Is there any case in classical mechanics where Newton's (strong) third law doesn't hold? Is there any case in classical (non relativistic) mechanics where the strong form of Newton's third law does not hold (that is, reaction forces are not collinear)? For example, if we consider a system of two point particles in equilibrium with each other upon which a constraint acts so that the reaction forces are directed in a direction that is not collinear. Is such a situation possible?
 A: If the forces are not precisely opposite one another, this would imply that conservation of (linear) momentum no longer holds; this has never been found in any experiment or observation.
Interestingly, the force must also point along the vector joining the two bodies: in other words, the cross product of the force with the position vector joining the two bodies at the point where the force acts, must be zero. If not, this would imply that conservation of angular momentum no longer holds. This, too, has never been found in any experiment or observation.
That's not to say it's impossible, or that designing experiments to look for it, in ways that are more sensitive than any yet done, are not worthwhile. We actually need people checking on this, to see if conservation of angular or linear momentum is all right, or just "mostly" right.
A: Well I think it does if 1 charged particle is fixed and 1 another similarity charged particle is moving away with constant velocity, then the force on the rest particle at an instant will be inversely proportional to r ( where r is the distance bw them) but force measured on the moving particle will be proportional to r-dr at that same instant since information needs some time to travel and in that time moving particle would have moved dr distance, so the actual force is delayed.It is based on the fact that everything has a speed limit which is c and hence here Newton’s third law doesn’t hold valid.Please correct me if I am wrong.
