Newton's Law of Gravitation

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8.2Newton's Law of Gravitation

In these revision notes for Newton's Law of Gravitation, we cover the following key points:

  • What is gravitation?
  • Where does gravitation differ from gravity, gravitational force and centre of gravity?
  • What does the Newton's Law of Gravitation say?
  • What are orbits?
  • What makes objects rotate in orbits?
  • How gravity affects our daily life?
  • What are artificial satellites?
  • How many types of artificial satellites are there? How do they differ from each other?

Newton's Law of Gravitation Revision Notes

Gravitation represents a movement, or a tendency to move, towards a centre of gravity, as in the falling of bodies to the earth.

In few words, gravitation is the tendency to move towards the centre of gravity of an object due to the attracting effect of gravity produced by it. This means gravity is the cause, gravitation the effect, centre of gravity the target point, gravitational force the tool used to realize the objective and gravitational field the medium of event's occurrence.

Newton's Law of Gravitation, states that:

Any particle of matter in the universe attracts any other with a force varying directly as the product of the masses and inversely as the square of the distance between them.

Mathematically, the Newton's Law of Gravitation is written as:

Fg = G × m1 × m2/R2

where m1 and m2 are the masses of the objects involved, and R is the distance between the objects centres of gravity.

The quantity G is the gravitational constant. Its numerical value is

G = 6.674 × 10-11 N × m2/kg2

There are two factors, which prevent the celestial bodies of our solar system (and all the other objects in general) from collapsing due to the attraction effect of gravitational force. First, the presence of other celestial bodies around them produces other gravitational forces that tend to balance the gravitational force produced by the Sun. Second, the centripetal force produced when the Earth rotates around the Sun, makes the Earth (and other celestial bodies) rotate in a fixed circular path known as orbit.

During a rotation in an orbit, the gravitational force is balanced by the centrifugal force. This helps us calculate the velocity of rotation necessary to keep an object (usually celestial bodies) moving in orbits.

Gravitational force between two normal objects on earth is very small compared to that produced by Earth. As a result, objects do not collapse with each other but only with the Earth's surface.

Artificial satellites are objects sent in the space by scientists for various purposes such as to observe natural phenomena such as weather, to distribute communication signals, for espionage, etc.

There are two types of artificial satellites: geostationary and geosynchronous satellites. Geo Synchronous satellites move in an orbit, which is synchronized with the earth's rotation to its own axis, while for the orbit of geostationary satellites we can say it is a circular orbit placed above the equator at an altitude of 35786 km with zero inclination with horizontal plane. In this case, such satellites remain fixed at a particular point in the sky.

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