Everybody in the universe attracts every other body with a force called force of gravitation. Thus, Gravitation is the force of attraction between any two bodies in the universe.
The attraction between the sun and earth, the attraction between a table and a chair lying in a room, etc., are examples of gravitation. Gravitation is the weakest of the four basic forces in nature. However, it is the most important force as it has played an important role in initiating the birth of stars and in controlling the structure and evolution of the entire universe.
Gravity:
Gravity is a special case of gravitation. If one of the attracting bodies is the earth, then gravitation is called gravity.
Gravity is the force of attraction between the earth and any object lying on or near its surface.
A body thrown up falls back on the surface of the earth due to earth’s force of gravity.
Free fall:
The motion of a body under the influence of gravity alone is called a free fall. If we neglect the resistance or friction offered by air, the fall of a body in air is a free fall. In fact, a body falls freely only in vacuum. The motion of a small heavy body in air may be taken as a free fall because air resistance on it is very small.
Acceleration due to gravity:
When a body falls freely towards the surface of the earth, its velocity continuously increases. The acceleration developed in its motion is called acceleration due to gravity.
The acceleration produced in a freely falling body under the gravitational pull of the earth is called acceleration due to gravity.
It is denoted by g. It is a vector having direction towards the centre of the earth. It does not depend on the mass, size and shape of the body. The value of g is constant at a given place. However, it varies from place to place on the surface of the earth. It depends on altitude, depth, rotation of the earth and shape of the earth.
Determination of ‘g’ using lasers:
Using lasers, it is possible to measure distances up to 10-9m and by electronic devices time can be measured upto 10-9s. Using these techniques, g can be determined to an accuracy of 1 part in 108 by observing the free fall of a body in vacuum. Near the surface of the earth, g = 9.8 ms-2 or 32 ft s-2.