All of us are aware that the material object are attracted towards the earth. For example, a body thrown up finally falls down toward the earth, the rain drops from cloud in sky fall toward the earth, the going uphill is lot more tiring than going downhill etc.

Galileo was the first to recognize the fact that all bodies, irrespective of their masses, fall towards the earth with a constant acceleration.

## Definition of Gravitation

**Gravitation is the force of attraction between any two bodies of the universe.**

It was discovered by newton in the year 1665, when he saw an apple falling down the tree.

Gravitation force is the weakest force among the four fundamental forces of nature, but is the most important force because it played an important role in initiating the birth of stars, and controlling the entire structure of the universe.

## Newton’s law of gravitation

It states that every body in this universe attracts every body with a force which is directly proportional to the product of their masses and is inversely proportional to the square of the distance between them.

Consider two point mass bodies A and B of masses m

_{1}and m_{2}.
Let r be the distance between their centers and F be the force of attraction between them.

According to newton’s law of gravitation,

F ∝ m

_{1}m_{2}/ r^{2}
or

F = G m

_{1}m_{2}/ r^{2}
Where G is a constant of proportionality and is called Universal Gravitational Constant.

## Definition of G

Let m

_{1}= m_{2}=1 and r = 1
Then from the above equation,

F = G × 1 × 1 / 1

^{2}= G
or

**G = F**
Thus, Universal Gravitational Constant is equal to the force of attraction acting between two bodies each of unit mass, whose centers are placed unit distance apart.

Gravitation constant is a scalar quantity. Its value is same through out the universe and is independent of the nature and the size of the bodies as well as the medium between the bodies.

**The value of G in SI is 6.67 × 10**

^{-11}N m^{2}kg^{-2}and in cgs system is 6.67 × 10^{-8}dyne cm^{2}g^{-2}.### Dimensional formula for G

G = F × r

^{2}/ m_{1}× m_{2}=(MLT^{2}) (L^{2}) / M × M**G = [M**

^{-1}L^{3}T^{2}]## Important characteristics

The gravitational force of attraction between two given bodies are

## Definition of Gravity

Gravity is the force of attraction applied by earth towards its center on a body lying on or near the surface of earth. Gravity is merely a special case of gravitation and is also called earth’s gravitational pull.

For illustration, the force of attraction between two bodies on the surface of earth is the force of gravitation, but the force of attraction between each body and the earth is the force of gravity.

Force of gravity acting on a body is measure of weight of the body. Thus weight of a body is defined as the force of attraction exerted by the earth on the body towards its center.

If m is the mass of a body placed on the surface of earth, where acceleration due to gravity is g,

then

**Gravity pull = weight of body = mg.**

The units and dimensions of gravity pull or weight are the same as those of force.

## Difference between Gravitation and Gravity

Gravity is the earth’s gravitational pull on a body, lying on or near the earth’s surface.

_{1}due to a body B of mass m

_{2}placed distance r apart is, F = G m

_{1}m

_{2}/ r

^{2}, where G is universal gravitational constant.

The force of gravity on a body of mass m is F = mg, where g is the acceleration due to gravity.

The direction of force of gravity on a body is along the line joining the body and the center of the earth and is directed towards the center of the earth.

The force of gravity on a body is zero at the center of the earth.

## Acceleration due to gravity

If a force F acts on a body on a mass m, produces an acceleration a, then according to Newton’s second law of motion

**F = ma**

If the force on body is due to gravity of earth, then acceleration in the body is called acceleration due to gravity, which is denoted by g, i.e., a = g

Then, F = mg or

**g =F / m**

If m = 1, then g = F

**Thus, Acceleration due to gravity is defined as the force of the gravity acting on unit mass of a body placed on or near the surface of earth.**

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