If two balls collide (elastically) and there is no friction between them, will their angular momentum change after the collision?
[Physics] Angular momentum after elastic collision
angular momentumcollisionfriction
angular momentumcollisionfriction
If two balls collide (elastically) and there is no friction between them, will their angular momentum change after the collision?
Best Answer
Without friction, the forces during the collision (glancing or head-on) are applied exclusively through their centres of mass. (Illustration available on Wikipedia.)
The torque is given by $\tau=\mathbf r \times \mathbf F$ - but if the forces are applied through the centre of mass, then $\mathbf r$ and $\mathbf F$ are parallel, and hence $\tau=0$.
Without a torque, angular momentum cannot change (because $\frac{\text{d}L}{\text{d}t}=\tau$), so that each ball will keep its angular momentum.
With friction, depending on the relative movement of the balls' surfaces during the collision, there could be a tangential component of the force, which would cause a torque on each ball. Therefore, angular momentum could be transferred. However, as joshphysics mentioned in a comment, the total angular momentum of the system would still be conserved, as there is no external net torque.