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.