# Physics formula for class 9 chapter-Gravitation

## Universal Law of Gravitation

According to newton's universal law of gravitational, every object in the universe attracts each other object with a force, which is

• Directly proportional to the product of the masses of the object and
• Inversely proportional to the square of the distance between their centers.

G = Fxd2/Mxm

The SI unit of G can be obtained by substituting the unit of force, distance and mass in equation (iii).

G = Nm2/kg x kg

G = Nm2/kg2 or G = Nm2 kg-2

Newton's third law of motion (I.e., action and reaction are always equal and opposite) applies to gravitational force as well.

### Kepler's Laws of Planetary Motion

1. 1st Law: Every planet revolves around the Sun in an elliptical orbit, with the Sun situated at any one of the foci of the ellipse
2. 2nd Law: In the elliptical orbit of the planet, the line joining the centre of the planet to the centre of Sun sweeps out equal areas in equal intervals of time.
3. 3rd Law: The Square of time period of revolution of a planet around the Sun is directly proportional to the cube of the semi-major axis or the elliptical orbit.

Newton guessed the inverse square rule (i.e., F ∝ 1/2) using Kepler's third law of planetary motion, and assuming that orbits of planets around the Sun are circular.

Gravity is the phenomenon of attraction between Earth and any other body

Force of gravity, F = GmM/r2

where M is mass of Earth and m is mass of the body at a distance r from the centre of Earth

The relation between g and G is g = GM/R2 GM where M is mass of Earth and R is radius of Earth. The value of g does not depend on mass of the body.