NHSA Module 3 Practice Test

The force that resists the start of sliding between two surfaces is static friction. When surfaces are at rest relative to each other, static friction acts to oppose any initial tendency to move, increasing in magnitude up to a maximum value (μs times the normal force). Only when the applied effort exceeds that limit does motion begin, at which point friction switches to kinetic friction, which is typically smaller. Rolling friction involves objects rolling rather than sliding, and fluid friction comes from moving through a fluid; neither describes the resistance to starting sliding. So static friction is the barrier to initiating motion.