Friction & Forces Cheat Sheet

The core ideas of Friction & Forces distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Static Friction

The force that resists the initiation of sliding motion between two surfaces in contact. It adjusts in magnitude up to a maximum value to exactly oppose any applied force.

Kinetic Friction

The constant resistive force acting on an object that is already sliding across a surface, typically less than the maximum static friction between the same surfaces.

Coefficient of Friction

A dimensionless number (mu) that characterizes the roughness between two surfaces. Static coefficient (mu_s) is for stationary contact; kinetic coefficient (mu_k) is for sliding contact.

Normal Force

The perpendicular contact force exerted by a surface on an object resting on it. Friction depends on the normal force because greater surface compression increases microscopic contact area.

Friction on Inclined Planes

On a ramp, the normal force equals mg cos(theta), so friction equals mu times mg cos(theta). The gravitational component pulling the object down the ramp is mg sin(theta).

Air Resistance (Drag)

A friction-like force exerted by air on a moving object, proportional to the object's speed (or speed squared at higher velocities). It acts opposite to the direction of motion.

Terminal Velocity

The constant maximum velocity reached by a falling object when air resistance equals gravitational force, resulting in zero net force and zero acceleration.

Friction and Energy

Friction converts kinetic energy into thermal energy (heat). The work done by friction equals the friction force times the sliding distance, and this energy is lost from the mechanical system.

Key Terms at a Glance

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

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

Coefficient of Friction (μ):A dimensionless constant characterizing the friction between a specific pair of surfaces. μ_s is for static contact; μ_k is for sliding contact.

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

Air Resistance (Drag):A friction-like force from air acting opposite to an object's direction of motion. Generally proportional to speed or speed squared.

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

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

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

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

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

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

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

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