How Much Energy Was Lost to Friction When Braking Downhill?

[davedave]

All my friends got different answers to the problem below. What is your answer?

An 800kg car moving at 6m/s begins to coast down a hill 40m high with its engine off. The driver applies the brakes so that the car's speed at the bottom of the hill is 20m/s. How much energy was lost to friction?

 

[mgb_phys]

Kinetic energy_start + potential energy = kinetic energy_end + friction