“Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.”Isaac Newton · Philosophiae Naturalis Principia Mathematica · 1687
An object stays at rest or moves uniformly if no net force acts on it.
Newton realized gravity does not stop at your roof. It pulls an apple from a tree and the Moon overhead the same way. The difference is speed: the Moon moves sideways so fast that it keeps falling toward Earth without ever hitting it. He linked sky and ground with one set of rules.
When you throw a ball, it flies straight until air and gravity slow it down. Newton saw that without these brakes, the ball would fly forever. The same rule applies to a car on the highway: if you don't hit the brakes, it keeps going. His idea connected motion on Earth with motion in space, showing that the laws are the same everywhere. How could we use this law to understand how planets move?
Why it matters The same equations still send satellites into orbit and rockets to other planets.