# I Angular momentum conserved for central forces not at origin?

1. Nov 7, 2016

### OmegaKV

My textbook says that for a central force at the origin, the angular momentum is constant, because the derivative rxF is zero since F points radially outwards so it is in the same direction as r. Ok, but what about the angular momentum about a point other than the origin, or the angular momentum with respect to the origin of a central force that is not located at the origin? Would angular momentum still conserved in these cases?

2. Nov 7, 2016

### Ibix

What would it mean if angular momentum were not conserved in those cases?

Can you do the maths to see what would happen?

3. Nov 8, 2016

### vanhees71

Of course then you have to relate the angular momentum to the new point of rotational symmetry. So you have to define
$$\vec{J}=(\vec{r}-\vec{r}_0) \times \vec{p},$$
if $\vec{r}_0$ is the position vector of the point around which the situation is rotatinal symmetric (isotropic). It's very obvious in view of Noether's theorem.

4. Nov 8, 2016

### rcgldr

Momentum is conserved in a closed or isolated system. If there is a central force not located at the origin, then from the frame of the origin, that force must be exerting a torque on something with respect to the origin, and would need to be taken into account.