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.

First, Galileo Galilei (who was an astronomer, mathematician, physicist and philosopher) saw a ball rolling down on a ramp, it keeping rolling on an horizontal surface and then it rolled up another ramp. He concluded that if there would be no friction acting on the ball, it would continue to roll up the other ramp and and would stopped when it reached the same height as it had first.
 Then,the ball rolled down on a ramp which its incline was decreased. Based on the first experiment, he cloncluded that the ball should have to roll farther to reached the same height as it had first.
 Finally, there was only one ramp. It was obvius that without a second ramp, the ball would never reach the first height, so he concluded that the ball would stay in constant motion if there is no friction or if no net force acted on it.