kel-
“to drive, to set in motion”drive, impel
Root for driving/setting in motion, yielding Latin celer (swift), Greek keles (racehorse).
Discussion
The PIE root *kel- (to drive, to set in motion, to urge forward) produced vocabulary for physical propulsion and temporal acceleration across several branches — words for striking, driving cattle, and the swift passage of events.
Latin celer (swift, quick — one who is driven forward) continues the root and gave English: accelerate (ad-celerāre, "to add speed to" — to increase the rate of driving forward), celerity (swiftness), and the personal name Celeris. The connection between driving and speed is transparent: what is driven moves fast.
Greek kélomai (κέλομαι, "to urge, to command, to drive") and the related kélēs ("a fast horse, a racer") preserve the root in both the command sense and the speed sense. The kelēs was specifically a horse bred for racing — the driven animal par excellence.
Latin calāre ("to call, to proclaim" — from *kelh₁-, a closely related or identical root) may share the same origin through the concept of driving people with the voice. From calāre came: calendar (Kalendae, the first day of the month when the pontifex "called" or proclaimed the dates), class (classis, originally a group "called" together for military service), and claim (clāmāre, "to call out" — an intensified form of the same root).
The semantic range — driving physically, urging verbally, proclaiming publicly — reflects a culture in which vocal command and physical propulsion were aspects of the same authoritative act: to drive cattle, to drive an army, and to drive a point home.
Notes
Pokorny 548.