Newton's First Law describes an object's tendency to resist changes in its motion, known as inertia. It states that an object at rest will stay at rest, and an object in motion at a constant speed and direction unless acted on by an unbalanced force. Galileo first theorized that constant motion was natural, while Aristotle believed an object needed a continuous force to maintain motion. Sir Isaac Newton acknowledged that gravity always affects an object's motion, and developed the law further by concluding that the laws of motion are the same in space and on Earth. The document also explains the differences between mass and weight.