Gravity : - The force of gravitation exerted by the earth is called gravity.

Universal law of gravitation : - Every particle in the universe attracts every other particle in the universe with a force, which is proportional to the product of their masses and inversely proportional to the square of the distance between them.

Centripetal Force : - The force that produces the acceleration and keeps the body moving along the circular path, acts toward the centre. This force is called the centripetal force.

Kepler's law : - 1. The orbit of a planet is an ellipse with the sun at the one of the foci. O is the position of the sun

2. The line joining the planet and the sun sweeps an equal area in equal intervals of time. Time travel from A to B is same as that from C to D, then the areas OAB and OCD are equal.

3. The cube of the mean distance of a planet from the sun is proportional to the square of its orbital period.

Acceleration due to gravity : - The acceleration produced in the bodies due to earth's gravity is called acceleration due to gravity.

Centre of Mass : - The centre of mass of a body is that point where the whole mass of the body can be thought to be concentrated.

Centre of Gravity : - The centre of gravity of a body is the point in the body at which the force of gravity on the whole of the body can be assumed to act.

Wobbling : - The irregularity in the motion of a star is called 'wobbling'.

Projectile : - A falling object having a horizontal velocity also, is called a projectile.

Mass : - The mass of a body is the quantity of matter contained in it.

Weight : - The force acting on a body due to gravity is called its weight.

Geotropism : - The response of plants to gravity is called geotropism.

Positive Geotropism : - The growth of primary roots in downward direction is known as positive geotropism.

Negative Geotropism : - The growth of shoot in the upward direction just opposite to the force of gravity is known as negative geotropism.