@mo-_- In this question there is no generator to supply power.
That means the energy dissipated as heat must be equal to the decrease in KE because there is no source of extra energy so the total energy must be constant.
But does the W = ∆KE method always work, when there is no generator? Or are there cases where I have to do the power integral i.e W = ∆KE doesn't work?
But the way to remember this is that the total energy is constant. That's a principle you can apply to any system where there is no extra source of energy.