A 3D Atwood machine: two masses, m1 and m2, hang from a massless inextensible string that passes over a pulley of radius R with adjustable moment of inertia I_p, rendered in genuine three-dimensional space with an orbitable camera. The net acceleration is a = (m1 − m2)g / (m1 + m2 + I_p/R²), and the string tension on each side, T1 = m1(g − a) and T2 = m2(g + a), is equal only when the pulley is ideal (I_p = 0). Drag to orbit, scroll to zoom, adjust both masses and the pulley's rotational inertia to watch acceleration, tension asymmetry and pulley spin change live.