inertial frame of reference
Quick Summary
An inertial frame of reference is a system in physics where Newton's laws of motion are valid. In simpler terms, it's a viewpoint from which an object at rest stays at rest, and an object in motion continues in motion at a constant speed in a straight line, unless acted upon by a force.12
Strictly speaking, these laws are only valid in a frame of reference that is stationary relative to the distant stars. However, any frame of reference moving at a constant velocity without rotation relative to such a stationary frame is also considered inertial. This concept is fundamental to the principle of Galilean relativity, which states that the laws of physics are the same for all observers in uniform motion.32
The Earth itself is not a perfect inertial frame because it rotates and accelerates. However, for many practical purposes, it can be approximated as one. The theory of special relativity further refines this by showing that the speed of light is constant for all inertial observers, leading to phenomena like time dilation and length contraction.32
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