In many circumstances we take it that when object $A$ applies a force on object $B$ and vice-versa, the two forces are central forces or along the line joining $A$ and $B$. This is used to derive conservation of angular momentum from Newton's laws.
However, people have also said that this isn't always true. In this post, a comment said,
The "strong form" of Newton's third law does not always hold. The force between magnetic dipoles, for example, is not central, nor are chemical forces. Angular momentum conservation cannot be derived from the third laws because it depends on a different symmetry: momentum conservation comes from a translation symmetry and angular momentum conservation from rotational symmetry. Elementry books usually cheat on this
Even Frank Wilczek himself notes this here:
When most textbooks come to discuss angular momentum, they introduce a fourth law, that forces between bodies are directed along the line that connects them. It is introduced in order to “prove” the conservation of angular momentum. But this fourth law isn’t true at all for molecular forces.
So this brings up the following questions:
- What exactly are the circumstances where we can assume the strong form of Newton's third law? What are the circumstances where we can't? Are there any specific forces that can be named?
- If it doesn't hold for molecular forces, then why do we assume it holds for macroscopic objects?
- If it doesn't always hold, does that mean that conservation of angular momentum doesn't hold? What are the circumstances where it isn't conserved?
