Newton's three laws
1. An object keeps moving in a straight line at constant speed unless a force acts on it. 2. Force equals mass times acceleration (F = ma). 3. Every action has an equal and opposite reaction. Together they explain everything from a rolling ball to a rocket launch.
Universal gravitation
Newton proposed that every mass attracts every other mass with a force F = G·m₁·m₂/r². The same law that makes an apple fall keeps the Moon in orbit. Because the force falls with the square of distance, gravity is weaker far from a planet but never reaches zero.
Mass and weight
Mass is the amount of matter in an object and is the same everywhere. Weight is the gravitational force on that mass and depends on where you are. A 70 kg astronaut weighs about 690 N on Earth but only about 113 N on the Moon.