"Action, Reaction, and Conservation of Momentum"
In 1687, Isaac Newton published the "Principia," arguably the most important work in the history of science. He achieved a grand synthesis, proving that the force dropping an apple to the ground is the same force keeping the Moon in orbit. This unified the heavens and the earth under one set of mathematical laws. For over 200 years, his laws reigned supreme and still govern most engineering today.
"An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." This resistance to change is called Inertia. It explains why you fly forward when a car brakes suddenly or why a spacecraft can coast forever in the void.
This is the core equation of dynamics: F = ma. Force equals mass times acceleration. It tells us that the acceleration of an object depends on the mass of the object and the amount of force applied. To move a heavier object (more mass), you need more force. This simple formula allows us to calculate everything from the thrust of a SpaceX rocket to the impact of a car crash.
"For every action, there is an equal and opposite reaction." Forces always come in pairs. When you walk, you push the ground backward, and the ground pushes you forward. When a rocket fires, it expels gas backward, and the equal reaction force propels the rocket upward into space.
Newton proposed that every mass in the universe attracts every other mass with a force called gravity. This force depends on the product of their masses and inversely on the square of the distance between them. This law unlocked the secrets of planetary motion.