Newton’s Law of Gravitation
Newton’s law of gravitation states that every particle in the universe attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The direction of the force is along the line joining the particles.
 

The universal constant of gravitation G is numerically equal to the force of attraction between two particles of unit mass each separated by unit distance.
• Important Characteristics of Gravitational Force
(i) Gravitational force between two bodies is a central force i.e., it acts along the line joining the centers of the two interacting bodies.
(ii) Gravitational force between two bodies is independent of the nature of the intervening medium.
(iii) Gravitational force between two bodies does not depend upon the presence of other bodies.
(iv) It is valid for point objects and spherically symmetrical objects.
(v) Magnitude of force is extremely small.

 variation of Acceleration Due to Gravity
The value of acceleration due to gravity changes with height (i.e., altitude), depth, shape of the earth and rotation of earth about its own axis.
(a) Effect of Altitude. As one goes above the surface of Earth, value of acceleration due to gravity gradually goes on decreasing. If gh be the value of acceleration due to gravity at a height h from the surface of Earth

 Gravitational Field
The space around a body within which its gravitational force of attraction is experienced by other bodies is called gravitational field.
• Intensity of Gravitational Field
The intensity of the gravitational field of a body at a point in the field is defined as the force experienced by a body of unit mass placed at that point provided the presence of unit mass does not disturb the original gravitational field.

Gravitational Potential
The gravitational potential at a point in the gravitational field of a body is defined as the amount of work done in bringing a body of unit mass from infinity to that point.
Gravitational potential at a point situated at a distance r from a body or particle of mass M 

• Whenever we throw an object towards the sky it will fall back onto the ground.
• For Example: - A ball comes down when thrown up. Rain drops fall towards the ground; Planets revolve in an elliptical orbit around sunetc.
• Planets revolving in the elliptical orbit.

• Rain drops falling on the earth.

  Leaves fall off the tree.

• There is a force due to which all things are attracted towards the earth. This force is known as Gravitation.
• Gravitation is the force of attractionbetween all masses in the universe, especially the force of attraction exerted by the earth on all the bodies near its surface.
• In this chapter we will take a look at gravitation force, its laws, and we will also study about the planetary motion.

Acceleration due to gravity of the earth

• Acceleration attained due to gravity of earth.
• All the objects fall towards the earth because of gravitational pull of the earth.
• And when abody is falling freely, it will have some velocity and therefore it will attain some acceleration. This acceleration is known as acceleration due to gravity.
• It is a vector quantity.
• Denoted by ‘g’.
• Its value is 9.8m/s2.

• Example:-Stones falling from a rock will have some velocity because of which some acceleration. This acceleration is due to the force exerted by the earth on the rocks.This is known as acceleration due to gravity.

   

  Stones falling from rock