Gravity

 Gravity

What do we human beings know about gravity? Most people have known gravity as a pull toward the center, but in reality, this is a simple and basic definition of gravity. Gravity is a vast and never-ending topic of exploration. Without gravity, the universe wouldn’t exist. The word gravity had come from the Latin word gravitas which means weight. The gravitational force is one of the fundamental four forces in the universe electromagnetic weak and strong nuclear forces. The gravitational force is the weak force among those fundamental forces. It is a natural phenomenon due to which all planets are stable in the universe. This force is also responsible for the earth to attract objects toward its center.

History of gravity

In ancient times scientists tried to proposed their theory to explain why objects are falling toward the Earth. The Greek philosopher Aristotle said that all matters in this universe had a natural tendency to move towards the center of the universe, which means towards the Earth. At that time, the Earth had believed to be the center of the universe. Later, Nicolas Copernicus said that the Earth is not the center of the universe. All planets are moving around the sun. Sun had to be at the center. In 1609, Johannes Kepler showed that the Earth moves around the sun in an elliptical path. But, he isn’t able to describe why the Earth moves around the sun in that way. Later on 5 July 1687, Sir Isaac Newton published Principia.  In Principia, he explained that a force was acting between them and named that force gravitational force. He also shows that the gravitational force depends on the distance between them inversely. A different approach to gravity had given by Einstein in the general theory of relativity. He said that an object curves the flat space around it due to its gravity.

Illustration of motion of the Earth around the Sun                     

Gravitational force

Sir Isaac Newton(source: Wikipedia)

The gravitational force refers to the force of attraction between two planets or astronomical bodies. The scientists were before Newton observed that planets were moving faster when they came close to the Sun and were moving slower when they were far from the Sun. But, they couldn’t explain the reason behind it. What type of force was acting on that phenomenon. Newton explained that the force acting between them is a gravitational force. It depends on the distance between them. He gave the following formula to calculate
that force.

Where F = Gravitational force,

            G = Gravitational Constant,

             m₁ = Mass of the first object,

             m₂ = Mass of the second Object

             r = Distance between them.

The Gravitational Force 

From this equation it is clear that the gravitational force F is inversely proportional to the distance between them.

This solve the unanswered question that how the force acting between object in the universe. Also, why planet moves around the sun in elliptical locus.

Acceleration due to gravity (g)

Newton’s first law states that an object will continue its state of motion until some external force applied on it. It means that the velocity of any object will change if we apply external force to it. From this, we can say that the gravitational force also causes a change in the velocity of an object. In physics, the change in velocity indicates acceleration. Thus, the acceleration related to gravitational force is known as acceleration due to gravity or simply g. The value of g depends on two factors mass of the planet or body and the radius of the planet or body.

Where G = Gravitational constant

             M = Mass of the planet

             R = Radius of the planet

Value of acceleration due to gravity at
different height form earth(source: Wikipedia)

This relation indicates that acceleration due to gravity directly depends on the mass of a planet and inversely to the square of the radius of the planet. That implies that the value of g on different planets is different. The value of g on earth has found 9.8 m/s², while its values on Mars and Jupiter had calculated as 3.7 m/s² and 24.5 m/s². Those data clearly show that the planet with a greater mass has a higher value of acceleration due to gravity than the body with a lesser mass. The value of acceleration due to the gravity of the earth is assumed to have a constant value of 9.8 m/s². But, it varies from higher and lower altitudes because the earth’s radius isn’t constant overall. The value of acceleration due to gravity is high at sea level than at high altitudes such as at higher peaks or mountains. But its value decreases under the surface of the earth due to a decrease in the effective mass of the earth.

Einstein's general relativity

Newton illustrates that gravity works as a pulling force towards the center. After that, for a long time, gravity was assumed only as a force. Later Einstein found that gravity does not only work as a force. A new concept of curve space come into focus after Einstein proposed the general theory of relativity in 1915. Einstein reveals that matter curves the space around it due to the effect of its gravity. The evidence of the general theory of relativity had observed by a team led by an English Astronomer Eddington in 1919. 

Solar eclipse in 1919(source: Wikipedia)

They studied the sky during the total solar eclipse. They observed that the star near the sun had appeared at a different position than expected to be in that position calculated by studying those stars at night. This experiment indicates that the sun bends the light coming from those stars due to its gravity. But, light always moves in the shortest distance between two points. If space is flat, the light will move in a straight line.  In this case, light moves in a curve line that is only possible when space has curved. This experiment concludes that gravity curves space. In the solar system, mercury moves in a different type of orbit than other planets. Its orbit continuously changes its position that is it had precession of the perihelion. This type of motion of mercury wasn't able to describe by Newton's gravitational formula. 

Motion of the Mercury around the Sun

A simple illustration of curve space can visualize by taking the example of a trampoline. If we put an object on the trampoline, it will change the flat surface into the curve surface. In that same way, gravity around an object curves the space. This example only illustrates the behavior of gravity in 2D space, but in reality, we are in 3D space. The correct visualization of 3D curve space is more complex.

Curved space due to the gravity of earth(source: Wikipedia)