of an object from the Earth's surface?

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