Slip

Relative sliding between contact surfaces, usually because the tangential force needed exceeds the friction limit.

Slip means two touching objects have relative motion at their contact point; the opposite state, where the contact point stays relatively still, is called sticking. Under the common Coulomb friction model, the tangential friction force f_t must satisfy f_t ≤ μf_n (μ is the friction coefficient, f_n is the normal force); the moment the tangential force actually needed exceeds this limit, the contact switches from sticking to sliding, at which point friction equals exactly μf_n and points opposite the direction of motion — geometrically, the contact force has left the friction cone. In manipulation, a grasp that slips can drop or spin an object out of position, so the fix is either more grip force or slip detection with a tactile sensor; in legged robots, a support foot that slips is a common cause of falling, so reinforcement-learning training often penalizes horizontal velocity at the contact foot. Deliberately controlled sliding is also a skill in its own right — pushing an object across a table, or letting it rotate between the fingers.

ExampleA two-finger gripper holds a 0.5 kg cup vertically, with its roughly 4.9 N of weight split between the two contact points' friction. If μ = 0.5, each finger needs at least about 4.9 N of normal force (2×0.5×4.9≈4.9 N), or the cup slides downward. Humanoid-Gym's foot_slip reward term penalizes a foot's horizontal velocity whenever it's in contact with the ground.

Also called

Sliding, Slippage, Foot Slip

Related

Coulomb Friction · Friction Cone · Friction Coefficient (Coefficient of Friction) · Slip Detection · Force Closure · Stance Phase / Swing Phase

Sources

Modern Robotics (Lynch & Park), Ch.12.2 Contact Forces and Friction
Humanoid-Gym: humanoid_env.py (_reward_foot_slip)