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Chapter 1: Vector Analysis Problem 1.1: Using the definitions in Eqs. 1.1 and 1.4, and appropriate diagrams, show that the dot product and cross product are distributive, a) when the three vectors are co-planar. b) in the general case.  Solution   Problem 1.2 Is the cross product associative? $$(\vec{A}\times \vec{B}) \times \vec{C} \overset{?}{=} \vec{A}\times (\vec{B} \times \vec{C})$$ If so, prove it; if not, provide a counterexample (the simpler the better). Solution   Problem 1.3 Find the angle between the body diagonals of a cube.  Solution Problem 1.4 Use the cross product to find the components of the unit vector $\hat{n}$ perpendicular to the shaded plane in Fig. 1.11. Solution   Problem 1.5 Prove the BAC-CAB rule by writing out both sides in component form. Solution Problem 1.6 Prove that $$[\vec{A}\times (\vec{B}\times \vec{C})]+[\vec{B}\times (\vec{C}\times \vec{A})]+[\vec{C}\times (\vec{A}\times \vec{B})] = 0$$ Under what conditions does $\vec{A}\times (\vec{B}\times \v
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Chapter 1 Vector Analysis: Problem 1.4

Problem 1.4 Use the cross product to find the components of the unit vector $\hat{n}$
perpendicular to the shaded plane in Fig. 1.11.

 

Solution:

We know that cross product of two vectors gives a vector which is perpendicular to both the initial vectors.

Hence to find the vector that is perpendicular to a plane we need two non collinear vectors in the plane and take cross product of them.

 Consider the two vectors as shown in the diagram,

 

 

$$\vec{r_1} = (0\hat{i}+2\hat{j}+0\hat{k})-(1\hat{i}+0\hat{j}+0\hat{k})$$

$$\vec{r_1} = -1\hat{i}+2\hat{j}+0\hat{k}$$

 

$$\vec{r_2} = (0\hat{i}+0\hat{j}+3\hat{k})-(1\hat{i}+0\hat{j}+0\hat{k})$$

$$\vec{r_2} = -1\hat{i}+0\hat{j}+3\hat{k}$$

 

$$\therefore \hat{n} = \frac{\vec{r_1}\times \vec{r_2}}{|\vec{r_1}\times \vec{r_2}|}$$ 

$$\vec{r_1}\times \vec{r_2} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k}\\ -1 & 2 & 0\\ -1 & 0 & 3\\ \end{vmatrix}$$

$$\vec{r_1}\times \vec{r_2} = 6\hat{i}+3\hat{j}+2\hat{k}$$

$$|\vec{r_1}\times \vec{r_2}| = \sqrt{6^2+3^2+2^2}=\sqrt{49} = 7$$

 

$$\therefore \hat{n} = \frac{6}{7}\hat{i}+\frac{3}{7}\hat{j}+\frac{2}{7}\hat{k} $$

 

If you have any doubt regarding the solution or you want solution of some problem which is not posted please let me know by commenting. This encourages me to answer more question because sometime it feels like all I am doing is just a waste. If it helps someone I will be happy to do it.

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