BCECE Engineering BCECE Engineering Solved Paper-2010

  • question_answer A force\[\mathbf{\vec{F}}=a\mathbf{\vec{i}}+3\mathbf{\vec{j}}+\mathbf{6\vec{k}}\] is acting at a point \[\mathbf{\vec{r}}=\mathbf{2\vec{i}}+\mathbf{6\vec{j}}+\mathbf{12\vec{k}}.\] The value of a for which angular momentum is conserved is

    A)  zero                                     

    B)  1

    C)  -1                          

    D)         2

    Correct Answer: C

    Solution :

    For angular momentum to be conserved? \[{{\vec{T}}_{ext}}=0\] \[\Rightarrow \]               \[\frac{a}{2}=\frac{3}{-6}=\frac{6}{-12}\] \[\Rightarrow \]               \[a=\frac{2\times 3}{-6}=-1\]

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