Friction does work, W=fs. Then the work-heat conversion, Q=W, leads to the temperature rise of the object Δ δT = Q/cm, and c is the specific heat. Of course, due to the different types of friction objects, the specific heat capacity may also be different. If two objects with different specific heats are rubbed, δ t = q/(C 1m 1+C2M2).

Then, when the temperature is higher than the ignition point of one of the objects, under the condition of sufficient oxygen, a chemical reaction (redox) occurs, that is, combustion.

In fact, from a microscopic point of view, friction does work, that is, it converts this work into the internal energy of an object. Under the condition that the state of matter has not changed, the increase of internal energy is mainly reflected in the increase of molecular average kinetic energy, which is macroscopically reflected by temperature.

Of course, for some objects, the state of matter may change when it is rubbed. Heat vaporizes an object and then ignites the vaporized object molecules. Or small debris generated by friction, which acts like molecules vaporized by objects, can also be ignited by the heat generated by friction.