# Linear and angular momentum

1. Feb 7, 2014

### dyn

Hi,

I,ve been looking through some notes on Lagrangian mechanics but they have got me confused so i'll start from the beginning.
If a point mass is moving in the x-y plane with constant velocity v at a y-coordinate r then it has linear momentum mv which is a constant but it has an angular momentum about the origin of mrv at x=0 ? If the point mass continues in the positive x direction towards ∞ then it its angular momentum must tend to zero ? What happens to conservation of angular momentum ?

2. Feb 7, 2014

### Jimmy

Are your sure that the angular momentum would tend toward zero? What happens to the angular velocity as the mass continues moving in the positive direction?

3. Feb 7, 2014

### dyn

I was using angular momentum=mrvsinθ and as x→0 then sinθ→0

4. Feb 7, 2014

### dyn

I meant as x→∞ then θ→0 and sinθ→0

5. Feb 8, 2014

### dyn

I've figured that now. The angle gets smaller but the modulus of the position vector gets bigger so the angular momentum stays the same.
So all linear motion that is not radial has an angular momentum about the origin ?
For a point mass is there such a thing as total momentum combining linear momentum and angular momentum ?

6. Feb 8, 2014

### Sunfire

Linear and angular momentum can't be summed up together as they have different units, so their mere sum won't make sense