University Physics with Modern Physics (14th Edition)

Published by Pearson
ISBN 10: 0321973615
ISBN 13: 978-0-32197-361-0

Chapter 1 - Units, Physical Quantities, and Vectors - Problems - Discussion Questions - Page 27: Q1.23

Answer

The cross product is not generally associative, as demonstrated with unit direction vectors $(\hat{i} \times \hat{i}) \times \hat{j}$ and $\hat{i} \times (\hat{i} \times \hat{j})$ However, in some special cases it can be. These cases include $(\hat{i} \times -\hat{j}) \times \hat{i}$ and $\hat{i} \times (-\hat{j} \times \hat{i})$

Work Step by Step

$(\vec{A} \times \vec{B}) \times \vec{C} \stackrel{?}{=}\vec{A} \times (\vec{B} \times \vec{C})$ Let $\vec{A} = \vec{B} = \hat{i}$ and $\vec{C} = \hat{j}$ $(\hat{i} \times \hat{i}) \times \hat{j} \stackrel{?}{=}\hat{i} \times (\hat{i} \times \hat{j})$ The cross product of parallel vectors is zero, and $\hat{i}\times \hat{j} = \hat{k}$ $0 \times \hat{j} \stackrel{?}{=} \hat{i}\times \hat{k}$ Anything crossed with zero is zero, and $\hat{i} \times \hat{k} = -\hat{j}$ $0\neq -\hat{j}$; the cross product is not associative An example of three vectors that are associative with the cross product is $\hat{i}, -\hat{j}, \hat{i}$ $(\hat{i} \times -\hat{j}) \times \hat{i} \stackrel{?}{=}\hat{i} \times (-\hat{j} \times \hat{i})$ $-\hat{k} \times \hat{i} \stackrel{?}{=}\hat{i} \times \hat{k}$ $-\hat{j} = -\hat{j} \checkmark$
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