Final answer:
To reduce speed while descending a hill, engage the brakes smoothly and use engine braking by downshifting, which converts kinetic energy into thermal energy due to friction.
Step-by-step explanation:
To reduce speed while going down a hill, one should engage the brakes smoothly and apply engine braking if driving a vehicle with gears. This involves gradually reducing your speed by shifting to a lower gear and allowing the engine to help slow the car, rather than relying solely on the brakes which may cause them to overheat. In physics, this relates to the concept of energy conversion and friction, where kinetic energy (the energy of motion) is transformed into thermal energy due to the friction between the brake pads and wheels or between the engine components when engine braking is applied.