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When we throw a ball up in the air it always comes back to the ground. This happens due to a force known as gravity. This same force makes it possible for us to walk on earth. Gravitation due to moon causes tides in oceans. In this blog, we would understand and learn about the force of gravitation in detail.

<h2 id="content-id-1665-0">Gravitation</h2>

Throw a ball vertically upwards in the air. It rises up and again falls downwards. The question arises here that why does the ball fall down. Why does it not go upwards?

This question got an answer in the year 1665 when Sir Issac Newton provided the theory of gravitation to the world. According to his theory, earth attracts everything towards itself with a force known as ‘gravity’. Due to this force of gravity the ball doesn’t go upwards after covering some vertical distance. Actually every massive object attracts every other massive object towards itself with this force.

Newton’s Law of Gravitation:

The gravitational force of attraction between any two bodies in the Universe is directly proportional to the product of their masses and inversely proportional to the square of distance between them. If two bodies having masses m1 and m2 whose centers are at a distance r from each other, the gravitational force will be: 

F ∝ M1 x M1

F ∝ 1/r2 (Inverse square law)

F ∝ M1M1/r2

F = G × [M1M2]/r2

The force is along the line joining the two particles. (Here G is the gravitational constant)

Universal Law of Gravity:

Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. The force is along the line joining the centres of two objects.

<ul>
	<li>G is the constant of proportionality, known as the universal gravitational constant.</li>
	<li>G = 6.673 × 10-11 Nm2/kg2 (In S.I system) = 6.67 × 10-8 dyne cm2/g2 (In C.G.S. system)</li>
	<li>Dimension of G = [M-1L 3T -2]</li>
	<li>The law of gravitation is applicable for all bodies, irrespective of their shape, size and position.</li>
</ul>

<h2 id="content-id-1665-1">Kepler's law of Planetary motion</h2>

First Law (Law of Orbits): The orbit of a planet is an ellipse with the sun at one of the foci.

Second Law (Law of Area): The line joining the planet and the sun sweeps equal areas in equal intervals of time.

Third Law (Law of Periods): The square of its time periods of revolution around the sun is directly proportional to the cube of the mean distance of a planet from the sun.

If r is the average distance of the planet from the sun and T is the time period of revolution then, T2 ∝ r 3 .

Earth’s Gravitational Acceleration (Acceleration due to Gravity): 

According to Newton’s second law of motion, a force acting on a body results in its acceleration. Thus, the gravitational force due to earth on a body results in its acceleration. This is called acceleration due to gravity and is denoted by ‘g’. Acceleration is a vector. g is directed towards the centre of the earth.

<img loading="lazy" alt="Gravitation Law Diagram Representation" src="https://study.kdcampus.live/admin/assets/uploads/ckeditor/Gravitation Law Diagram Representation.webp" style="height:114px; width:200px">

<h2 id="content-id-1665-2">Mass and weight</h2>

The quantity of matter in a body is known as the mass. Mass is a quantitative measure of inertia. Higher the mass, higher will be the inertia. Mass is an intrinsic property of matter and doesn’t change as an object is moved from one location to another.

The weight of an object is defined as the force with which the earth attracts the object.

The force (F) on an object of mass m on the surface of the earth can be written as Weight (W) = F = mg, (g = GM/R2 ) Weight being a force, its SI unit is Newton and is a vector quantity. Its direction is towards the centre of the earth. As the value of g is not the same everywhere, the weight of an object changes from place to place, though its mass remains constant everywhere. Since the value of g on the moon is 1/6th of the value of g on earth, the weight of an object on the moon is 1/6th of the weight on the earth.

<h2 id="content-id-1665-3">Escape velocity</h2>

When we keep on increasing the velocity of an object thrown upwards, a stage will arrive when the object will reach height so large that it will escape the gravitational field of the Earth and will not fall back on the Earth. This initial velocity is called the escape velocity.

<ul>
	<li>Escape velocity of earth = √(2 × 9.8 × 6.4 × 106 ) = 11.2 km/s.</li>
	<li>For the Moon, the Escape Velocity is 2.4 km/s</li>
	<li>Escape Velocity of satellite Ve = √(2gr) [R = radius of earth]. Ve = √2V0 i.e. √2 times of the orbital velocity.</li>
</ul>

Therefore, if the orbital velocity of a satellite is increased to √2 times (increased by 41%), the satellite will leave the orbit and escape.

Weightlessness in Space:

Space travellers as well as objects in the spacecraft appear to be floating. Why does this happen? Though the spacecraft is at a height from the surface of the earth, the value of g there is not zero. In the space station the value of g is only 11% less than its value on the surface of the earth.

Hope all your doubts related to this topic are clear. Stay for more such informative blogs.

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