JEE Main & Advanced Physics Vectors Question Bank Multiplication of Vectors

  • question_answer
    The torque of the force \[\overrightarrow{F}=(2\hat{i}-3\hat{j}+4\hat{k}\,)N\] acting at the point \[\overrightarrow{r\,}=(3\hat{i}+2\hat{j}+3\hat{k})\]m about the origin be [CBSE PMT 1995]

    A)                 \[6\hat{i}-6\hat{j}+12\hat{k}\]

    B)                             \[17\hat{i}-6\hat{j}-13\hat{k}\]

    C)                 \[-6\hat{i}+6\hat{j}-12\hat{k}\]

    D)                 \[-17\hat{i}+6\hat{j}+13\hat{k}\]

    Correct Answer: B

    Solution :

                        \[\overrightarrow{\tau }=\overrightarrow{r\,}\times \overrightarrow{F}\] \[=\left| \begin{matrix}    \hat{i}\,\, & \hat{j}\,\, & {\hat{k}}  \\    \,3\,\,\, & \,\,2\,\,\,\,\, & 3  \\    \,2\,\,\, & -3\,\,\,\,\,\, & \,\,4\,\,  \\ \end{matrix} \right|\,\]                 \[=\left[ (2\times 4)-(3\times -3) \right]\,\,\hat{i}+\left[ (2\times 3)-(3\times 4) \right]\,\hat{j}\]                 \[+\left[ (3\times -3)-(2\times 2) \right]\,\hat{k}\]\[=17\,\hat{i}-6\,\hat{j}-13\,\hat{k}\]


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