A robot arm must accelerate to be extended. Constant$-$acceleration motion is simple to analyze but rarely applies in the real world because acceleration cannot be instantly turned on and off. Robot designers have found that motion can be both practical $($implemented by software and real motors$)$ and sufficiently smooth if it has constant jerk. Just as velocity is the time derivative of position and acceleration is the time derivate of velocity, jerk is defined to be the time derivative of acceleration. That is$,$ jerk$$jx$=$daxdtjx$=$daxdt is the rate of change of acceleration, a name that comes from the fact that motion with a changing acceleration can be quite jerky. Motion with constant jerk is motion in which the acceleration has a constant rate of change. What are the units of jerk?