Average thermal power produced

[yashboi123]

Homework Statement

A 20.0 kg rock is sliding on a rough, horizontal surface at 8.00 m/s and eventually stops due to friction. The coefficient of kinetic friction between the rock and the surface is 0.200.

Relevant Equations

P = F . v
P = Fvcos(x)

I'm curious as to why the average power is not negative in this scenario, since I thought the friction force is in the opposite direction of velocity. As far as I see friction force is the only acting force in the problem, but I may be wrong. I solved this by simply doing (Friction Force * (v/2)), the average velocity.

 

[haruspex]

I'm curious as to why the average power is not negative in this scenario

Because it asks for the thermal power produced, not the work per unit time done on the rock. The friction force the rock exerts on the surface is in the same direction as its velocity.