How Everything Works
How Everything Works How Everything Works

Question 916

I have read that sometimes two very slick things rubbing together have more friction than two rough things. Is that true? Why? — A
Friction is caused by contact and collisions between the tiny projections that exist on all surfaces. When you put one block on top of another, the tiny projections on the bottom of the upper block touch the tiny projections on the top of the lower block. If you then try to slide one block across the other, these projections begin to collide with one another and they oppose the sliding motion.

If the two blocks have rough surfaces, then the projections that are colliding are obvious to your eyes. But if the two blocks have very smooth surfaces, you can't see their surface projections. However, the invisibility of these projections doesn't make them insignificant. Even the smoothest surfaces are rough at the atomic scale. When you press two smooth surfaces against one another, their microscopic projections still touch one another and those projections still collide when you try to slide the surfaces across one another. In short, smooth surfaces still experience friction.

But it's also possible for attachments to form between portions of the two smooth surfaces when they touch. This molecular adhesion makes it even harder to slide the two surfaces across one another. You can feel this adhesion when you press two pieces of very clean glass against one another—they form bonds that partially stick them together. Actually, this sort of sticking would be quite common if it weren't for water. Almost all surfaces are coated with a layer or two of water molecules. These water molecules lubricate the interface between any two surfaces and make it hard for those surfaces to stick to one another. But if you get rid of the water molecules, the sticking becomes quite severe. This effect causes trouble in my laboratory, where sliding mechanisms that move easily in air stop working properly when we put them in a vacuum chamber and remove the water on their surfaces.


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