Say that one object (A) exerts a force on another object (B). Then ALWAYS B will exert a force of the same strength on A, but pointing the opposite way. If you want to call these forces action and reaction, it doesn’t matter at all which one you call action and which one you call reaction.
So if A starts to accelerate East, then B will accelerate West, assuming nothing else is exerting forces on them. When the two objects have similar masses, the accelerations are comparable and easily noticeable. If one object is small (say a tennis ball) and the other big (say the Earth), then the force will make the small object accelerate lots more than the big object. When that happens, it looks like only one object is feeling the force, but if you could measure carefully you’d see that actually the force still goes both ways.
Sometimes people get confused about all this when one of the objects is a person. Then it’s tempting to think of the person as somehow different from the other object- say a basketball. They are different, of course, but this law (called Newton’s third law of motion) applies equally to both of them.
(published on 10/22/2007)