Velocity is a vector quantity; that is, it has magnitude (speed) and direction. If a body is travelling at a certain speed and in a certain direction undisturbed it has constant velocity. Thus if a body is travelling at a constant speed in a circle, its velocity is constantly changing.
We know from Newton's first law that a body will retain its velocity unless another force acts upon it. Thus when a body travels in a circle a force must be applied to stop it from travelling in a straight line. This force is the centripetal force, the only force necessary for a circular motion. What is interpreted sometimes as a centrifugal force is the tendency of the object to follow in a straight line, which would bring it outside of its circular trajectory.
See also: Coriolis force
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