Gravitational field Intensity or Strength
Definition: The gravitational field intensity or strength of an object at any point in the gravitational field is defined as the force experienced by a unit mass placed at that point.
If force acting on a body of mass m placed at a point in gravitational field is F, then gravitational field intensity at that point is –
g or E = F/m … … … (1)
It is seen from this equation that as the value of m increases E decreases. It is a vector quantity. It has both magnitude and direction. If more than one field intensity is acting at any point, then the resultant field intensity is obtained by vector addition of individual intensity. Direction of the field intensity indicates the direction of the field. Sometimes, only gravitational field is written instead of gravitational field intensity. In S. I. system unit of intensity is Nkg-1.
In order to understand the direction of gravitational intensity let a body of mass m be placed at a point P [Figure]. For determining gravitational field intensity of that body at point A let us assume that a body of unit mass is present at that point. Now the unit mass placed at point A will experience attractive force along PA. So, direction of the intensity at point A will be along AP. Similarly, the direction of intensity at point B will be along BP. Hence according to equation (1), field intensity in vector form can be written as,
EG = F/m
and in that case force, F= EG x m
Now we will see what is the meaning of intensity 4 Nkg-1 at a point in gravitational field? In answer we can say that if at any point in the gravitational field 1 kg mass is placed it will experience a force of 4 N or if 4 kg mass is placed in that field it will experience a force of 1 N.