Final answer:
The speed of a vehicle increases on a downgrade because gravity converts gravitational potential energy into kinetic energy, causing acceleration. Brakes counter this by converting kinetic energy into heat, but can overheat due to the mass discrepancy between the vehicle and the brake material.
Step-by-step explanation:
The speed of a vehicle naturally increases on a downgrade due to the force of gravity. As a vehicle moves downhill, the gravitational pull translates potential energy into kinetic energy, causing the vehicle to accelerate. This process is akin to an object falling freely under gravity, except that in the case of a vehicle, the motion is along the slope of the hill rather than straight down.
Vehicle brakes can counteract this acceleration by converting the kinetic energy into internal energy (heat), but this means that brakes can suffer from excessive wear and overheating, especially for heavy vehicles like trucks. This is due to the fact that the mass of the truck is quite large relative to the brake material that is absorbing the energy. As the kinetic energy of a truck converts to the internal energy of the brakes, the temperature can increase rapidly, which may not allow enough time for the heat to dissipate into the environment effectively.