Gravity 

Heard the famous story of Newton’s apple? When an apple fell from a tree, Sir Isaac Newton began to wonder why objects always fall downward. That simple observation led to the discovery of gravity, the universal force that keeps everything in check. Without this force, nothing, from the atmosphere to the oceans to the life we know, would have existed. So let’s explore this fine-tuned event happening with every object of the universe. 

1.0What is Gravity? 

Gravity, or simply gravitation, is the fundamental force of nature, causing attraction between any two objects in the universe that have mass. It is a non-contact force, meaning it does not need the bodies to be in physical contact to be affected by this force. 

Understand it like this: gravity is the reason why the moon doesn’t float away in space like a slingshot or why the Earth rotates around the sun.  

On Earth, one of the most well-known examples of gravity is the attractive force between the surface objects and the centre of this planet. It gives objects their specific weight and makes things fall when dropped from a certain height. 

2.0History of Gravity

The theory of gravity, as we know it today, has evolved over centuries. In the pre-Socratic period, gravity had more philosophical explanations than scientific ones. It was in the 4th century that Aristotle postulated that heavy objects naturally fall faster than lighter ones. This fact was proven wrong by Galileo Galilei in the 16th and 17th centuries by his experiments, which proved that objects, regardless of their weight, fall at the same time in a vacuum. However, not an ounce of gravity was discovered at that time. 

It was in the 17th century that Sir Isaac Newton formulated the Law of Universal Gravitation, citing the term gravity. He stated that every object in the universe is bound by a certain force of attraction known as gravity. 

However, in the 20th century, Albert Einstein redefined the term for Gravity in his General Theory of Relativity, where he proposed that gravity is the bending of space and time caused by massive objects. 

3.0Measuring Gravity: Newton’s Law of Universal Gravitation

Gravity is a force of attraction between two bodies, which can be measured using the fundamental principle of Newton’s Law of Universal Gravitation. According to this law, 

• The force between any two attracting bodies in the universe is directly proportional to the product of their masses and, 
• Inversely proportional to the square of the distance between their centres. Mathematically, it can be expressed as: 

$F=G\frac{m_1{m}_2}{r^2}$

Here, 

• F = Gravity or the gravitational force. 
• G = Universal gravitational constant with the value of 6.67×10⁻¹¹Nm²/kg²
• $m_1{m}_2$ = masses of two attracting objects. 
• r = distance between the centres of those two masses. 

The equation helps measure the gravitational forces between the Earth and the Sun, the Earth and the Moon, and the forces between any other celestial objects.  

4.0Acceleration Due to Gravity

Acceleration due to gravity is basically the outcome of gravity itself. That is, it is the acceleration which an object experiences when it falls freely under the influence of the gravity of Earth or any other celestial body. In simple terms, it is the rate at which the speed of an object increases during free fall. 

$\begin{array}{rl}& \begin{array}{rl}F& =ma\\ \frac{GMm}{r^2}& =ma\end{array}\\ & \text{ Then, cancelling }m\text{ on both sides: }\\ & a=\frac{GM}{r^2}=g\end{array}$

The acceleration due to gravity can be calculated using the modified version of Newton’s law of gravitation, like this: 

$g=G\frac{M}{R^2}$

Here, 

• G is the Universal gravitational constant.
• g is the acceleration due to Gravity
• M is the mass of the celestial object which is applying force on an object on its surface. 
• R is the radius of the celestial body. 

For Earth, the value of acceleration due to gravity on any object in free fall is 9.8m/s², which differs based on the height and altitude from the centre of the Earth. 

That is, as the Earth is not a perfect sphere, but rather an ellipse. This is the reason why Earth’s centre is farther away from the equator than the pole, creating the difference of “g” in both places. 

5.0Weight and Gravity: 

Weight and Gravity are closely related to each other, as the weight of any object directly depends on the gravitational pull exerted by the Earth on its mass. In other words, Weight is the force exerted by a planet on an object containing mass. The relation between the weight and gravity can be expressed as: 

Weight(W)=Mass(m) X Acceleration due to Gravity(g)

Acceleration due to gravity is the reason why the weight of a body differs at different places, and astronauts experience weightlessness. The SI unit for weight is in Newton, just like force. 

6.0Redefining Gravity by Albert Einstein: 

Although Newton explained gravity as the force of attraction between two masses, Albert Einstein changed this whole concept with his General Theory of Relativity. As per him, gravity is not a force but a curvature of space and time around which mass and energy bend. Understand it like this: 

• Consider space and time as a stretchy fabric called the curvature of space-time. 
• Now, when a heavy object sits on this fabric, it will stretch and make a dent or curve in the fabric. 

One of the most common examples of gravity proving this theory is the attraction and rotation between the Earth and the Moon, where the Moon moves along the curves formed by the Earth’s mass on the space-time curvature. 

The theory also gave the concepts of black and white holes and warmholes, which were once considered myth but are now a reality(At least the Black hole is).