If 4 kg block is held after 2 s of start then how high 2 kg block will rise from the beginning before coming to rest momentarily?

  velocity acquired by each block after 2 s is $\mathrm{v}=\frac{2\mathrm{g}}{3}$

Height attained by 2 kg in 2 s,

At the instant the 4 kg block is held, the 2 kg block has attained a velocity of 2g/3 and this block will continue to move upward with constant retardation 'g'. So the further rise of 2 kg block till it comes to rest is given by

h₂ = V² / 2g = (2g/3)² / 2g = 2g / 9.

The total height = h₁ + h₂ = 2g / 3 + 2g / 9

= 8g / 9