For example, Newton's third law is that if a body acts another body, so the other body reacts to the acting body.
Now consider a mental action, for example one person who loves another person: that action doesn't always cause a reaction, the other person doesn't love a person who loves.
Is not the law of action and reaction, of cause and effect, also true for mental actions as it is for physical actions?
Is Newton's action- reaction law true for only on the body? Isn't it true for our mind?
In relation to kamma the physical universe has a tendency to balance itselt out towards the zero-state. The various conservation laws can be an example of that. Take for example the law of conservation of angular momentum. If you alter one of the vectors the others vectors will "react" and change in accordance in order to maintain the zero-state.
The most common example used is the ice skating figure doing a pirouette. Here the skater begins with arms stretched out and thereby having a large moment of inertia. Then the skater pulls the arms closer to the body and thereby decreasing the moment of inertia. In order to conserve the angular momentum something else has to change and that is the angular rotational component.
In the same way when we commit actions by body, speech or mind there will be an reaction. Like when disturbing the universe it will wobble a bit and then balance itself out again.
When that is said i do not think that your question is really about buddhism and it maybe better to ask the question on Physics SE since you will probably get a better answer there.
When answering your question i tried to make the reaction-mechanism of our actions a bit clearer by making a comparison between the physical universe and kamma (i do not know if such a comparison is valid and this one is merely for the sake of understanding the mechanism). In buddhism we call that cause and effect as its also been mentioned previously in this thread.
Lanka
Physical laws govern relatively simple systems. A human being is a vastly more complex system, with a huge number of physical laws coming into play with even the simplest of interactions. Thus, you shouldn't try to apply action/reaction in this area.
If I bump into something, my mental reaction is the result of a huge number of factors, including how I perceive the event, my state of mind and my history with regards to those events. Each of those in turn is hideously complex, and if you do arrive at basic physical laws, it will be at a much lower level than what presents itself to me (and you'd end up with a huge amount of them).
This fallacy is all too common. Many people try to apply scientific concepts to Buddhist ones, and it's a mistake. Newton's laws don't give you ready predictions for human behavior, conservation of energy does not support rebirth, etc...
If you are really interested in studying this, I recommend studying scientific laws on their own, then moving up to complex systems, and emergence. If nothing else, it will give you an idea of what a gross oversimplification people make when they make this mistake.
