Forces acting on the ball in the reference frame of the cup. θ is the contact angle, and the red arrows show the capillary force F_γ, the apparent weight modified by the Archimedes’ P (with g* the apparent gravity), the added mass force F_a and the drag force F_D. ∆z represents the difference between the position of the ball at equilibrium (dotted circle) and during the free fall of the cup (black circle).