Conservation of Angular Momentum from Newton's third law

[vjraghavan]

I am reading Introduction to Mechanics by Kleppner and Kolenkow. I am now reading the articles dealing with conservation of angular momentum. I am not satisfied with the articles dealing with how the third law does not lead to conservation of angular momentum. Could anyone please throw some light there?

 

[Shooting Star]

I don't have the book right now with me, so I'm a bit unsure of exactly what you don't like. I presume that you are talking about system of particles. The third law states that the action and reaction forces are equal and opposite, but does not say that they lie along the line joining the particles. Only in the latter case will the total angular momentum of a system be conserved if the external torque is zero.

 

[vjraghavan]

I think I got it now. I read into the book incorrectly. :D

 

[arildno]

I don't have the book right now with me, so I'm a bit unsure of exactly what you don't like. I presume that you are talking about system of particles. The third law states that the action and reaction forces are equal and opposite, but does not say that they lie along the line joining the particles. Only in the latter case will the total angular momentum of a system be conserved if the external torque is zero.

Of course, in the special (&idealized) case of CONTACT forces, the joining line segment is of zero length, and hence, the conservation of total angular momentum is preserved anyhow we look at it...

 

[Shooting Star]

So, the case you are talking about is more pertinent for collisions, it seems to me?

 

[arildno]

So, the case you are talking about is more pertinent for collisions, it seems to me?

That's a typical case, yes.