When the active drive exceeds drag, the leftover net force accelerates the cell rather than holding it at steady speed. The drag stays 3 times 4 = 12 N while drive 18 N plus signed drag -12 N leaves a net force of 6 N exactly for the stated toy force ledger; real active-matter systems have variable drag, stochastic forcing, nonuniform media, imaging limits, and biological variability.
Changing only the drive shows the force-balance arithmetic. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Hold the drag source fixed
The drag source still gives 12 N from the same speed and coefficient.
3⋅4=12
Extra drive leaves net force
Drive 18 N plus the signed drag -12 N leaves net force 6 N.