The dynamic friction coefficient is typically smaller than the static friction coefficient due to the nature of the surfaces in contact and their interactions at the microscopic level.
When two objects are at rest relative to each other, the friction between them is called static friction. Static friction occurs because the irregularities on the surfaces interlock or adhere to some extent, requiring a larger force to overcome their resistance and set the objects in motion. The static friction coefficient represents the ratio of the maximum static friction force to the normal force between the surfaces.
Once the objects start moving relative to each other, the friction between them becomes dynamic or kinetic friction. Dynamic friction occurs because the surfaces slide or move against each other, and the interlocking or adhesive effects are reduced. The dynamic friction coefficient represents the ratio of the magnitude of the kinetic friction force to the normal force.
The reason the dynamic friction coefficient is typically smaller than the static friction coefficient is that once the objects are in motion, the interlocking or adhesive effects are weakened. The surfaces are no longer in prolonged contact, and the frictional resistance is reduced. Additionally, during motion, there may be the formation and breaking of microscopic bonds between the surfaces, leading to lower resistance compared to the initial static condition.
It's important to note that the actual values of friction coefficients can vary depending on the specific materials and surface conditions involved. The general trend, however, is for the dynamic friction coefficient to be smaller than the static friction coefficient.