The speed of circular motion can be measured by the ratio of the time taken by the object to pass the Arc length. If the object is moving from M to N, at some point t passes through point A. In order to describe how fast or slow the object moves when it passes near point A, we can take A short period of time, △t, from this moment on, during which the object moves from A to B by Arc length of △L. The ratio △l/△t reflects the speed of the object, called line speed, expressed by v, that is, v=△L/△t. Line speed is also divided into average and instantaneous values. The time interval taken is very small, so that the receive is instantaneous line speed.

Note that when △t is sufficient hours, the arc AB is almost straight line, there is almost no difference between the arc length of AB and the length of the line segment of AB, and then △l is the displacement of the object from A to B. So, here v is really the instantaneous speed in straight line motion, but now it's used to describe circular motion.

Line speed is a vector with magnitude and direction. An object moving in a circle changes its line speed direction from time to time and always points in the tangential direction of the point.

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Line speed: V = 2*PI*r/T

Here:

r = radius

T = Time cycle