Explain how Galileo used a thought experiment of a ball on a ramp to conclude that an object would stay in constant motion if no net force acted on it.

1) He placed a ball in center of ramp (deepest part), and thought that "it would not attain speed until I push it" ...

2) He placed a ball on the top of the ramp, and thought "it would be continue in motion until I stop it" ...

Then he stated first law of Inertia ... which you know very well!