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Inertial frame of reference

An inertial reference frame is a coordinate system in which Newton's first and second laws of motion are valid. Hence, within the inertial frame, an object or body accelerates only when a physical force is applied, and (following Newton's first law of motion), in the absence of a net force, a body at rest will remain at rest and a body in motion will continue to move uniformly %u2014ie. in a straight line and at constant speed.
Inertial frame of reference - Wikipedia, the free encyclopedia