Final answer:
The retraction of the caliper piston in a disc brake system is mainly due to the seal's elasticity and the decrease in hydraulic pressure when the brake pedal is released.
Step-by-step explanation:
In a disc brake system, what returns the caliper piston away from the rotor during brake release is a combination of the pressure release in the hydraulic system and the elasticity of the seal around the piston inside the caliper. When the driver releases the brake pedal, the pressure in the brake fluid decreases, which allows the seal to retract the piston back to its resting position. Additionally, in some systems, there is a mechanical retraction system that assists this process but the primary mechanism is the reversal of the hydraulic pressure that was previously applied to push the piston outwards.
The braking process involves the conversion of kinetic energy of the vehicle to heat through friction between the brake pads and rotors. This analogy can be seen with the example of a book sliding across a floor and giving up its kinetic energy as heat due to friction. Similarly, when brakes apply force to stop a vehicle, heat generates in the brake pads and rotors due to friction, which relates to the energy transformation from the vehicle's motion to thermal energy.