Increase in the internal energy of the block-surface system due to friction [closed]

Question

I found this question:

https://plainmath.net/force-motion-and-energy/13968-block-dragged-horizontal-surface-acting-degree-horizontal-displaced

and I'm trying to understand why he used this formula to calculate the internal energy of the block-surface system due to friction. But I can't get it. He used

$$W_f = -ud(mg-F\sin{x})$$

Why is that? Could someone explain it to me?

Answer 1

While your question is a bit unclear, it appears you are asking why kinetic friction increases internal energy. The short answer is the work done by kinetic friction on an object converts the kinetic energy of its motion as a whole into an increase in the motion of the molecules of the material (often referred to as "heat"). In other words, the kinetic energy of the motion of the object as a whole is converted to molecular kinetic energy of the object by an amount equal to the kinetic friction work. That energy is referred to as internal kinetic energy.

Hope this helps;