# Homework Help: What is the actual definition of rotational motion?

1. Dec 30, 2013

### influx

Ok so I roughly know what rotational motion is (e.g. the spinning of a DVD) but I wish to know the text book definition for it. I tried Googling it but to no avail. The first link on Google gives this:

http://en.wikipedia.org/wiki/Rotation_around_a_fixed_axis

But as mentioned above, that is a special case only, so what is a general case of rotational motion? IE is there a definition for it?

Thanks

2. Dec 30, 2013

### SteamKing

Staff Emeritus
The general case is where the axis of rotation itself may be translating or rotating w.r.t. another reference.

3. Dec 30, 2013

### D H

Staff Emeritus
A free object does not in general rotate about a fixed axis. The object's angular momentum will be constant if no external torques are exerted on the object, but even that does not mean that it will rotate about a fixed axis. Angular momentum and angular velocity will point in different directions if the instantaneous rotation is not about one of the object's principal axes. The object will tumble. Or, as Goldstein puts it in his text Classical Mechanics, "The polhode rolls without slipping on the herpolhode lying in the invariant plane."