IIT JEE Blogs

You must have seen the night-sky composed of stars, planets, moon and you could have also been lucky enough to see a shooting star or meteor as it is termed. If you make a careful study, you could also observe, that the pattern of night-sky keeps on changing with time as well as with seasons in a seemingly complex manner. Our ancient Indian scientists had made careful observation about this changing pattern and had deduced that planets revolve around the sun, a fact which was later rediscovered by Copernicus.

It took the genius of Newton to reduce this complicated motion into a very simple universal law – the law of Gravitation, a law, which not only applies to celestial bodies but also applies to that famous apple which, Newton saw falling.

OBJECTIVE

After learning this chapter we will be able to understand the law of gravitational force, gravitational potential energy. We will also be able to learn the concept of planetary motion of different planets and other heavenly bodies. In this chapter we will revolve around the Universal Law of Gravitation and its applications and try to gain an insight into one of the basic forces in the Universe.

PRE-REQUISITE

Þ            Concept of acceleration

Þ            Earth’s gravity

Þ            Concept of circular motion

Þ      Concept of centripetal and centrifugal forces

Þ      Newton’s third law of motion (Action and Reaction)

CORE CONCEPTS

When you throw a ball up, the ball goes up, its velocity is retarded and it finally comes to rest. Henceforth, it gains acceleration and returns back to the ground. This retardation and subsequent acceleration is a consequence of the universal gravitational force. There is a specific law, which guides the above phenomena, known as Newton’s Law of Gravitation. It states:

Every particle in this Universe attracts every other particle with a force, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Let m1 and m2 be the masses of the two particles and r be the separation between them (see figure given below).

“The space around a body within which its force of gravitational attraction is perceptible (by any other body in this space) is called its gravitational field.”

The intensity E, of the gravitational field of a mass ‘m‘ at a point at distance ‘r‘ from it is the force experienced by a unit mass placed at this point in the field. (Assuming that the presence of unit mass does not affect the gravitational field of the mass m)