What does Feynman mean here about static and sliding friction?

Question

I'm reading the Feynman lectures and was confused by a section in Volume I Chapter 12 (on forces). Feynman is discussing static and sliding friction:

In experiments of the type described above, the friction is nearly independent of the velocity. Many people believe that the friction to be overcome to get something started (static friction) exceeds the force required to keep it sliding (sliding friction), but with dry metals it is very hard to show any difference. The opinion probably arises from experiences where small bits of oil or lubricant are present, or where blocks, for example, are supported by springs or other flexible supports so that they appear to bind.

I don't understand what he means in the last sentence about "small bits of oil" and "blocks supported by springs". What exactly is the setup he's imagining with blocks supported by springs, and what does "bind" mean in this context? Why would these small bits of oil or supports by springs cause people to (falsely?) believe that static friction exceeds sliding friction?

Answer 1

In the lubrication case, in the real world as two surface slide over each other there are hydraulic effects that cause the coefficient of friction to drop signicantly with velocity before gradually increasing with velocity. This is a well documented phenomenon in journal bearings and led to the development of the Stribeck curve crudely illustrated below where the x-axis is relative velocity and the y-axis is the coefficient of friction.

The spring situation can be illustrated by imagining a block which is pulled by a spring. Initially the spring will stretch until enough tension is present for th block to overcome friction. So a person pulling on a handle attached to the spring will feel that the block is resisting motion even though it is just an artifact of the effect of the spring.