When you rub two objects together, such as your hands or a stick against a surface, it can generate sound through a process called friction. The rubbing motion creates forces between the two surfaces, causing them to interact at a microscopic level.
When you rub an object, the irregularities on its surface, such as bumps, ridges, or roughness, come into contact with the irregularities on the surface of the other object. As you apply force and move the objects relative to each other, these surface irregularities slide, slip, or stick momentarily, creating frictional forces between them.
As the surfaces interact and slide, some of the applied mechanical energy is converted into other forms, including sound energy. The friction between the surfaces causes the air molecules in the surrounding medium (such as the air) to vibrate. This vibration is then transmitted as a pressure wave through the air, which we perceive as sound.
The vibrating air molecules compress and rarefy in a pattern that corresponds to the frequency and amplitude of the vibration. These pressure variations propagate outward from the source, forming sound waves that travel through the air and reach our ears. When these sound waves reach our eardrums, they cause the eardrums to vibrate at the same frequency, and our brain interprets these vibrations as sound.
In summary, rubbing objects together generates friction, which causes the surfaces to interact and vibrate the surrounding air molecules, producing sound waves that we hear.