3.2.1—Frictional forces and viscous/drag forces including air resistance (no

Dry friction acts at contact and is limited by μR; viscous or drag forces depend on motion through a fluid and may vary with speed, often as kv or kv² in ideal models.

Identify the regime and direction before choosing a formula. Friction can be static up to a limit, while drag is zero when relative speed is zero.

A falling object experiences weight downward and air resistance upward once it moves; a block at rest can have static friction smaller than μR.

“Resistance” is not always μR, and drag does not have a fixed value independent of speed.