Elevator with broken cable converting mechanical energy to thermal energy

[dstahn]

The cable of a 2017-kg elevator has broken, and the elevator is moving downward at a steady speed of 1.64 m/s. A safety braking system that works on friction prevents the downward speed from increasing. At what rate is the braking system converting mechanical energy to thermal energy?

Im not real sure what equations to use. Sorry I'm not very helpful. This one just kinda stopped me in my tracks for some reason.

 

[rl.bhat]

Since elevator is falling with constant velocity, the frictional force must be equal to...?
Then rate of doing work = Fr*v.
Now what is relation between the mechanical energy and the thermal energy?