Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. ... If that velocity is zero, then the object remains at rest.
Before Galileo and Newton, many people thought that objects lost speed because they had a built-in natural tendency to do so. But those people weren't taking into account the multiple forces here on Earth - for example, friction, gravity, and air resistance - that cause objects to change their speed. If we could see the motion of an object in deep interstellar space, we would be able to observe the natural tendencies of an object that is free from any external influence. In deep interstellar space we would observe that if an object had a speed, it would continue to move with that speed until there was some force causing a change in its motion. Likewise, if an object were at rest in interstellar space, it would remain at rest until there was a force causing a change in its motion.
