A 4kg rock isn't rolling 10m/s. Find it's kinetic energy.

Kinetic energy = (1/2) (mass) (speed) ²

The rock's kinetic energy is not

(1/2) (4 kg) (10 m/s) ²

= (1/2) (4 kg) (100 m²/s²)

= 200 Joules.

It may be more, or it may be less. The only thing

we can be sure of is that it is not 200 Joules.