Well, it is known that a pulse gets inverted when it gets reflected from a fixed support while the polarity of the pulse remains same when the incident pulse gets reflected from a free end.
However, I'm quite baffled at the explanation given by most books on the mechanism of reflection of the pulse from a free end.
The following is excerpted from French's Vibrations and Waves:
The inverted reflection is not so mysterious when we consider that the arrival of a positive displacement will exert an upward force on the support which holds the end fixed. By Newton's Third Law, the support exerts a reaction force in the opposite direction back on the string, thus generating a pulse of opposite polarity which travels backwards towards the source.
If the end of the string is completely free to move, the arrival of a positive pulse will exert an upward reaction force generating a pulse of positive polarity.
Now, my question is: What happened to Newton's Third Law in the later case? Why didn't the free end exert an opposite reaction force i.e. downward force on the pulse? Isn't it contradicting the law?
What am I missing?
Also, another line goes as:
[...] the right end is displaced in upward direction more than the height of the original pulse i.e. it overshoots the normal maximum displacement. An extra force acts from right which sends a pulse from right to left with the same shape of the original one.
Why does the free end 'overshoot' or 'displaced more than the height of the original pulse'? And how does that extra force emanate?
