Newton's Third Law Exceptions?

Question

Lately I've been brushing up on some of my old Physics texts from college. Most recently, I've been rereading parts of "Classical Dynamics of Particles and Systems (5th ed.)" by Thornton and Marion. In the second chapter, the authors discuss Newton's Laws of Motion, and point out about the Third Law that >We must hasten to add, however that the Third Law is not a general law of nature. The law does apply when the force exerted by one (point) object on another (point) object is directed along the line connecting the objects. Such forces are called central forces. (Pg 50) I know that gravity and the electrical attraction between static charges are central forces, and so I know that these forces certainly obey Newton's Third Law. However, aside from briefly mentioning that velocity-dependent forces do not generally follow the Third Law, the authors do not elaborate. So my question is two-fold. First, is Newton's Third Law not universally true? It was my understanding the the Third Law was a necessary consequence of the homogeneity of space, and conservation of momentum. Second, what would be a familiar example of a non-relativistic force that does not follow the Third Law. Am I correct in understanding that drag would not necessarily follow the Third Law, because the magnitude of the force is velocity-dependent?

Answer 1

The third law is not dependent on what caused the force. Air friction is good example of force that depends on velocity. As an object travels through air, each collision puts a force (impulse, to be more precise) on both the object and the molecule of of air. The force acts to decelerate the moving object, and to accelerate the particles of air.