The movement of an object at rest if no outside forces act on it is described by Newton's First Law of Motion, also known as the Law of Inertia.

Here's what it states:

* 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.

In simpler terms, an object will remain still unless something pushes or pulls it. And if it's already moving, it will continue moving at the same speed and in the same direction unless something stops it or changes its direction.