Gadfly, I accept your bite, but Anonymous' redefinition of Newton's third law must not be allowed to pass without comment.

Anonymous claims:

Newton's Third Law does not tell us forces exist in pairs. Newton's third law tells us the sum of the vector forces is zero.

Here's what Wikipedia says, in agreement with every reputable source I have seen in the 48 years I have been studying physics:

Third law
Whenever a particle A exerts a force on another particle B, B simultaneously exerts a force on A with the same magnitude in the opposite direction. The strong form of the law further postulates that these two forces act along the same line. This law is often simplified into the sentence "Every action has an equal and opposite reaction".

The Wikipedia article also includes Newton's original Latin wording and the precise English translation, which is accurately summarized in the quoted portion directly above.

I put it this way in the introduction to my history of 20th-century physics (page xx), where I discuss the importance of conservation laws:

Conservation of Momentum. The oldest conservation law in physics results from two of Newton's three laws of motion. 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....

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.

Anonymous, would you care to tell us why we should accept your definition rather than Newton's original words?

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