## Gravitational field

Gravitational field – 1. The gravitational force between two masses or the electrostatic force between two charges or magnetic force between two magnetic poles are found to exist between them, even in the absence of contact with each other. Hence, these forces or interactions are sometimes called as action at a distance. Each of these interactions can be very well explained in terms of the concept of field.

2. According to concept of field, a material body, when placed in space, changes it in some way and set up the G. F. The space surrounding a metrical body, in which its gravitational force of attraction can be experienced, is called its *gravitational field*. It is actually the G. F of the body, which exert force on any material body introduced in its field.

3. **Intensity of gravitational field**: The intensity of the gravitational field of a body at any point in the field is defined as the force experienced by a unit mass placed at that point. This definition is given on the assumption that the unit mass itself does not make any change in the original G. F. The intensity of gravitational field of a body is also called as strength of the gravitational field. It is a vector quantity and always represented by vector E.

4. Thus if a force F Newton’s act a mass of m Kg placed in a G. F, then the G. F strength will be (F/m) Newton/kg. Its dimensions are [LT^{-2}]. For example, the gravitational force acting on a mass m placed at a distance r from a mass M is given by F=(GMm/r^{2}). Hence the G. F strength at this point will be

**E =F/m=GMe/r ^{2}…(1)**

5. The direction of **gravitational field** at the point where mass m is placed is always directed towards the center of mass of the body that produces it. It also follows that the intensity of G. F decreases as the distance r increases and becomes zero at infinity.

6. The gravitational field at a point on the surface of the earth is to obtained by setting r =Re in equation ……….(1).

**E= GMe/Re ^{2}=g**

Where g is the acceleration due to gravity at the surface of earth.