CAIE A-Level Physics AS 3.2.1 Frictional Forces and Viscous Drag Forces Including Air Resistance No Questions

The cylinder in (b) is released from the string at time $t=4.0 \mathrm{~s}$. The cylinder falls, from rest, vertically downwards through the water. Assume that the upthrust acting on the cylinder remains constant as it falls.

State the name of the force that acts on the cylinder when it is moving and does not act on the cylinder when it is stationary.