Final answer:
Both Tech A and Tech B are correct. Tech A is correct that drum brakes can fail when driven through deep water, and Tech B is correct that with an increase in temperature, the drum expands away from the shoes, which can cause the brake pedal to drop.
Step-by-step explanation:
Between Tech A and Tech B, both technicians are correct in their assertions. Tech A is correct that drum brakes can fail to slow the vehicle if driven through deep water due to the water interfering with the friction needed for the brakes to function effectively. When brakes get wet, the presence of water can reduce the contact between the brake shoes and the drum, leading to reduced braking efficiency until the water is dispersed and the brakes dry out. On the other hand, Tech B is correct that when drum brakes get hot, the brake pedal can drop because the drum expands away from the shoes. This occurs because heat causes the metal in the drum to expand. This thermal expansion can increase the gap between the brake shoes and the drum, necessitating greater pedal travel to engage the brakes effectively.
The physics behind their observations relate to the principles of energy conversion and thermal expansion. In the case of overheating, as a car descends a hill, the truck brakes convert gravitational potential energy into increased internal energy (higher temperature) of the brake material. However, if the brake temperature increases too quickly, this does not allow for sufficient heat transfer from the brakes to the environment, leading to issues like thermal expansion. Hydraulic brakes use Pascal's principle to ensure that the force applied by the driver on the brake pedal is efficiently transmitted to the brakes. However, excessive heat from prolonged use or rapid deceleration can compromise this system.