"Acceleration is the rate of change of . ...The units are ...velocity per unit time, or distance-per-time per time. ...[F]alling bodies ...near ...earth fall with an acceleration or 9.8 meters-per-second per second ...after the first second ...falling at speed ...9.8 meters per second, after two seconds... 19.6 ...and so on. [T]he units of velocity are length/time... and units of acceleration... (distance/time)/time, or equivalently distance/time2 ...[T]he acceleration near... Earth would be written 9.8 m/sec2 for short."