# How can I calculate rotational speed?

##### 1 Answer

Hope it helps...

#### Explanation:

Rotational speed

Practically this can be a bit dfficult so what we do to measure the rotational speed of, say, a motor shaft is to visually count the number of rotations per minute.

We observe a point on the rim of the rotating shaft and we count the number of rotations (say 10) and check the time elapsed.

We divide the number of rotations by the time and get the rotational speed in, say:

Now, we can improve a bit by considering that during a complete revolution the angular displacement will be

So now we can go from revolutions per minute to angular dsplacement

Consider an example:

the shaft of a motor rotates

The rotational speed in radians per second that corresponds to

since

[Aside: remember that once we have the angular displacement in radians it is only a matter of considering the radius