The question is about why a brake drum that is over the maximum size should not be put back in service.
The answer explains that when a truck goes downhill and the brakes are used, the brakes convert the truck's gravitational potential energy into internal energy, which raises the temperature of the brake material.
If the brake drum is over the maximum size, it cannot absorb as much heat and can cause overheating.
When a truck goes downhill and uses its brakes to control its speed, the brakes convert the truck's gravitational potential energy into internal energy, which raises the temperature of the brake material.
This prevents the gravitational potential energy from turning into kinetic energy and keeps the truck from speeding up.
However, if the brake drum is over the maximum size, it does not have the ability to absorb as much heat, which can lead to the brakes overheating.