When an apple falls on my head, my first thought it “ouch!” But not Sir Isaac Newton. By his time, people already understood the concept that gravity makes things fall, but Newton showed that gravity is in fact a universal principle, that ALL objects had a gravitational pull to each other. The force of that pull was directly related to the objects mass. The Earth has far more mass than the apple so it pulls the apple towards it, but the apple is also exerting a small gravitational force upon the Earth, too.

The equation to calculate the force of universal gravitation between two objects is:

where “G” is the universal gravitation constant of 6.67×10^{-11 }Nm^{2}/kg^{2}

“m1” is the mass of object 1

“m2” is the mass of object 2

“r” is the distance from the center of object 1 to the center of object 2.