Gravity is a phenomenon which makes things fall back to the earth surface due to the gravitational force of the earth. The phenomena of gravitational pull came into the picture when Galileo Galilei in early 17^{th} century performed an experiment by falling vertically balls through the tower of Pisa. Then Sir Issac Newton in 1687 gave the universal law of gravitation.

In the universal law of gravitation, F is the gravitational force by which object with mass m1 is attracted towards m2. Gravitational force can be calculated by the formula, as force is in direct relation to the product of both masses, and inverse in relation with the distance between them calculated from the center.

Further, the theory of relativity was given by Albert Einstein in the year 1915, he described gravity as not been any force but it is an envelope that surrounds an object. The theory was although superior to Newton theory but still, calculations are performed by Newton’s formula because it is simple.

Gravity is all depends upon mass and distance between the objects. It provides you weight, which is location dependent as suppose if you are on the planet having a mass less than earth, then you will weigh less.

The mass of object m1 is not considered as it is negligible in comparison to the mass of earth m2. Hence, that forms equation 4.

The mass of the earth is 5.98×10^{24} Kg and distance from the center of the radius of the earth is 6.38x 10^{24} m then the value of g comes to be 9.8 m/s^{2}. If the object is present at 2/3 radius of the earth, then g comes out to be 2.5m/s^{2}.

Gravity is advantageous because of which all universe exists, all the planets move in their orbits because of gravity. The tides of the sea are also influenced by it and it helps us to maintain enough distance from the sun, to keep away from its heat.

NASA is measuring the variations of gravity at different places on earth surface under mission Gravity Recovery and Climate Experiment (GRACE). Currently, two spacecraft are working for it. Variations of gravity can bring out very important information like what differences have occurred in the crust of earth after the onset of earthquakes.