Question

A car with mass 3250 lbm drives with 60 mi/h when the brakes are applied to quickly decrease its speed to 20 mi/h. Assume the brake pads are 1 lbm mass with heat capacity of 0.2 Btu/lbm R and the brake discs/drums are 8 lbm steel where both masses are heated uniformly. Find the temperature increase in the brake assembly.

Answer 1

C.V. Car. Car looses 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(V22 − V21) + 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.