1.1.1 Frames of Reference
The first step is to clarily what we mean by motion. When we say that something is moving , what we mean is that it's its position relative to something else is changing. A passenger moves relative to an airplane, moves relative to the earth; the earth moves relative to the sun; the sun moves relative to the galaxy of stars (the Milky Way) of which it is a member, and so on. In each case, a frame of reference is part of the description of the motion. To say that something is moving always implics a specific frame of reference.
An inertial frame of reference is one in which Newton's first law of motion holds. In such object at rest remains at rest and an object in motion continues to move at constant velocity (constant speed and direction) if no force acts on it. Any frame of reference that moves at constant velocity relative to an inertial frame is itself an inertial frame.
All inertial frames are equally valid. Suppose we see something changing its position with respect to us at constant velocity. Is it moving or are we moving? Suppose we are in a closed laboratory in which Newton's first law holds. Is the laboratory moving or is it at rest? These questions are meaningless because all constant velocity motion is relative. There is no universal frame of reference that can be used everywhere, no such thing as absolute motion.
The theory of relativity deals with the consequences of the lack of a universal frame of reference. Special relativity, which is what Einstein published in 1905, treats problems that involve inertial frame of refcrence. General relativity, published by Einstein a decade later, describes the relationship between gravity and the geometrical structure of space and lime. The special theory has had an enormous impact on much of physics.
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