The frictional force that opposes motion of two surfaces sliding past each other is called kinetic friction.

Here's a breakdown:

* Friction: A force that resists motion between two surfaces in contact.

* Kinetic Friction: The specific type of friction that arises when two surfaces are already moving relative to each other.

Key characteristics of kinetic friction:

* Direction: Always acts opposite to the direction of motion.

* Magnitude: Dependent on the normal force (force pressing the surfaces together) and the coefficient of kinetic friction (a property of the materials in contact).

* Constant: Once motion starts, kinetic friction remains relatively constant regardless of speed.

Let me know if you'd like to learn more about friction, or explore its different types (like static friction)!