the answer
Note that the x-t, ut, and a-t graphs shown
in several figures in this chapter have sharp
kinks at some points implying that the
functions are not differentiable at these
points. In any realistic situation, the
functions will be differentiable at all points
and the graphs will be smooth.
What this means physically is that
acceleration and velocity cannot change
values abruptly at an instant. Changes are
always continuous.​