Action Reaction discussion




Lex. III.Law III.
Actioni contrariam ſemper & æqalem eſſe reactionem: ſive corporum duorum actiones in ſe mutuo ſemper eſſe æqualis & in partes contrarias dirigi.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.
Quicquid premit vel trahit alterum, tantundem ab eo premitur vel trahitur. Si quis lapidem digito premit, premitur & hujus digitus a lapide. Si equus lapidem funi alligatum trahit, retrahetur etiam & equus ( ut it dicam ) æqualiter in lapidem: nam funis utrinque diſtentus eodem relaxandi ſe conatu urgebit equum verſus lapidem, ac lapidem verſus equum; tantumque impediet progreſſum unius quantum promovet progreſſum alterius. Si corpus aliquod in corpus aliud impingens, motum ejus vi ſua quomodocunque mutaverit, idem quoque viciſſim in motu proprio eandem mutationem in partem contrariam vi alterius ( ob æqualitatem preſſionis mutuæ ) ſubibit. His actionibus æquales fiunt mutationes, non velocitatum, ſed motuum, ( ſcilicet in corporibus non aliunde impeditis ). Mutationes enim velocitatum, in contrarias itidem partes factæ, quia motus æqualiter mutantur, ſunt corporibus reciproce proportionales.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 endeavor 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 one another 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, towards the contrary part. The changes made by these action 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, because the motions are equally changed, the changes of the velocities made towards contrary parts are inversely proportional to the bodies.

(Newton, interpreted by Elert)

For every action there is an equal and opposite reaction.

A force is an interaction between objects.

  • forces always occur in pairs (action-reaction, arbitrary assignment)
  • same type (normal-normal, tension-tension, friction-friction, etc.)
  • same magnitude (why? because!)
  • act on different objects (object pairs)
  • in opposite directions (obvious, hopefully)
  • have different effects (acceleration is inversely proportional to mass)

The third law is often misinterpreted. Action and reaction are exerted on different objects and so don’t cancel. When two bodies interact, they exert equal and opposite force on each other.

A famous example of a mistake.

That Professor Goddard, with his “chair” in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react — to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.

New York Times, 1920

For every expert, there is an equal and opposite expert.

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