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NOTE-Guys , you need to find out the incorrect option among the given four options. and not the correct option

A D C B

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1 solution

Steven Chase
Aug 5, 2017

Use conservation of energy to find the speeds.

m g ( l + 2 l ) = 1 2 m ( l 2 + 4 l 2 ) ω 2 3 g l = 5 2 l 2 ω 2 ω = 6 g 5 l v B = l ω = 6 g l 5 mg (l + 2l) = \frac{1}{2} m (l^2 + 4l^2) \omega^2 \\ 3gl = \frac{5}{2} l^2 \omega^2 \\ \omega = \sqrt{\frac{6g}{5l}} \\ \boxed{v_B = l \omega = \sqrt{\frac{6gl}{5}}}

We therefore know that Option A is correct. Find the rod tensions using the following Newton's 2nd Law equations:

T B C m g T A B = m v B 2 l = 6 m g 5 T A B m g = m v A 2 2 l = 12 m g 5 T A B = 17 m g 5 T B C = 28 m g 5 T_{BC} - mg - T_{AB} = \frac{m v_B^2}{l} = \frac{6mg}{5} \\ T_{AB} - mg = \frac{m v_A^2}{2l} = \frac{12mg}{5} \implies \boxed{T_{AB} = \frac{17mg}{5}} \\ \implies \boxed{T_{BC} = \frac{28mg}{5}}

Therefore, Option C is correct and Option B is wrong.

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