Why are there no backward secondary wavefronts?
A secondary wave source is of different nature from a primary wave source. Consider an one dimensional transverse wave on a string:
Primary wave source is activated by the force from above, and then the wave propagates to both directions. Secondary wave source is activated by the particle on the left.
Primary wave source energy is from outside the string. Secondary wave source energy is from an adjacent string molecule.
When the secondary source S reaches its maximum, although it drags both P and Q, they are in different situations. At that moment, while the instantaneous velocity of Q is upward, that of P is downward.
Note that the red arrow at P is the force on P by S. It is not the net force on P.
— Me@2022-03-09 11:14:07 PM
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