There are many equations and formulas that are important in physics because of what they imply or solve like E=mc^2, but Newton’s Second Law of motion has to be one of the most important.
Force = Mass x Acceleration (Newtons)
With the use of this equation he proved that objects that are heavier fall at the same rate towards earth as objects that are lighter because the earth pulls them down with the same force. To put it in perspective a car and a feather would fall at the same universal rate of 9.8m/s^2 even though one is much heavier. Newton also figured out that the moon is held to the earth by the same force that pulls an apple off a tree (Newton's famous story of the apple