# Gravitation

Every object in the universe attracts every other object. They attracts with a force which is directly proportional to the product of their masses and inversly proportional to the square of the distance between them.

Gravitational Force, F∝(m_{1}m_{2})/R^{2}

F=(G×m_{1}m_{2})/R2, where G= gravitational constant, the value of G = 6.67 × 10^{-11} Nm^{2}/kg^{2}

## Falling Objects

Earlier it is thought that the lighter objects fall slowly and the heavier objects fall more rapidly. Later Galileo found that this thought was wrong. Galileo dropped two stones of different masses from the top of the leaning rower of pisa, and found that they hit the ground at the same time. From this Galileo concluded that the acceleration of an object falling freely towards the earth does not depend on the mass of object it only depends on the surface area, so the feather of a bird have larger area then the stone of same weight, so it faces more resistance of air and its speed gets slow down. In vacuum, all teh objects fall at the same rate.

## Acceleration due to Gravity

The uniform acceleration produced in afreely falling body due to the gravitational pull of the earth is known as acceleration due to gravity and is denoted by the letter 'g'. The value of 'g' is taken as 9.8 m/s2.

The value of acceleration due to gravity 'g' is maximum on the surface of earth. It decreases as we go above or inside the surface of earth. 'g' is minimum at equator and maximum at poles.

## Mass

The mass of a body is the quantity of matter contained in it. The unit of mass is kilogram. The mass of a body is constandt and remains same everywhere in universe.

## Center 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.

## Weight

The earth attracts every body towards its centre with a certain force. The weight of a body is the force with which it is attracted towards the centre of the earth.

The weight of body is given by W= m ×g. Weight is a vector quantity. The gravitational pull of the moon is about one-sixth of that of the earth, therefore, the weight of an object on moon will be about one-sixth of what it is on the earth.

## Weightlessness

The weight of a body is the force with which the earth attracts it. An object is said to be weightless when it is galling freely under the action of gravity.

## Projectiles

A falling object having a horizontal velocity is called a projectile and it moves in curved path. When we through a stone horizontally, it have two type of motions i.e. horizontal and vertical(towards earth). This type of motion is known as projectile motion.

## Escape velocity

The volicty at which we through an object, that object doesnot return to earth and escape itself in space is known as escape velocity. The escape velocty of earth is 11.16 km/sec.