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

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A diagram with labeled vector illustrations. Panel A shows vector addition with two green arrows A and B forming a parallelogram, and their sum C as a red diagonal. Panel B shows vector subtraction, with A and negative B forming resultant D. Panel C demonstrates vector components using angles and projections. Panel D uses the right-hand rule with thumbs to show the direction of the cross product E = A x B and its opposite, -E = B x A. — mechanics Figure 1: (A) The vector sum C = A + B = B + A. (B) The vector difference A + (−B) = A − B = D. (C, left) A cos θ is the component of A along B and (right) B cos θ is the component of B along A. (D, left) The right-hand rule used to find the direction of E = A × B and (right) the right-hand rule used to find the direction of −E = B × A.

Galileo Galilei’s experiments with inclined planes were pivotal in overturning the Aristotelian view that all motion requires a continuous proximate cause to persist.12

By observing balls rolling down one inclined plane and up another, Galileo noted that they consistently sought to regain their initial height, regardless of the specific inclines used. He reasoned that if the second plane were perfectly horizontal, the ball would be unable to regain its original height and would, therefore, continue rolling indefinitely.2

From this, he deduced that objects do not require a force to maintain motion; rather, a body in motion will naturally persist in that state unless acted upon by an external influence, such as friction. This insight provided the foundation for the law of inertia, which explains why Earth’s motion is not apparent to us: because we and our surroundings are moving together, we perceive ourselves to be at rest. This principle was later refined by René Descartes to apply to motion in a straight line and became the basis for Newton’s first law of motion.12