An object may have
a) varying speed without having varying velocity
b) varying velocity without having varying speed
c) nonzero acceleration without having varying velocity
d) nonzero acceleration without having varying speed.
speed of an object is constant means magnitude of its velocity remains constant but direction of its velocity may change. So when veloctiy varies, speed may not vary, on the contrary when speed varies, magnitude of velocity varies; hence velcotiy must vary. Again when an object moves along a curved path with constant speed its acceleration at a point must be non-zero and directed towards the centre of curvature of the path at that point. This acceleration only changes the direction of velocity but not the magnitude, hence speed remains constant but velocity changes.
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