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Added mass is the effective inertia of fluid set in motion when an immersed body accelerates. The body's changing flow field requires kinetic energy; integrating the associated pressure over its surface gives an additional acceleration-dependent force, even in an inviscid model.

For a sphere of volume $V$ accelerating in otherwise stationary, unbounded, incompressible ideal fluid of mass density $\rho$, the added mass is $m_a=\rho V/2$. The fluid's acceleration-dependent force is $-m_a\dot{U}$, so a horizontal force balance takes the form $(m+m_a)\dot{U}=F$ when other forces are absent.^[ref]

An oscillating-body experiment separates this inertial response from velocity-dependent drag by comparing force with both acceleration and velocity. Walls and free surfaces change the flow, and a nonspherical body can have direction-dependent added mass described by a tensor; displaced-fluid mass alone is therefore not a universal coefficient.