According to Galileo, an object moving without opposing friction or other forces will continue moving at a constant speed in a straight line. This is a fundamental concept in physics, known as inertia.

Here's why:

* Before Galileo: People believed that objects naturally came to a stop unless a force acted upon them. This was based on everyday observations where friction and air resistance always acted against moving objects.

* Galileo's insight: Through experiments, Galileo realized that the reason objects slow down is due to forces like friction and air resistance. In a perfect vacuum, or with zero friction, an object would continue moving forever at the same speed and in the same direction.

This concept was later formalized by Isaac Newton in his First Law of Motion, which states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.