What does the term "friction" refer to?

The term "friction" refers to the force that resists the relative motion of two surfaces in contact. In the context of air brakes, friction plays a crucial role in how effectively a vehicle can slow down and come to a stop. When the brakes are applied, brake pads or shoes press against the brake drum or disc, creating friction that slows the rotation of the wheels. This process converts kinetic energy into thermal energy, allowing the vehicle to decelerate. Understanding this force is essential for safe driving and effective braking performance, as it directly impacts a vehicle's stopping capability and overall control.

Other options pertain to different aspects of vehicle operation. The speed at which a vehicle can stop is influenced by various factors, including friction, but it does not define what friction itself is. Tire pressure is related to the contact surface and can affect traction, but it is not a characteristic of friction itself. The weight of the vehicle affects braking effectiveness as well, but it is a separate consideration from the concept of friction.