Final answer:
In a drum brake system, the brake shoes are the components that expand outward against the drum to stop the vehicle when the brakes are applied. This process is governed by Pascal's principle, where hydraulic pressure causes the shoes to press against the drum and create the necessary friction for stopping.
Step-by-step explanation:
The part in the drum brake system that expands outward against the drum to stop a vehicle when brakes are applied is B shoes. When the brake pedal is pressed, the force is leveraged and intensified through a hydraulic system in accordance with Pascal's principle. The hydraulic pressure causes the brake shoes to press against the inside of the rotating drum, creating friction that slows and eventually stops the vehicle. Hydraulic brakes operate based on the principle that pressure exerted anywhere in a confined fluid is transmitted equally in all directions throughout the fluid. The brake pedal is connected to a set of wheel cylinders via a hydraulic system. When the pedal is applied, the force is transmitted through the hydraulic fluid, causing the wheel cylinders to expand and the brake shoes to move outward, making contact with the drum.