Explanation:

mass of the body= 1 kg

u, velocity=3m/s

v, velocity= 6m/s

we know that the kinetic energy of body osgiven by. 1/2 mv²

now,

the gain in the kinetic energy will be given

as ,

(final)- (initial kinetic energy)

1/2 mv²- 1/2 mu²

now putting the respective values

we get

1/2 1. (6)²-1/2 1.(3)²

1/2. 36-1/2.9

18.-. 4.5

13.5