Newton's Third Law of Motion either re-explained or properly explained.

[Varun Gupta]

Newton’s Third Law of Motion either re-explained or properly explained:

(I’ve already put it in first two or my three blogs given below and will be shortly putting in third too.

1. http://eyevarun.spaces.live.com

2. http://eyevarun.blogspot.com

3. http://eyevarun.wordpress.com )
We have been reading Newton’s Third Law of Motion since school times and the following two statements of it would be etched in our minds like anything:

“Action and reaction forces are equal and opposite”.

“To every action force there is an equal and opposite reaction force”.

Newton’s Third Law is said to be applicable everywhere and we believe it to the core. However is it really so?

Let’s ask a question to a physics teacher, which I actually asked to an eminent scientist and former Ph.D. student of IIT (Indian Institute of Technology) recently.

Q. Sir, when we push an object like a mobile phone by our finger, the finger pushes the mobile phone away. Here we can easily see that to an action force of our finger there was no reaction force, otherwise the mobile phone wouldn't have moved. So there was no reaction force to the action force here. Rather here the Newton's Second Law was applicable but not the Third. Sir, please explain why Newton's Third Law was not applicable over here as it is said to be applicable everywhere.

(You too think about the answer, before you read his answer.)

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A. Do you feel a little pressure on the finger? That is the reaction.
Well, you would be really thinking that he gave the very right answer. But is it really so?

Let’s try to think again.

Actually the little pressure on the finger is not the reaction force, rather it is the force of friction between the mobile phone and surface on which mobile is kept, which is acting in opposite direction to the movement as always and thus creating pressure on my finger. When my finger overcomes the reaction force then the action force which I'm applying moves the mobile phone. Though, yes if I have applied say 10N of action force and friction force is say 2N then the actual action force in action is only 8N.

I can clarify it further with one more example. Say I'm moving a table with my hand then the force of friction on my hand will be more. However, if I place some oil or butter below table then the friction force will be less and thus also the pressure on my hand. Also to clarify, if the force on my hand were to be reaction force then it should have been same in both cases, as reaction force must be equal to action force and also in opposite direction. To clarify further the force of friction will be zero in space, and thus no pressure on the finger. Thus, it's not the reaction force which must always be equal to the action force.

So it's very much clear that Newton's Third Law is not applicable over here and only Newton's Second Law is very much applicable.

Is Newton’s Third Law wrong then?

Well let me re-explain it in proper way.

Reaction force is actually generated to action force only when the action force is left unused. Like, if I push a stone downwards towards earth or a wall, then my action force is actually not being used at all. The more I push, the more is the force on my hand as well, which is nothing but the opposite as well as equal reaction force to my action force.

So the proper statement of the Newton’s Third Law should be, “Action force if left unused, creates an equal and opposite reaction force”.

There is one more thing into it and it is that there are conditions when some of action force is used and some is left unused, like if on boat I push water backwards, the reaction force generated moves boat ahead. Here some of the action force is being dissipated in frictional and other forces and as I can’t actually move water out of the river then the action force that is left unused creates an equal and opposite reaction force which thus propels the boat ahead.

Thus, a better statement of Newton’s Third Law must be, “Action force if left unused, creates an equal and opposite reaction force, applicable even in cases wherein part of the action force is left unused, wherein the reaction force would only be to the part of action force left unused”.
I’ve used this result too in my book while citing conditions wherein action force when left unused creates an equal and opposite reaction force. Though, I’ve not re-explained Newton’s Third Law in my book, as I really felt that it must be explained properly in some or the other good book, despite we being subjected to the age old statements in almost every book.

But in case it has not yet been explained properly then I’m glad that I’m the one to do it.

Also to tell you this re-explained Newton’s Third Law is not my theory, rather I’ve just cited the final result of the proper law wherein I needed.

This short insight into proper version of Newton’s Third Law must also give you a good peep into the fact that I’ve already said about my book, that I’ve explained everything in my book with proper proofs.

And very good news is that my book is releasing very very soon.
Varun