A trap displacement is the signed distance from the trap center to the bead position, so the sign tells which side of center the bead sits on. With the trap center at 0 m and the bead at 3 m, the displacement is 3 - 0 = 3 m exactly for the stated toy position ledger; real trap measurements have position noise, calibration drift, Brownian motion, finite camera and sample timing, and measurement uncertainty.
A trap force starts from a checked center-to-bead displacement. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The bead position is measured from the trap center
Set the trap center at 0 metres and the bead at 3 metres.
3−0
Displacement is a signed difference
The measured displacement is 3 metres to the right of the trap center.