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THERMO Spoken Here! ~ J. Pohl © ~ 2019 | www.ThermoSpokenHere.com (E2220-225) |

The beginning models of physics of matter having motion are those of a matter aproximated as existing "at a point." This is an easy beginning point. Motions of systems that are and remain solid are easier studies than are motions of liquids and gases. Here, beginning with that simple vantage of matter, techniques of calculus are explained then used to describe more complicated motion of matter - motion of liquids and gases.

**Derivative of an Integral with a Variable Limit:**

♦ A theorem from of calculus states the derivative with respect
to x of the integral of a function of "f(u)," ("u" being a dummy variable)
equals the differential of the integrand of the integral as a function of "x." Thus given:

(1) 1 |

Then:

(2) 2 |

While this result is commonly understood there is a tendency to forget how the result is obtained. Beginning with the following equation, correct steps are taken to arrive at the result.

(3) 3 |

The derivative of the constant is zero. Inspecting the
next term we observe that it is a derivative of an integral that has a
variable upper limit. Leibnitz has shown that this derivative equals another
integral plus a term for the upper limit (term_{upper}) minus a
term for the lower limit (term_{lower}). Thus

(4) 4 |

Again the integral is zero. The "upper" and "lower" limit terms are evaluated in a similar fashion. The term for the upper limit is the derivative of the upper limit with respect to "x" times the integrand, f(u) evaluated at the upper limit. Thus:

(5) 5 |

Consequently,

(6) 6 |

which was to be proved.