Everything in classical mechanics grows from three laws Newton set out in 1687. They are simple to state and endlessly deep in their consequences.
The first law: inertia
An object keeps doing what it is doing unless a force acts on it. A body at rest stays at rest; a body in motion keeps moving in a straight line at constant speed. This is inertia, and it overturns everyday intuition: a sliding puck stops not because motion naturally fades, but because friction is a force pushing back. Remove all forces and motion simply continues forever.
The second law: F = ma
The second law says how forces change motion. A force produces an acceleration (a change in velocity) in proportion to its strength and in inverse proportion to the object's mass:
Push twice as hard and you get twice the acceleration; push the same on something twice as massive and you get half. Mass is exactly this reluctance to be accelerated. This one equation, applied over and over, predicts the entire path of anything you can name the forces on.
The third law: action and reaction
Every force comes in a pair. If you push on a wall, the wall pushes back on you just as hard, in the opposite direction. A rocket flings gas downward and the gas flings the rocket upward; you step back off the ground and the ground pushes you forward. Forces never act alone: they are always mutual, equal and opposite.
Putting them to work
Together the laws form a recipe. Identify every force on an object, add them up to get the net force, and the second law hands you the acceleration; from the acceleration and the starting motion, the future path follows. This procedure (forces in, motion out) is the working heart of mechanics, and the rest of the course is really about the great quantities that survive as objects move: energy and momentum.