A4kg 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.

Kinetic energy will be converted into electric potential energy if the stationary particle is positive.