We know, power = Force × velocity
e.g., P = Fv .........(1)
we also know Force = mass × acceleration
or, Force = mass × change in velocity/time taken
if intial velocity = 0,
then, Force = mass × velocity/time taken
e.g., F = m(v/t) ..........(2)
put equation (2) in equation (1),
P = m(v/t)v = mv²/t
now, mass = density × volume
or , mass = density × base area × length
e.g., m = , put it in above equation.
so , here, A is nothing mass per unit length
hence, P = 100 × (2)3 = 100 × 8 = 800W