How did Galileo’s experiments help develop the idea of inertia?

Galileo Galilei developed the concept of inertia by challenging the prevailing Aristotelian view that all horizontal motion required a constant, direct cause to persist. Through his experiments with balls rolling down inclined planes, Galileo observed that a ball rolling down one plane and up another sought to regain its initial height.

He reasoned that if the second plane were horizontal rather than inclined, the ball would be unable to regain its original height and would therefore continue rolling indefinitely. From this, he deduced that a body in motion does not require a continuous force to remain in motion; instead, it tends to maintain its state of motion unless acted upon by an external influence, such as friction.

This insight was fundamental to explaining why Earth’s rotation is not apparent to us: because we are in motion along with the planet, our natural tendency is to retain that motion. This principle, later generalized by René Descartes, became the foundation for Newton’s first law of motion.12