Final answer:
Using brakes on a steep downgrade is only a supplement to engine braking. The principle is that brakes can overheat due to the heavy mass of trucks, so engine braking is employed to reduce brake wear and prevent overheating.
Step-by-step explanation:
The statement, "The use of brakes on a long steep downgrade is only a supplement to the braking effect of the engine," relates to the concept in physics that during a descent, brakes on a vehicle convert gravitational potential energy into internal energy, which manifests as heat. This heat build-up can jeopardize the effectiveness and safety of the brakes, particularly on heavy vehicles like trucks. To reduce the reliance on brakes and prevent them from overheating, truck drivers utilize engine braking. This method involves using the engine's resistance to slow the vehicle down, effectively supplementing the function of the brakes. Advanced vehicles, such as hybrid and electric cars, employ regenerative braking where the mechanical energy from braking is converted into electrical energy and stored in batteries.
However, in situations where the mass of a truck is large compared to the brake material, engine braking becomes especially essential as a large temperature increase may occur too swiftly during a downgrade, preventing sufficient heat transfer from the brakes to the surrounding environment.