Kim,

Missed your second question re: foot and board the first time.

So, a little more on Third Law. I've always disliked the standard textbook phrasing of Newton's Laws. They seem to be designed for memorizing and parroting back rather than for true understanding. "For every action, there is an equal and opposite reaction."

In the first place that statement is abbreviated to the point where it is an oxymoron. There is no such thing as being equal _and_ opposite. What it really means to say is that there is an equal force (same size) in the opposite direction. If an abbreviation is necessary, it should omit the word equal, because opposite _means_ same size already. For example, 2 and -2 are opposites, both the same distance from zero (2 spaces), but in opposite directions (2 spaces right, 2 spaces left). But that's mathematics. Back to physics.

Part of the difficulty people have in learning physics is a result of sloppy thinking like that above. Yes, it's true that the quoted statement is from Newton himself, but he _knew_ what he meant, as eventually everyone who struggles with physics long enough does; or anyone else who is much smarter than the average bear. However, that does not help beginning students to get a clear hold.

Simplest terms, you push (or pull) something, it pushes (or pulls) back just as hard. You push the wall, it pushes back. Action: you push wall. Reaction: wall pushes you. You can always identify an "action-reaction pair" by reversing the subject and the direct object.

And the aspect which nobody ever explicitly pointed out to us non-geniuses (at least when I was a student): an action-reaction pair _ALWAYS_ acts on two different objects. This is the answer to the old paradox about the horse and wagon. The horse, at rest, begins to exert a force on a wagon. But, according to Newton's 3rd, the wagon exerts the same amount of force in the opposite direction on the horse. Ergo, the horse can never move the wagon. Explain.

This stumps most beginning physics students. (Remember the class average of 32 in Bev & Jerry's post.) So, how to understand this?

Action: horse pulls wagon. Reaction: wagon pulls horse. Notice we're interested in the wagon. Is there a force on the wagon? Yes, horse pulls wagon. So the motion of the wagon _CHANGES_. The force that the wagon exerts on the HORSE has nothing to do with the motion of the WAGON. Get it?

Action: Foot pushes board. Reaction: Board pushes foot. The rest is slightly more complicated because in this example the forces are changing. At the beginning of the kick, when the foot first contacts the board the force exerted by the foot is small but rapidly increasing. The board pushes back matching the increasing force exerted by the foot until the board can't push any harder, then it breaks. This is because the intermolecular forces which hold the board together (as mentioned by Amos) have been exceeded. So the board, by pushing on the foot has managed to slow the foot down, but because it isn't strong enough does not manage to stop the foot.