Question

A car with mass 1275 kg drives at 60 km/h when the brakes are applied quickly to decrease its speed to 20 km/h. Assume the brake pads are 0.5 kg mass with heat capacity of 1.1 kJ/kg K and the brake discs/drums are 4.0 kg steel. Further assume both masses are heated uniformly. Find the temperature increase in the brake assembly.

Answer 1

C.V. Car. Car loses kinetic energy and brake system gains internal u. No heat transfer (short time) and no work term.

m = constant;

E2 - E1 = 0 - 0

= mcar 1/2(V2 2 − V2 1) + mbrake(u2 − u1)

The brake system mass is two different kinds so split it, also use Cv

since we do not have a u table for steel or brake pad material.