# When conservation does not apply

1. Mar 18, 2007

### pakmingki

im just talking in general about conservation of linear momentum, angular momentum, mechanical energy, etc.

So, for angular momentum, if angular momentum is constant, then initial angular momentum = final angular momentum.

But, what if there is a net torque? How do you take that into account when calculating the final angular momentum?

2. Mar 19, 2007

### KingNothing

It's quite analogous to linear momentum and linear force. When a torque is applied over a certain amount of time, it adds angular momentum. For example, imagine pedaling a bicycle. The motion of the wheels remains mostly constant, then once you start pedaling it speeds up, increasing angular momentum.

In general, torque can be thought of as a rate of change in angular momentum.

3. Mar 19, 2007

### Claude Bile

Conservation laws only work for isolated systems. If there is an external force or torque for example, conservation of momentum does not apply.

For the case where there is an external torque, then;

Initial L + delta L = Final (L)

To find delta L (change in angular momentum) you need to know the time over which the torque was applied.

Claude.