Gravitational Field Intensity

Introduction to gravitational field intensity

Gravitation is the force acting between two bodies in the universe. Gravitation force is attractive in nature. Sir Isaac Newton discovered gravitation in the year 1665, when he saw an apple falling down the tree. Gravitation is the weakest force among the four fundamental forces in nature. Gravitation played an important role in initiating the birth of stars, planets etc. and controlling the entire structure of the universe.

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Gravitational Field Intensity


As we know that, the earth attracts everybody towards its centre. According to the Newton’s law of gravitation, the force of attraction varies inversely as the square of the distance of the body from the centre of the earth. So, there is a space around the earth in which gravitational pull can be experienced. Hence, the space around a material body in which its gravitational force can be experienced is called its gravitational field. The intensity of gravitational field of a body at a point in a field is defined as the force experienced by the body of unit mass placed at that point provided the presence of unit mass does not disturb the original gravitational field. Gravitational field intensity is always directed towards the centre of gravity of body whose gravitational field is considered. Intensity of the gravitational field is denoted by.

Let M be the mass of the body with centre of gravity O. Let F be the gravitational force of attraction placed at P in the gravitational field of the body, where OP = x. According to the Newton’s law of gravitation

F = GMm0 / x^2

Intensity of gravitational field at P will be

I = F / m0 = GMm0 / x^2 m0

I = GM / x^2

In vector form,  `vecI`  = GMx^ / x^2

Here, negative sign shows that the gravitational intensity is of attractive force.

If the gravitational field is due to the earth and point P lies on the surface of the earth then x = R (radius of the earth), so

I = GM / R^2 = g (acceleration due to gravity at the surface of the earth)

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Units of Gravitational Field Intensity

The SI unit of gravitational field intensity is Newton per kilogram or metre per squared second and in CGS system the unit of gravitational field intensity is dyne per gram or centimetre per squared second. The gravitational field intensity is a vector quantity.