Why is sliding friction less than static friction?
Answer
Sliding friction is less than static friction because a moving body's surfaces are in contact for too short a time to form strong interlocking bonds between their irregularities.
The working
Static case
When a body is at rest, its surface irregularities settle and interlock firmly with those of the other surface, forming strong momentary bonds.
Moving case
Once the body slides, the two surfaces stay in contact only briefly at each point, so the irregularities cannot interlock as deeply.
Result
Because fewer and weaker contact bonds form during sliding, less force is needed to keep the body moving than to start it moving.
Coefficient comparison
This is why the coefficient of kinetic friction μk is always less than the coefficient of static friction μs for the same pair of surfaces.
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