Answer
6.75 J
Work Step by Step
According to the principle of conservation of mechanical energy,
$K_{2}+U_{2}=K_{1}+U_{1}$
$\implies U_{2}-U_{1}=K_{1}-K_{2}$
We found in part (a) that $K_{1}=6.75\,J$
When water balloon reaches its maximum height, velocity becomes 0. That is, $v_{2}=0$. Therefore
$K_{2}=\frac{1}{2}mv_{2}^{2}=0$ which gives
$U_{2}-U_{1}=6.75\,J-0=6.75\,J$
