Why is acceleration is called the rate of change of velocity why not rate of change of speed?
In common usage, “acceleration” does often refer to change in speed. I’ve had arguments with (non physics-type) people who’ve insisted that a particle moving at constant speed in a circle is not accelerating, because its speed isn’t changing. But in physics, “acceleration” has to refer to the vector rate of change of velocity. The main reason is so that the definition will be consistent with observed physical law. For example, Newton’s 2nd law, which says that “Force equals mass times acceleration,” does not hold (or rather it holds only in limited special cases) if you define acceleration as the rate of change of speed. On the other hand, if you define acceleration as the vector rate of change of velocity, then F=ma holds in all inertial reference frames.
