RAJASTHAN ­ PET Rajasthan PET Solved Paper-2008

  • question_answer
    A force\[\overrightarrow{F}=5\hat{i}+10\hat{j}+16\hat{k}\] acts at a point, whose position vector is\[2\hat{i}-7\hat{j}+10\hat{k}\]. The moment of F about the point\[-5\hat{i}+6\hat{j}-10\hat{k}\]is

    A)  \[41\hat{i}-8\hat{j}+55\hat{k}\]

    B)  \[-408\hat{i}-12\hat{j}+135\hat{k}\]

    C)  \[-36\hat{i}+14\hat{j}-35\hat{k}\]

    D)  None of the above

    Correct Answer: B

    Solution :

     Let\[\overrightarrow{F}=5\hat{i}+10\hat{j}+16\hat{k}\] Here, \[\overrightarrow{r}=2\hat{i}-7\hat{j}+10\hat{k}-(-5\hat{i}+6\hat{j}-10\hat{k})\] \[=7\hat{i}-13\hat{j}+20\hat{k}\] \[\therefore \] Moment \[=\overrightarrow{r}\times \overrightarrow{F}\] \[=\left| \begin{matrix}    {\hat{i}} & {\hat{j}} & {\hat{k}}  \\    7 & -13 & 20  \\    5 & 10 & 16  \\ \end{matrix} \right|\] \[=\hat{i}(208-200)-\hat{j}(112-100)+\hat{k}(70+65)\] \[=-408\hat{i}-12\hat{j}+135\hat{k}\]


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