Re: action and reaction force

April is upon us, bringing with it warmer days, daylight savings time, and resumption of baseball, our national pastime and a sublime demonstration of Newton's 3rd law of motion in action. (follow the links for some lessons on the physics of baseball).

The 3rd law, "to every action there is an equal and opposite reaction", is of course, the statement triggering your question.

The "action" is a force is that which impels an object to move; a "reaction" is the force which opposes such movement.

I'm not sure at which philosophical level you're asking "why". At the deepest level, an honest person must answer, "I don't know". The late Richard Feynmann, winner of the Nobel prize for physics for his work on quantum electrodynamics spent much of his life in search of the fundamental "whys?" behind the elementary forces and particles of natures. Throughout his writings he acknowledges limitations to our understanding of the physical world. While the job of science is to push these limitations back, always and ultimately we are left with an explanation of "what" happens, but not "why".

You see, what physics does is give us MODELS for how the world works. That is, it can describe WHAT happens and HOW, but not WHY (except to give us more details on WHAT and HOW). As Feynmann pointed out, no one has ever seen or touched or heard or smelt an electron. Yet we can be quite successul thinking about the world and acting AS IF they really existed. We see evidence they leave behind, AS IF they really existed. We can manipulate them to achieve desired results AS IF they possessed certain well-known properties (e.g., if you're reading this on a CRT, it is because your computer's manufacturer has been successful in designing a device that works AS IF electrons actually were tiny, negatively charged particles of a particular mass).

The experimental evidence says that the world behaves AS IF every force were opposed by an equal and opposite reaction force.

For example: reach out your hands and press against a nearby wall. Now step away from the wall, and press out your hands again. Why do you feel a force in the first case, and not the second? Nothing relevant has changed about you but your position. The answer is that in the first case, you feel the reaction force of the wall. Indeed, without a reaction force I will claim that a forces becomes invisible, ineffective abstractions. Without reaction forces, you could thrust your hands through the wall as if it were tissue paper; you could toss aside trucks and cars as easily as you swat a floating soap bubble. Without reaction forces, no work could be done -- any more than you do any real work as you thrust out your hands but fail to reach the wall.

A thought experiment: have you every seen a pair of mimes pretend to be a person and mirror reflection? Let's suppose these mimes are absolutely identical twins. One presses her hands to the imaginary mirror, and the other presses back, matching the force exactly. Who is imparting the action, and who is imparting the reaction? What if instead of an imaginary mirror, a clear sheet of thin plastic (like sandwich wrap) served as a wall between the two mimes (so that they could see but not touch directly). What if the wall were opaque, but the two mimes were so highly trained that they could match each other's movements with seeing the other? And finally, what if the thin, opaque wall were replace by concrete? At what point do we distinguish between "reaction force" and the force from the other twin mime?

At a slightly less abstract level I'll submit that two of the primary sources of reaction forces in everyday life are inetrial forces (reaction to the acceleration imparted by an applied force, equal to F=ma) and the electric forces due to the bonds holding atoms and molecules together. We'll leave the interaction of electric and magnetic forces (the electromagnetic force) as an exercise for the student :-)

For example, our baseball players each get a chance to hit the baseball with a bat. When the bat connects, there is a "SMACK!" which the player feels in the hands. This smack is nothing like the feel one would get were the player to hold the bat stationary and allow the ball to hit it (as in a bunt). As you know, when hitting the ball, the player is applying a force to accelerate in a new, and hopefully, more desireable direction. The ball's inertia resists this change to the extent described by Newton's 2nd law, F=ma. The sting in the player's hands is not the force which has been applied to the ball, but the ball's reaction to that force.

This covers inertial forces; what about electrical forces? Recall that like charges repel and unlike charges attract. The forces between negatively charged electrons and positively charged protons hold together atoms in molecules and molecules to form a liquid or solid. When you press your hands against a wall, there is no momentum change, and hence, no inertial force. What does happen is that you bend or compress the wall a bit. At the molecular level, you've stretched apart some unlike-charge pairs and compressed together other like-charge pairs. This stretching and compression converts your mechanical force to potential energy stored in the electric field existing between charge pairs. The force you feel is the electric force opposing the release of this stored energy.

Of course, you may ask, WHY is there inertia? Why does "charge" and "electric force" exist? These are deep questions. That's why the hardest of all questions is "why": that is because every answer , no matter how satisfying, leads to additional questions of "why?". As any parent of a 3-year old can attest, sooner or later the chain of answers must stop with, "Because it just DOES! I don't know WHY."

Most 3-year olds eventually abandon the relentless pursuit of "why?". The others grow up to become research scientists.

May you always keep asking "why?"

Steve Czarnecki

P.S. "Why not?", the favorite of engineers, is a good question, too.