Answer:
Inertia
Step-by-step explanation:
According to Galileo, the motion of a projectile can be described according to two principles.
Firstly, he argued that when the projectile is fired, it moves in straight line until it looses its impetus and falls to the ground.
The horizontal motion of the projectile is explained by the theory of inertia. According to Galileo, the two independent motions of the projectile all together leads to a mathematical curve which completely describes the motion of a projectile.