I Newton's Third Law: Action & Reaction Along the Line Connecting Two Objects

[MatinSAR]

TL;DR Summary

Is this law always true or not?

Newton's Third Law: Action & Reaction
His third law states that for every action (force) in nature there is an equal and opposite reaction. If object A exerts a force on object B, object B also exerts an equal and opposite force on object A. In other words, forces result from interactions.(NASA.gov)

Can we assert that this law is universally applicable and always true? Shouldn't the forces between objects act along the line that connects them?
I think I read in a book that Newton's Third Law fails when forces do not act along the line connecting the objects, or when the force depends on particle velocities, such as in the case of magnetic forces. But I'm not sure if it's true or not because one of my professors said that in such cases weak form of the law is valid.

 

[anuttarasammyak]

You are right. It is also expressed as law of momentum conservation.

 

[tech99]

If we have have a current-carrying wire and a nearby magnetic pole, the forces on the wire and the magnetic pole seem to be equal and in opposite directions but not on the same line i.e. we have a turning moment. It strikes me that if force and reaction are in any circumstances unequal, we will have a means of propulsion through space, which seems implausible.

 

[pines-demon]

TL;DR Summary: Is this law always true or not?

Newton's Third Law: Action & Reaction
His third law states that for every action (force) in nature there is an equal and opposite reaction. If object A exerts a force on object B, object B also exerts an equal and opposite force on object A. In other words, forces result from interactions.(NASA.gov)

Can we assert that this law is universally applicable and always true? Shouldn't the forces between objects act along the line that connects them?
I think I read in a book that Newton's Third Law fails when forces do not act along the line connecting the objects, or when the force depends on particle velocities, such as in the case of magnetic forces. But I'm not sure if it's true or not because one of my professors said that in such cases weak form of the law is valid.

As far as I can recall Newton's 3rd law fails in "naively" electromagnetism not because the force is not directed in the line between the two particles but because there is "force" carried by the electromagnetic fields. What I mean is that Newton's third law was for a long time equivalent to conservation of momentum. However when dealing with electrodynamics, momentum is carried by the field so it is not that easy to speak of particular forces anymore.

Example, consider two particles held in place separated by a distance $d$. Now take away one of the charges, as you move the charge away it creates electromagnetic waves. These EM waves reach the other particle, this particle that is still at its original place feels all kind of forces, but in the meantime the other particle is gone.

 

[tech99]

I don't think there is an issue surrounding EM radiation, as we know it has momentum and we get a force when emitting the energy and when the waves strike suitable objects.

 

[pines-demon]

I don't think there is an issue surrounding EM radiation, as we know it has momentum and we get a force when emitting the energy and when the waves strike suitable objects.

Indeed that's what I said, if you do not know that EM carries momentum/force then you might be led naively to think that electromagnetism violates momentum conservation.