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How did Galileo Galilei’s inclined-plane experiments lead to the idea of inertia?

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Title page of Isaac Newton's Philosophiae Naturalis Principia Mathematica, 1687, featuring large Latin text and publication details in a bordered layout. — Newton’s laws of motion The title page of Isaac Newton's *Philosophiae Naturalis Principia Mathematica* (1687; *Mathematical Principles of Natural Philosophy*), the work in which the physicist introduced his three laws of motion.

Galileo Galilei’s experiments with inclined planes were pivotal in challenging the Aristotelian view that all horizontal motion required a constant, direct cause to persist.

By observing balls rolling down one inclined plane and up another, Galileo noticed that the balls consistently sought to regain their initial height, regardless of the specific angles of the inclines. He reasoned that if the second plane were removed and replaced with a horizontal surface, the ball would be unable to regain its original height and would, therefore, continue rolling indefinitely.

From this, he deduced that objects do not require a proximate cause to maintain motion; rather, a body in motion will naturally tend to stay in motion unless acted upon by an external force, such as friction. This insight provided the foundation for the law of inertia, which later became the first of Isaac Newton’s three laws of motion. This principle was essential for explaining why Earth’s rotation is not apparent to us, as we and our surroundings move together in a shared state of motion.12