The law of motion you're describing is Newton's First Law of Motion, also known as the Law of Inertia.

Here's the formal statement:

An object at rest will remain at rest, and an object in motion will remain in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

This means:

* Objects at rest will stay at rest unless something pushes or pulls them.

* Objects in motion will keep moving at the same speed and direction unless something slows them down, speeds them up, or changes their direction.