ONE MORE STEP TOWARD BETTER TOMORROW : CHAPTER 3

Saturday, 9 August 2014

CHAPTER 3 - Worked Out Examples

Example: 3

How many lines can we draw that are equally inclined to each of the three coordinate axis?

Solution: 3

Intuitively, we can expect the answer to be $8$ , one for each of the $8$ octants. Lets try to derive this answer rigorously.

Assume the direction cosines of the lines to be $l$ , $m$ , $n$ . Thus,

${l^2} + {m^2} + {n^2} = 1$

$\ldots(1)$

	$\left\| l \right\| = \left\| m \right\| = \left\| n \right\| = \dfrac{1}{{\sqrt 3 }}$
	$\Rightarrow \,\,\,\,\, l = \pm \dfrac{1}{{\sqrt 3 }},\,\,\,m = \pm \dfrac{1}{{\sqrt 3 }},\,\,\,n = \pm \dfrac{1}{{\sqrt 3 }}$

It is obvious that $8$ combinations of $l$ , $m$ , $n$ are possible. Hence, $8$ lines can be drawn which are equally inclined to the axes.

Example: 4

Find the direction cosines of the line segment joining $A({x_1},\,{y_1},\,{z_1})$ and $B({x_2},\,{y_2},\,{z_2}).$

Solution: 4

Refer to Fig – $4$ . Note that the $x$ , $y$ and $z$ -components of the segment $AB$ are $AD$ , $CB$ and $DC$ respectively. If the direction cosines of $AB$ are $l$ , $m$ , $n$ and the length of $AB$ is $d$ , we have

$ld = {x_2} - {x_1},\,\,\,md = {y_2} - {y_1},\,\,nd = {z_2} - {z_1}$

Thus, the direction cosines of $AB$ are given by

${l = \dfrac{{{x_2} - {x_1}}}{d},\,\,\,m = \dfrac{{{y_2} - {y_1}}}{d},\,\,\,n = \dfrac{{{z_2} - {z_1}}}{d}}$

This result is quite important and will be used frequently in subsequent discussions.

Example: 5

Find the projection of the line segment joining the points $A({x_1},{y_1},{z_1})$ and $B({x_2},{y_2},{z_2})$ onto a line with direction cosines $l$ , $m$ , $n$ .

Solution: 5

Let us first consider a vector approach to this problem. The vector $\overrightarrow {AB}$ can be written as

$\overrightarrow {AB} = \left( {{x_2} - {x_1}} \right)\hat i + \left( {{y_2} - {y_1}} \right)\hat j + \left( {{z_2} - {z_1}} \right)\hat k$

A unit vector $\hat u$ along the line with direction cosines $l$ , $m$ , $n$ will be

$\hat u = l\hat i + m\hat j + n\hat k$

Therefore, the projected length of $\overrightarrow {AB}$ upon this line will be.

	$d = \left\| {\overrightarrow {AB} \cdot \hat u} \right\|$
	$= \left\| {l\left( {{x_2} - {x_1}} \right) + m\left( {{y_2} - {y_1}} \right) + n\left( {{z_2} - {z_1}} \right)} \right\|$

This assertion can also be proved without resorting to the use of vectors. For this, we first understand the projection of a sequence of line segments on a given line.

Assume ${P_1},{P_2},{P_3}\ldots {P_n}$ to be $n$ points in space. The sum of projections of the sequence of segments ${P_1}{P_2},{P_2}{P_3},\ldots {P_{n - 1}}{P_n}$ onto a fixed line $L$ will be the same as the projection of ${P_1}{P_n}$ onto $L$ . This should be obvious from the following diagram:

The projection of the segment ${P_1}{P_n}$ onto $L$ is ${Q_1}{Q_n}.$ The sum of projections of segments ${P_1}{P_2},{P_2}{P_3}\ldots {P_{n - 1}}{P_n}$ onto $L$ is ${Q_1}{Q_2} + {Q_2}{Q_3} + \ldots + {Q_{n - 1}}{Q_n} = {Q_1}{Q_n}.$

We use this fact in our original problem as follows:

The projection $d$ of $AB$ onto any line $L$ (with direction cosines say $l$ , $m$ , $n$ ) will be sum of projections of $AC$ , $CD$ , $DB$ onto $L$ . Since $AC$ , $CD$ , and $DB$ are $l\left( {{x_2} - {x_1}} \right),\,m\left( {{y_2} - {y_1}} \right)$ and $n\left( {{z_2} - {z_1}} \right)$ respectively, we get the total projection of $AB$ onto $L$ as

$d = \left| {l\left( {{x_2} - {x_1}} \right) + m\left( {{y_2} - {y_1}} \right) + n\left( {{z_2} - {z_1}} \right)} \right|$

Saturday, 9 August 2014

CHAPTER 3 - Worked Out Examples

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