Does anyone remember this?

Posted by Graeme Davidson on April 7, 2008

It was always one of my favourite subjects at school, and still holds a lot of truth for me today.

Newton’s third law: law of reciprocal actions 

Lex III: Actioni contrariam semper et æqualem esse reactionem: sive corporum duorum actiones in se mutuo semper esse æquales et in partes contrarias dirigi.

All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.

This law of motion is commonly paraphrased as: “For every force there is an equal, but opposite, force”.

A more direct translation is:

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. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. If a body impinge upon another, and by its force change the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of the bodies; that is to say, if the bodies are not hindered by any other impediments. For, as the motions are equally changed, the changes of the velocities made toward contrary parts are reciprocally proportional to the bodies. This law takes place also in attractions, as will be proved in the next scholium.

In the above, as usual, motion is Newton’s name for momentum, hence his careful distinction between motion and velocity.

The two forces in Newton’s third law are of the same type, e.g., if the road exerts a forward frictional force on an accelerating car’s tyres, then it is also a frictional force that Newton’s third law predicts for the tyres pushing backward on the road.

Newton used the third law to derive the law of conservation of momentum; however from a deeper perspective, conservation of momentum is the more fundamental idea (derived via Noether’s theorem from Galilean invariance), and holds in cases where Newton’s third law appears to fail, for instance when force fields as well as particles carry momentum, and in quantum mechanics.

“For every force there is an equal, but opposite, force.”

I love that.  That should mean that anything any one of us should do, has an equal, but opposite force meaning we should all think more carefully about what we do as actions have to have an effect elsewhere.

Somewhere along the lines here, I am trying to say that people should have a lot more thought about empathy.  This is a concept recognised as “reading” another person, and completely translating each movement and syllable into understandable conversation.    It is the ability to put yourself into someone else’s shoes and understand where they are coming from and what they are thinking.   This is what I strive to do every day when communicating to prospective clients.

Understanding that they have needs and desires for their job and realising that everything that I do or say to them will affect the way we are perceived - thus, every action has an equal, but opposite, action.

Maybe not exactly what Newton saw in his head….but I love it.