a. Universal gravitational constant:
G is defined as the force of attraction between two bodies of mass 1 kg each kept at a distance of 1 m apart from their centers.
b. Gravitational field:
The gravitational field is the field around any body or heavenly body where the force of attraction by that body is experienced.
c. Acceleration due to gravity:
Acceleration produced on a freely falling body due to the influence of gravity is called acceleration due to gravity.
When a body is falling freely under the effect of gravity only, it is called freefall.
When any body is falling freely, the weight of the body becomes zero which is called weightlessness.