# Question: consider the braking process of a car of mass m...

###### Question details

Consider the braking process of a car of mass m = 1500 kg and moving at a velocity of V = 100 km/hr. All four wheels are equipped with disk brakes as shown below. Each disk brake has two brake pads pressing against the rotor. The brake pad has a surface area of Ap = 16 cm2 and a thickness of t = 0.5 cm. During the braking process, about 5 % of the energy dissipated apprears as stored thermal energy of the brake pads and another 5 % goes into shearing the brake pad material itsef. The latter is the dust that appears on the wheels (Where does the rest go?). All surfaces of the brake pads other than the one in contact with the rotor can be considered as adiabatic. The density of the brake pad material is 𝜌 = 4000 kg/m3 and its specific heat is c = 760 J/kg·K. Before applying the brakes, the brake pads are at the ambient temperature of Tatm = 10 OC. The brakes are applied and the velocity of the vehicle is reduced to zero at a constant rate of deceleration. Assume that all of the braking action occurs at the front wheels only.

Construct a proper thermodynamic model that will enable you to accomplich the following taskes:

(a) Describe the nature of the work transfer interaction experienced by on brake pad. Make note of the fact that the brake pad is acted upon by both normal and shear forces. Calculate the rate of work transfer resulting from this interaction.

(b) Calculate the rate of heat transfer experineced by one brake pad.

(c) Calculate the average temperature of the brake pads at the end of the braking process.

(d) Calculate the entropy change of one brake pad at the end of the braking process.

(e) What is the total entropy generation in the braking process when the vehicle has run down to equilibrium?

(f) What ultimately happens to the entropy that is generated in the braking process?