KaptainK wrote:Let us begin with a translation of what Newton actually wrote in his third law and the first two explanatory sentences immediately following that law.

“LAW III. To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone.”

Those two explanatory sentences involve forces and do not necessarily imply motion. Subsequent sentences separately deal with opposite and equal motions. If KaptainK still equates action only with motion and not with force, then I must take it as a failure on my part to have adequately explained to KaptainK why Newton’s third law also applies to forces. I will therefore follow the maxim “When all else fails, consult the manual.” In this case the manual is Webster’s New Twentieth Century Dictionary Unabridged, Second Edition. The word action derives from the Latin noun actio, that derives from the Latin verb agere, to do, drive. The first definition given for action is:
“1. the doing of something; hence, the state of acting or moving; exertion of power or force, as when one body acts on another.”

Newton wrote his Philosophiae Naturalis Principia Mathematica in Latin. He would certainly have known the exact meaning of actio as ‘the doing of something.’ Whether a body moves or not, if it exerts a force it is doing something.

In the Principia, prior to presenting his Laws of Motion, Newton defined his terms. His third definition is:

“DEFINITION III. The vis insita, or innate force of matter, is the power of resisting, by which every body, as much as in it lies, continues in its present state, whether it be of rest, or of moving uniformly forwards in a right line.”

In the third sentence of explanatory text following that definition Newton wrote:
“Upon which account, this vis insita may, by a most significant name, be called inertia or force of inactivity. But a body only exerts this force when another force, impressed upon it endeavors to change its condition;…”

By his own words Newton’s inertial force, his ‘force of inactivity,’ was indeed meant to oppose any motivating force, and was meant to be subject to his third law of motion. However, it is important to remember that inertial force does not prevent a body from moving and arises only if the body is being accelerated; and if a body is being accelerated it must be moving.

Newton also wrote:
“DEFINITION IV. An impressed force is an action exerted upon a body, in order to change its state, either of rest, or of uniform motion in a right line.”
When he subsequently presents
“LAW II. The change of motion is proportional to the motive force impressed; …”
there can be no question that “the motive force impressed” includes only actions exerted upon a body and not actions exerted by a body such as its inertial force.

Motive force impressed is not the same as the net force KaptainK mistakenly applied in that equation. A zero net force does not preclude acceleration if one of its components is inertial force. Only if the sum of all impressed forces is zero will the acceleration be zero.


<font size=-1>[ This Message was edited by: Richard J. Hanak on 2002-08-13 20:05 ]</font>