Suppose you drag a piece of sandpaper along the surface of a wall (let's keep it simple). Here you are doing positive mechanical work on the sandpaper, this makes it gain energy. Friction does negative work on the sandpaper which makes it lose energy. Now, we know that because the sandpaper is in motion it gains some kinetic energy. But how does the sandpaper acquire heat, i.e., why does its temperature increase?
I've heard some people say it's the negative work of friction which makes the sandpaper lose energy, which ultimately transfers some of its thermal energy as heat to the environment. But if it loses thermal shouldn't its temperature actually decrease?
Or is it the other way around? Not all of the mechanical work gets transferred into kinetic energy. Rather some of it actually gets transported into the sandpaper as heat. But then again shouldn’t the negative work of friction make it lose some of that heat to the environment?
How does the temperature of the sandpaper increase when mechanical work is done on it?
Best Answer
The negative work done by kinetic friction takes the macroscopic kinetic energy of the object it does work on and converts into the microscopic kinetic energy of the molecules of the sandpaper and wall materials, as reflected by an increase in the temperature of the surface of the materials. In effect, the rubbing action between materials increases molecular motion, and thus kinetic energy, of the molecules of the materials.
The temperature increase of the surface of the sandpaper and wall materials is not due to heat. Heat is energy transfer due solely to temperature difference between objects. If the sandpaper and wall are initially at the same temperature there can be no energy transfer in the form of heat. The increase in temperature is due to friction work.
Consider the fact that you can warm the surfaces of your hands by rigorously rubbing them together. The temperature increase of your skin is due to friction work, not heat. On the other hand, if you put your hands in front of fire, they will also warm up. But in this case it is due to radiant heat transfer from the fire to you hands, due to the initial temperature difference between you hands and the fire.
Hope this helps.