The physics
A box lands at rest on a running belt. The belt slides under it. So kinetic friction drags the box forward at a = μg.
slip time: t = v / (μ g)
In that time, the box moves v² / (2μg) over the ground. The belt moves vt = v² / (μg), twice as far.
slip on the belt = v t − v² / (2 μ g) = v² / (2 μ g)
Heat comes only from rubbing, so it uses the slip. Q = μmg × slip = ½ m v².
The motor gives m v² in all. Half becomes the box's kinetic energy, and half becomes heat.