"dV" is a "small change" in speed.

"dV/dt" is "the rate of change of speed with time"-that is, acceleration. (differential, also known as "derivative of velocity v")

Write the expression: a = dv/dt-(1)

X stands for displacement, and "dX/dt" is "the rate of change of displacement with time"-that is, speed.

Write the expression: v = dx/dt-(2)

Substituting (1) into (2) gives a = (d 2x)/(dt 2)-this is the "second derivative" of "displacement versus time".

Actually, (d 2 v)/(dt 2) is the result of differentiating time again by "dv/dt (acceleration)".

After reading the supplement, say two words:

D(dV/dt)/dt distinguishes "dV/dt" from time again. -It can also be called "the second derivative of velocity v versus time t". The "calculation rule" here is the process of "deduction", nothing else.