並非天外飛來一筆，突然談起 SymPy mechanics 之向量指南︰

Guide to Vector

 

而是若不能好好掌握它的架構，將如何用之表達概念、寫作程式耶？同時提醒讀者，有時即使多便詳實細讀，亦難免疑惑生焉！為什麼呢？？常因文本作者通熟其道，以至於忽略了初學者往往不知該系統之理念想法以及設計假設也！！

比方說︰

Introduction to Kinematics

 

有一段講『點』 Point 的文本︰

 

事實上『參考系』 Reference Frame 不過定義了一個抽象的『向量空間』 ，內有『零向量』 ，可以藉著 正交單位基底向量 表述任意向量 耳。

既然『參考系』沒有『原點』或『固定點』屬性？？！！

所謂『點』又是什麼呀！！？？

如果思考『觀察者』之作為，豈不會以選擇任一『靜止點』方便度量『位置向量』乎？★

須經過嘗試與實證︰

 

方將了然下述文本於胸矣！☆

Points are a translational analog to the rotational ReferenceFrame. Creating a Point can be done in two ways, like ReferenceFrame:

>>> O = Point('O')
>>> P = O.locatenew('P', 3 * N.x + N.y)
>>> P.pos_from(O)
3*N.x + N.y
>>> Q = Point('Q')
>>> Q.set_pos(P, N.z)
>>> Q.pos_from(P)
N.z
>>> Q.pos_from(O)
3*N.x + N.y + N.z

 

Similar to ReferenceFrame, the position vector between two points is found by the shortest path (number of intermediate points) between them. Unlike rotational motion, there is no addition theorem for the velocity of points. In order to have the velocity of a Point in a ReferenceFrame, you have to set the value.

>>> O = Point('O')
>>> O.set_vel(N, u1*N.x)
>>> O.vel(N)
u1*N.x

 

自可解碼『慣性系』間關係呦◎