* Newton's First Law of Motion (Inertia): 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*.

* Net Force: A net non-zero force means there's a resulting force acting on the object. This force will cause a change in the object's motion.

In simpler terms: If a force is pushing or pulling on an object, that object will either start moving, speed up, slow down, or change direction. It cannot maintain a constant speed in a straight line.