All objects tend to stay still and move in a straight line at a constant speed, so the motion state of an object is determined by its motion speed. Without external force, its motion state will not change. This property of an object is called inertia. So Newton's first law is also called the law of inertia. The first law also clarifies the concept of force. It is clear that force is the interaction between objects, and it is pointed out that force changes the motion state of objects. Because acceleration describes the change of object's motion state, force is related to acceleration, not to speed.