A frame of reference is a perspective from which a system is observed. In physics, it provides a set of axes relative to which an observer can measure the position and motion of all points in a system, as well as the orientation of objects in it. There are two types of reference frames: inertial and non-inertial. An inertial frame of reference translates at a constant vectorial velocity, which means that it does not rotate (translates) and its origin moves with constant velocity along a straight line (constant vectorial velocity, null vectorial acceleration). In inertial frames, Newton's first law (inertia) holds true. A non-inertial frame of reference, such as a curving car, an accelerating car, or a rotating carousel, accelerates and/or rotates. Newton%u2019s first law does not hold true in a non-inertial reference frame, as objects appear to accelerate without the appropriate forces. A constant velocity is not enough to obtain an inertial reference frame. Frames translating at a constant (linear) velocity along a curved trajectory or rotating at a constant (angular) velocity are non-inertial due to centripetal acceleration.
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