Introduction to Quantum Mechanics 2nd Edition

Published by Cambridge University Press
ISBN 10: 1107179866
ISBN 13: 978-1-10717-986-8

Chapter 1 - Section 1.3 - Probability - Problems - Page 12: 1.2

Answer

In general, the cross product is not associative.

Work Step by Step

Let $\vec{A},\vec{B},\vec{C}$ be 3 non-zero vectors Suppose $\vec{A} = \vec{B}\ne\vec{C}$ , then $\vec{A}\times\vec{B}=0$ $\hspace{1cm}(\because$ they are along the same direction$)$ But, $\vec{B}\times\vec{C}\ne0$ and is perpendicular to both $\vec{B}$ and $\vec{C}\hspace{0.4cm}$ (By definiton of the cross product) Thus, it is also perpendicular to $\vec{A}\hspace{0.4cm}$ ( $\because\vec{A}=\vec{B}$ ) $\therefore\vec{A}\times(\vec{B}\times\vec{C})$ is a non zero quantity, whereas $(\vec{A}\times\vec{B})\times\vec{C}=0\hspace{0,4cm}(\because\vec{A}\times\vec{B}=0)$ $\therefore(\vec{A}\times\vec{B})\times\vec{C}\ne\vec{A}\times(\vec{B}\times\vec{C})$ ,i.e., the cross product between the vectors is not associative.
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