Question

Unlike Aristotle, Galileo realized that in motion there was a role played by ___________

Answer 1

What today is known as first law of the motion (First Newton's law)(Law of inertia), is basically what Galileo understood.
Aristotele thought that in order for something to move, a force had to be applied constantly.
But nowadays, we know that's not true, as a matter of fact there's something preventing the object from moving "forever".
That thing is called..
friction :)