Junior high school physics textbooks all point out that the sliding friction force is only related to the pressure between the objects and the roughness of the contact surface, but has nothing to do with the contact area between the objects.
In fact, under normal circumstances, when two objects come into contact and are squeezed, the actual contact area between the two objects is much smaller than the apparent contact area between the two objects (that is, what we usually call the contact area).
The research results show that the larger the actual contact area between the two, the greater the friction force. The actual contact area between the two is only related to the size of the positive pressure, the properties of the material and the roughness of the surface, and has nothing to do with their apparent contact area. When the material properties and surface roughness of the objects remain unchanged, the greater the positive pressure, the larger the actual contact area and the greater the friction force; when the positive pressure is the same, the surface contact area between objects changes, for example, changing the surface on a plane Changing the cuboid from vertical to flat does not change the actual contact area, and the friction remains unchanged. Therefore, in general, friction has nothing to do with the surface contact area of ??an object.