Question

Why does the brake drum of an automobile get heated up while moving down at constant speed?

Answer 1

when automobile moves with constant speed, constant amount of kinetic energy with automobile. now after applying brake, a frictional force act between automobile and surface, due to this kinetic energy is converted into heat energy. that's why the brake drum of an automobile get heated up while moving down at constant speed.

Let v is the constant speed of automobile of mass m . $\mu$ is the coefficient of friction between automobile and surface.
then, fractional force experienced by automobile after applying brake , $f_r=\mu mg$

and then, heat produced $v\mu mg$ per second.

Answer 2

now after applying brake, a frictional force act between automobile and surface, due to this kinetic energy is converted into heat energy. that's why the brake drum of an automobile get heated up while moving down at constant speed.