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

= m_car 1/2(V₂ ² − V² ₁) + m_brake(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.