An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This principle fundamentally changed the way scientists understood motion. Here's what it means:

* Objects don't need a constant force to keep moving: Before Galileo, it was thought that a force was needed to keep an object moving. Galileo realized that an object in motion will continue moving at a constant velocity unless something acts upon it.

* Inertia is a property of all objects: Inertia is the tendency of an object to resist changes in its motion. The more massive an object, the more inertia it has.

* Friction is a force: In our everyday experience, objects eventually stop moving because of friction, a force that opposes motion. Galileo understood that friction is a force, but it's not the only force that can affect an object's motion.

Galileo's principle of inertia paved the way for Isaac Newton's laws of motion and our modern understanding of how objects move.