Tag: mathematical methods

Questions Related to mathematical methods

Three vectors satisfy the relation $\displaystyle \overrightarrow { A } .\overrightarrow { B } =0$ and $\displaystyle \overrightarrow { A } .\overrightarrow { C } =0$, then $\displaystyle \overrightarrow { A } $ is parallel to:

  1. $\displaystyle \overrightarrow { C } $

  2. $\displaystyle \overrightarrow { B } $

  3. $\displaystyle \overrightarrow { B } \times \overrightarrow { C } $

  4. $\displaystyle \overrightarrow { B } .\overrightarrow { C } $


Correct Option: C
Explanation:

Using : $\vec{A} \times (\vec{B} \times \vec{C})  = \vec{B}  (\vec{A} . \vec{C})  - \vec{C} (\vec{A}.\vec{B})$

Given : $\vec{A}.\vec{C}  = 0$  and  $\vec{A}.\vec{B}  = 0$ 
$\therefore$         $\vec{A} \times (\vec{B} \times \vec{C})  = \vec{B}  (0)  - \vec{C} ( 0)   = 0$
Thus, $\vec{A}$ is parallel to  $(\vec{B} \times \vec{C})$.

Vectors $\bar { A }$, $\bar { B }$ and $\bar { C }$ are such that $ \bar { A } \bullet \bar { B } =0$ and $ \bar { A } \bullet \bar { C } =0$. Then the vector parallel to $\bar { A }$ is

  1. $\bar { A } \times \bar { B }$

  2. $\bar { A }+ \bar { B }$

  3. $\bar { B} \times \bar { C }$

  4. $\bar { B}$ and $\bar { B}$


Correct Option: C

The vector $\overrightarrow { B } = 5\hat { i } + 2\hat {  j}-S \hat {  k} $ is perpendicular to the vector $\overrightarrow {  A}= 3\hat {  i} +\hat { j } + 2\hat {k  } $ if S=

  1. $1$

  2. $4.7$

  3. $6.3$

  4. $10.5$


Correct Option: D

$\vec {A}$ and $\vec {B}$ are vectors expressed as $\vec {A} =2\hat {i}+\hat {j}$ and $\vec {B} =\hat {i}-\hat {j}$. Unit vector perpendicular to $\vec {A}$ and $\vec {B}$ is

  1. $\dfrac{\hat {i}-\hat {j}+\hat {k}}{\sqrt{3}}$

  2. $\dfrac{\hat {i}+\hat {j}-\hat {k}}{\sqrt{3}}$

  3. $\dfrac{\hat {i}+\hat {j}+\hat {k}}{\sqrt{3}}$

  4. $\hat {k}$


Correct Option: D

Two particles are simultaneously projected in opposite direction horizontally from a given point in space where gravity g is uniform.If $u _1 and u _2$ be their initial speeds, then the time t after which their velocities are mutually perpendicular is given by

  1. $\dfrac{\sqrt{u _1 u _2}}{g}$

  2. $\dfrac{\sqrt{u^2 _1 + u^2 _2}}{g}$

  3. $\dfrac{\sqrt{u _1(u _1 + u _2)}}{g}$

  4. $\dfrac{\sqrt{u _2(u _1 + u _2)}}{g}$


Correct Option: A
Explanation:

${v _1} = {u _1}\hat i - gt\hat j$

${v _2} =  - {u _2}\hat i - gt\hat j$

$for\,\,\,\,\,\,{v _1} \bot {v _2}$

${{\bar v} _1}.{{\bar v} _2} = 0$

$({u _1}\hat i - gt\hat j).( - {u _2}\hat i - gt\hat j) = 0$

$ - {u _1}{u _2} + {g^2}{t^2} = 0$

${g^2}{t^2} = {u _1}{u _2}$

$t = {{\sqrt {{u _1}{u _2}} } \over g}$

If the magnitude of two vectors are $8$ unit and $5$ and their scalar product is zero, the angle between the two vectors is

  1. Zero

  2. ${ 30 }^{ o }$

  3. ${ 60 }^{ o }$

  4. ${ 90 }^{ o }$


Correct Option: D

If $\overrightarrow { A } +\overrightarrow { B } =\overrightarrow { R }$ and $\left( \overrightarrow { A } +2\overrightarrow { B }  \right)$ is perpendicular to $\overrightarrow { A }$, then

  1. $R=2B$

  2. $R=B/2$

  3. $R=B$

  4. $R=B/\sqrt { 2 }$


Correct Option: A

The angle between the vectors $(\overline{\mathrm{A}}$ x $\overline{\mathrm{B}})$ and $(\overline{\mathrm{B}}\times\overline{\mathrm{A}})$ is:

  1. $0^{0}$

  2. $180^{0}$

  3. $45^{0}$

  4. $90^{0}$


Correct Option: B
Explanation:

($A$$\times $$B$) = - ($B$$\times $$A$) which are equal and opposite in direction.
Hence it will have angle in between $180^0$

In a clockwise system, which of the following is true?

  1. $\hat { j } \times \hat { k } =\hat { i } $

  2. $\hat { i }\ .\hat { i } =0$

  3. $\hat { j } \times \hat { j } =1$

  4. $\hat { k } \ .\hat { i } =1$


Correct Option: A
Explanation:

In clockwise system:
$\hat{i} \times\hat{ j} = \hat{k}$
$\hat{ j} \times \hat{k} = \hat{i}$
$\hat{k} \times \hat{i} = \hat{j}$

The value of $ (\bar { A } +\bar { B } )\times (\bar { A } -\bar { B } )$ is 

  1. $0$

  2. ${A}^{2}-{B}^{2}$

  3. $\bar { B } \times \bar { A }$

  4. $2(\bar { B } \times \bar { A })$


Correct Option: D
Explanation:
$(\overrightarrow A+\overrightarrow B)\times(\overrightarrow A-\overrightarrow B)=\overrightarrow A\times \overrightarrow A-\overrightarrow A\times \overrightarrow B+\overrightarrow B\times \overrightarrow A-\overrightarrow B\times \overrightarrow B=2(\overrightarrow B\times \overrightarrow A)$