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How did Galileo explain projectile motion?

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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 revolutionized the understanding of projectile motion by breaking away from Aristotelian physics, which incorrectly categorized such movement as "violent" and requiring a continuous proximate cause.12

Instead, Galileo demonstrated that the motion of a projectile is the result of two simultaneous and independent components:
* Horizontal motion: An inertial motion that, in the absence of interference, would continue indefinitely at a constant speed.2
* Vertical motion: A downward motion characterized by constant acceleration due to gravity.12

By combining these two independent motions, Galileo proved in his work Dialogues Concerning the Two New Sciences that the resulting trajectory of a projectile is a parabola. This insight was a foundational contribution to kinematics, providing a mathematical description of motion that contradicted the prevailing Aristotelian view and helped set the stage for the development of classical mechanics.32