Dynamics

Kinematics told you how things move — position, velocity, acceleration, all of it bookkeeping. It never once asked why velocity changes in the first place. Newton did, and in 1687 he wrote the answer down in the Principia — three laws of motion that still run the everyday world.

His answer: velocity changes only when something pushes or pulls. He called that push or pull a force.

And the same push doesn't move everything the same. Shove a pebble and a boulder with equal force — the pebble leaps, the boulder barely leans. Mass is just how stubbornly a thing resists being accelerated. That's the whole of F = ma, read sideways as a = F / m:

Force = Mass × Acceleration

Now watch the planet on the right. The sun is pulling it hard, constantly — so why doesn't it just fall in? Because it's also moving sideways. It is falling, every single moment, toward the sun — and every moment its sideways motion carries it just far enough to miss. An orbit is a fall that keeps missing the ground.

That picture is Newton's own. He imagined a cannon on an impossibly tall mountain, firing harder and harder. Fire it weakly and the ball arcs down and lands. Fire it hard enough and the ground curves away beneath it just as fast as it falls — so it never lands. It falls forever, circling. That, he realized, is the Moon.