Yes, I know what happens when something hits the ground . But I always understood "acceleration" to mean an increase in speed, not a sudden impact. However; in PHYSICS ; the rate of change of velocity per unit of time. I know you used it in this context, but in view of the general definition, this type of speed change is used in context of relativity, no? In this application, the actual direct rate of change in speed is a sudden and catastrophic "deceleration"? No? Not trying to be difficult, just wanting to get a clear understanding of the terms and when they are appropriate.