Normal Force and Tangential (Shear) Force

法向力与切向力（剪切力）Common

The contact-force component perpendicular to a surface is normal force; the component parallel to it is tangential (shear) force.

When two objects touch, the contact force between them can be split into two parts: a normal force, perpendicular to the contact surface, which presses the two surfaces together and keeps them from interpenetrating, and a tangential force, parallel to the surface — also called shear force — usually supplied by friction, which resists relative sliding. The two are linked by Coulomb's law of friction: |f_t| ≤ μf_n, where f_t is tangential force, f_n is normal force, and μ is the friction coefficient. If the tangential force needed exceeds μ times the normal force, the surfaces slip; in 3D, that constraint is exactly the friction cone. When a gripper holds a cup from the sides, the cup's entire weight is supported by tangential friction at the fingertips — if the grip force (normal force) isn't enough, the cup slides out. This is why tactile sensors usually measure both normal and shear force together, with changes in shear force serving as an important early signal of impending slip; legged robots likewise have to keep the ground reaction force inside the friction cone.

ExampleA two-finger gripper holds a 0.3 kg cup from the side, with a friction coefficient of 0.5: each finger needs to provide about 1.5 N of upward tangential force, so each finger's normal (gripping) force must be at least about 3 N — with some margin added in practice.

Also called: Normal Force, Tangential Force, Shear Force
Related: Contact Force · Coulomb Friction · Friction Cone · Friction Coefficient (Coefficient of Friction) · Slip Detection · Tactile Sensor
Sources: Wikipedia: Normal force
Wikipedia: Friction