Look at the drawing given in the figure, which has been drawn with ink of uniform line-thickness. The mass…

Look at the drawing given in the figure, which has been drawn with ink of uniform line-thickness.
The mass of ink used to draw each of the two inner circles, and each of the two line segments is $m$. The mass of the ink used to draw the outer circle is $6 \mathrm{~m}$. The coordinates of the centres of the different parts are, outer circle 10 , $0)$, left inner circle $(0,0)$, right inner circle $(a, a)$, vertical line $(0,0)$ and horizontal line $(0,-a)$. The $y$-coordinate of the centre of mass of the ink in this drawing is
  1. $\frac{a}{10}$
  2. $\frac{a}{8}$
  3. $\frac{a}{12}$
  4. $\frac{a}{3}$

Solution

$y_{C M}$ $ \begin{gathered} =\frac{m_1 y_1+m_2 y_2+m_3 y_3+m_4 y_4+m_5 y_5}{m_1+m_2+m_3+m_4+m_5} \\ =\frac{(6 m)(0)+(m)(a)+m(a)+m(0)}{6 m+m+m+m+m}=\frac{a}{10} \end{gathered} $

Asked in: JEE Advanced 2009 (Paper 1)

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