Final answer:
Newton's first law is more accurately considered in terms of velocity because velocity covers both speed and direction of an object's motion. An object's velocity needs to remain constant (both in speed and direction) for the object to continue moving without any external force, validating Newton's first law in an inertial reference frame.
Step-by-step explanation:
Newton's first law, also known as the law of inertia, is better understood in terms of velocity rather than speed. This is because velocity not only considers the magnitude of the motion (how fast an object is moving) but also its direction. Therefore, a change in an object's direction without a change in its speed is still considered a change in its velocity.
In Newton's first law, an object at rest stays at rest, and an object in motion stays in motion with the same speed and direction unless acted upon by a net external force. Hence, velocity is a crucial aspect of the law, not just speed.
The concept of inertial reference frames is also integral to understanding this law. An inertial frame is one in which Newton's first law holds. If the velocity of a body relative to a given frame is constant, which means both speed and direction are steady, then that frame is an inertial reference meaning Newton's first law is valid in that frame.
Learn more about Newton's first Law and Velocity