# Help with Rotational Motion

1. Sep 8, 2011

### Xinyee

Can anybody please explain to me the concept of rotational motion?? I'm so lost in that chapter... ><

2. Sep 8, 2011

Which part of rotational motion? Frankly, its a vast concept so it would help if you could be more specific, please.

3. Sep 8, 2011

### Xinyee

Erm... The part about moment of inertia and angular momentum...

4. Sep 8, 2011

I take it you've understood linear momentum? I'll proceed with that assumption.

In linear momentum, when a body resists it's state of motion (i.e. when we have to apply some force to get it to move or to bring it to rest) we call it inertia. Similarly, when a body is in constant rotation, it opposes any changes we try to make in its rotation. This opposition is called its 'Moment of Inertia.'

Now this is a purely geometric concept as it depends on the distribution of mass within the body and the position of the axis of rotation (i.e. the imaginary line around which the body rotates.) This means the moment of inertia of the same body takes different values based on where you consider the axis of rotation to be.

To put it simply, the moment of inertia of the entire body (I) is the total sum of the products of the masses at all points in that body and the square of their respective distances from the considered axis about which they are rotating. That is,

I=mr2

The angular momentum (L) of a rotating body is the counterpart of linear momentum. To state it at a very basic level, linear momentum (p) of a body is the product of its mass and velocity. Similarly, angular momentum is the product of the mass and angular velocity (= velocity at which the point mass is rotating.)