What type of frame of reference is considered to be at rest or moving with a constant velocity?

An inertial frame of reference is defined as a frame that is either at rest or moving with a constant velocity. The significance of this definition lies in Newton's first law of motion, which states that an object will remain at rest or move in a straight line at constant speed unless acted upon by a net external force. In an inertial frame, the laws of physics take their simplest form, allowing for straightforward application of these laws.

When observing motion from an inertial frame, one can accurately predict the behavior of objects based on their initial conditions and the forces acting on them. This predictability contrasts with non-inertial frames, which may experience apparent forces like centrifugal force, complicating the analysis of motion. Understanding the characteristics of inertial frames is crucial in both classical mechanics and various applications in physics, confirming why this choice is the correct answer.