Classical mechanics
Before Isaac Newton, people thought that things do not move forever: they will always stop even if nothing touches them. On Earth, this seems correct: if you roll a ball on the grass, the ball will stop; if you push a book across a table, the book stops moving. But this does not happen everywhere. In outer space, rockets and planets move and they do not slow down or stop. So something on Earth makes objects stop moving, and this is a force called friction. Every object that touches another object feels friction. Even when you throw something like a baseball, the baseball feels friction because of the air. This is called drag or air resistance. In outer space, there is no friction because outer space is a vacuum: there are no objects there, including no air.Gravity is another force that changes how objects move on Earth, but in outer space gravity is very small unless you are close to a planet or a star.