Acceleration of a freely falling sphere with added mass
= Acceleration of a freely falling sphere with added mass
A sphere of density $\rho_s$ falling in inviscid fluid of density $\rho$ has effective inertia $(\rho_s+\rho/2)V$ and net weight $(\rho_s-\rho)Vg$. Hence
$$
\dot U=\frac{\rho_s-\rho}{\rho_s+\rho/2}g.
$$