F.6 Physics Question (20PTS)

A simple qualitative theory to explain friction phenomenon is as follows.
Nothing in the wrold is perfectly smooth. Depending on the roughness (or smoothness) of a surface, there are lumps on it. The more rough the surface is, the more lumps there are.

When two surfaces (e.g. a block rested on a table) are in contact, lumps from each surface will lock onto each other. This produces friction when the surfaces tend to slide, as the locked lumps have to be broken. This process requires a force, which is the frictional froce that we observed.

For a heavier weight, the force pressing the surfaces together (i.e. the normal reaction) increases. The lumps are thus locked more tightly, and even welded together. Sliding between the two surfaces would become more difficult. This explains why frictional force increases with the normal reaction.

With the above understanding, when the two surfaces are stationary, the lumps can be locked more easily than when the two surfaces are in motion. As locking of these lumps produces frictional froce, this is why static friction is higher than kinetic friction.

... More on kinetic friction and statics friction is the momentum exerted on the moving object, which is the force helping to break those "lumps" hence making the kinetic friction smaller.