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Roland,

The "action-reaction" terminology is misleading. I always explain that Newton's Third Law notes that forces always exist in equal and opposite pairs. Calling one an "action" and the other a "reaction" can lead to a misunderstanding that one causes the other when in fact they exist as a pair. You aren't alone in misinterpreting the terminology in that way.

In the introduction to my history of twentieth-century physics, I explain the historical importance of conservation laws, noting that Newton's Second and Third Laws lead to conservation of momentum as follows.

The third law, commonly known as the law of action and reaction, states that forces always occur in equal and opposite pairs. Whenever one body exerts a force on another, then the second body exerts the exactly same amount of force on the first, but in the opposite direction. For instance, while the Earth's gravitational attraction holds the Moon in its orbit, the Moon's gravity pulls back on the Earth with equal intensity. Because the Earth is so much more massive than its satellite, the effect of the Moon's gravity on Earth is not to create an orbit but rather a wobble, which is most notable in the ocean tides.

Newton's second law states that when a force acts on a body, it produces a change in a quantity called momentum, which is commonly expressed mathematically as the product of mass and velocity. The longer the force acts on a body, the greater the change in the body's momentum. When two bodies exert equal and opposite forces on each other, they produce equal and opposite changes in momentum. Thus if no other forces are acting on them, the change in total momentum of the two bodies must be zero.

(Reading that now, I probably overstated the case for the Earth not orbiting the Moon. The fact is that they both orbit their common center of mass, which is inside the Earth, and that makes the Earth's orbit around that c.m. a wobble.)

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)