In the case of friction, as well as the resistor, conservation of energy cannot be violated. if it appears energy is appearing from nowhere, there is something being overlooked. when negative non-conservative work is being done, energy is converted into a form that is not directly recoverable mechanically. where did the energy go? is it ever possible to reclaim some of this lost energy? if so, how?

the decrease in energy is due to the change in the internal energy of the body, which reflects an increase in its temperature.    

Explanation:

The principle of the conservation of stable energy that energy cannot be created or destroyed but only transformed.

In the case of friction or any other type of dissipative force, the decrease in energy is due to the change in the internal energy of the body, which reflects an increase in its temperature.

This process is due to the breakdown of molecular bonds between the bodies in contact and the spent energy is not fully recoverable from the system.

c)as speed changes, wavelength and frequency change.

everything changes!