What happens to an object in motion according to the principle of inertia?

The principle of inertia, as defined by Newton's First Law of Motion, states that an object in motion will remain in motion with the same speed and in the same direction unless acted upon by an external force. This means that if no net external force is applied to the object, it will continue its motion indefinitely.

In the context of the question, this principle highlights that the object's state of motion remains constant; it doesn't inherently stop, change direction, or decrease speed without some form of external influence. Therefore, the answer accurately reflects this fundamental concept of physics, clearly illustrating the nature of objects in motion and their behavior under the influence of forces.