Friction & Forces Glossary

A friction-like force from air acting opposite to an object's direction of motion. Generally proportional to speed or speed squared.

The net inward force needed for circular motion. On flat surfaces, static friction provides this force.

A dimensionless constant characterizing the friction between a specific pair of surfaces. μ_s is for static contact; μ_k is for sliding contact.

Any force that requires physical contact between two objects, including friction, normal force, tension, and applied force.

The incline angle at which an object just begins to slide. At this angle, tan(θ) = μ_s.

Negative acceleration; a decrease in speed. Friction typically causes deceleration by opposing the direction of motion.

The friction force on an object that is already sliding, equal to μ_k × N. Typically less than maximum static friction.

The perpendicular contact force a surface exerts on an object. On a flat surface N = mg; on an incline N = mg cos(θ).

The resistive force on a rolling object, caused by deformation at the contact surface. Usually much smaller than sliding friction.

The friction force that prevents two surfaces from beginning to slide. It adjusts up to a maximum of μ_s × N.

The constant speed at which the drag force on a falling object equals its weight, producing zero net force and zero acceleration.

The energy generated by friction when surfaces slide against each other. Kinetic energy is converted into heat through friction.