Contact Force

The force two objects exert on each other when touching, split into a normal (pressing) part and a tangential (friction) part.

Contact force is the force that arises between two objects because they're physically touching, as opposed to action-at-a-distance forces like gravity or electromagnetism. It's usually split into two components: a normal force perpendicular to the contact surface (a pushing force only — it can press but never pull), and a tangential force parallel to the surface (friction), whose magnitude is limited by the friction coefficient and the normal force, as described by Coulomb friction. A robot's walking, grasping, pushing, and inserting all happen through contact forces: a legged robot supports itself and moves forward using the ground reaction force at its feet, and a gripper lifts an object using the normal force and friction at its fingertips. A physics engine has to solve for contact forces at every simulation step; on a real robot, they're measured with six-axis force sensors, foot force sensors, or tactile sensors, and controllers commonly write “the contact force must stay inside the friction cone” as a constraint.

ExampleWhen training quadruped walking in Isaac Lab, each foot gets a contact sensor reading net_forces_w — the total contact force on that foot in world coordinates. A reading near zero means the foot has left the ground, which is used to infer gait phase and shape reward terms.

Related

Normal Force and Tangential (Shear) Force · Coulomb Friction · Friction Cone · Ground Reaction Force (GRF) · Contact Model · Six-Axis Force/Torque Sensor

Sources

Contact force - Wikipedia
Contact Sensor - Isaac Lab Documentation