The heavier block

Let the mass of the lighter block be m

Mass of heavier block = 2m

Tension in the string, T = 16 N

Consider the following diagram:

If acceleration produced in the system = a

And acceleration due to gravity, g = 10m/s²

When the system is set into motion,

For the heavier block, 2mg = T + 2ma

For the lighter block, T = mg +ma

After the system is released from rest, change in height of the heavier block in the first second, h = 1/2 at² (where t=1sec)

⇒ h = 0.5 × g/3 × 1²

⇒ h = g/6 m

Decrease in gravitational potential energy corresponding to this =(2m-m) ×g×h

= 19.6 J