Final answer:
An Inertial Reference Frame is a concept in physics where objects at rest or moving at a constant velocity in a straight line continue to do so unless acted on by an external force
Step-by-step explanation:
An Inertial Reference Frame is essential for understanding motion in physics. This concept requires that a body at rest or in uniform motion in a straight line will remain in that state unless an external force is applied. When a coordinate system is not accelerating, and all forces are real, it is referred to as an inertial frame of reference.
This is opposed to fictitious forces that arise when observing a system from an accelerating frame of reference, such as in a turning car where the passengers feel a force pushing them to the side, which is not a real force but a result of the car's turning motion. Moreover, a reference frame moving at constant velocity relative to another inertial frame is also inertial, meaning it maintains the same physical laws of motion as the first frame. In contrast, a frame that is accelerating relative to an inertial frame is not an inertial frame of reference.
. This definition is a crucial part of classical mechanics and is explained by Newton's first law of motion.