Every earlier drift-diffusion lesson in this book (chapters three and six) fixed drift speed at 2 m/s and only ever scanned time. This scan holds time (3 s) and diffusion (2 m²/s) fixed and checks three drift speeds instead: 1 m/s gives mean position 3 m, 2 m/s reproduces chapters three and six's own checked drift speed exactly at mean position 6 m, and 3 m/s gives mean position 9 m. In every row the diffusive spread stays exactly 12 m², since it depends only on time and diffusion — drift speed moves the mean without touching the spread.
Every earlier drift-diffusion lesson fixed drift speed at two metres per second and scanned time; this scan holds time and diffusion fixed and scans drift speed itself. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Every earlier drift row fixed speed at two
Chapter three and chapter six both kept drift speed at 2 metres per second and only ever scanned time. Here time stays 3 seconds and diffusion stays 2 square metres per second, but drift speed itself drops to 1 metre per second, giving mean position 3 metres.
1⋅3=3
Drift speed moves the mean without touching the spread
Time and diffusion never change across these rows. Drift speed alone rescales the checked mean-position arrow, while the diffusive spread ledger, built from time and diffusion alone, stays exactly 12 square metres in every row.
2⋅3=6
A faster drift pulls the mean further at the same spread
At drift speed 3 metres per second, the same time and diffusion give mean position 9 metres, while diffusive spread stays 12 square metres. Three rows separate drift speed from the time factor already scanned in chapter six.