High School Physics/Newtons Apple
Human beings have a tendency to understand their surroundings and figure out an explanation for the various phenomenons that take place. When they observed that an object in motion slowly de-accelerates they concluded that it is the nature of an object to remain at the state of rest. Similarly observing the sun, the moon, and even the rotating of stars around the pole star they concluded that earth is the center of the universe and all other objects revolve around it.
Later Newton stated three important laws explaining the motion of an object. There is even a popular story that Newton devised his theory of gravitation by observing the fall of an apple from a tree. His theory of gravitation and his three laws of motion redefined the way we look at the world and laid the foundations of basic physics.
The theory of gravitation states that the force of gravity is directly proportional to the product of masses of the two bodies and inversely proportional to the square of the distance between them. The constant which gives us the exact value of the force of gravitation is called the gravitational constant and is symbolised by the sign 'G'.
G = 6.67*10^-11 m^3 kg^-1 s^-2