Frictiehoek

Frictiehoek, often referred to as the friction triangle in Dutch, is a schematic tool used in statics and mechanics to analyze the contact forces between a body and a supporting surface. It represents the forces acting at the contact: a normal force N, perpendicular to the surface, and a friction force F, parallel to the surface and opposing motion.

In the friction triangle, the forces form a right triangle. The vertical leg corresponds to the normal force N, the horizontal leg to the friction force F, and the hypotenuse to the resultant force R that acts at the contact. The angle between the normal N and the resultant R is the friction angle, denoted here as θ. This angle satisfies tan(θ) = F/N = μ, where μ is the coefficient of friction (static or kinetic depending on the situation). Therefore θ = arctan(μ).

The friction triangle is used to determine whether motion will occur under an applied tangential force. If the external tangential force does not exceed μN, the contact remains at rest; when F reaches μN, impending motion occurs. This visual tool helps resolve forces quickly in problems involving restraints, inclines, or horizontal pushes.

Notes: the coefficient of friction μ varies with materials, surface conditions, and velocity; μ_s denotes static friction and μ_k kinetic friction. In geotechnical engineering, a related concept is the internal friction angle φ' (phi prime), where tan φ' relates to shear strength over normal stress.

Example: with μ = 0.3, the friction angle θ ≈ arctan(0.3) ≈ 16.7 degrees. If N = 100 N and F = 30 N, F/N = 0.3, yielding θ ≈ 16.7°.