Test Index

Systems of Particles and Rotational Motion

© examsnet.com
Question : 8 of 33
Marks: +1, -0
A non-uniform bar of weight W is suspended at rest by two strings of negligible weight as shown in figure. The angles made by the strings with the vertical are 36.9° and 53.1° respectively. The bar is 2 m long. Calculate the distance d of the centre of gravity of the bar from its left end.
Solution:  
Let AB be the uniform bar of weight W suspended at rest by the two strings OA and O′B which make angles 36.9° and 53.1° respectively with the vertical.
OAA=9036.9=53.1\therefore \angle OAA' = 90^{\circ} - 36.9^{\circ} = 53.1^{\circ}
Similarly OBB=36.9\angle O'BB' = 36.9^{\circ}
AB=2m,AC=dm.AB = 2\,\text{m}, AC = d\,\text{m}.
Let T1 and T2T_1 \text{ and } T_2 be the tensions in the strings OA and O′B respectively and their rectangular components are shown in the figure.1As the rod is at rest, so the vector sum of the forces acting along A′B′ axis and ⊥ to it are zero i.e.
T1cos53.1+T2cos36.9=0(i)- T_1 \cos 53.1^{\circ} + T_2 \cos 36.9^{\circ} = 0 \ldots (i)
and T1sin53.1+T2sin36.9W=0(ii)T_1 \sin 53.1^{\circ} + T_2 \sin 36.9^{\circ} - W = 0 \ldots (ii)
Taking the torques about A and equating the sum of torques to zero, we get
(T2sin36.9)×2+Wd=0- (T_2 \sin 36.9^{\circ}) \times 2 + Wd = 0
or T2=Wd2sin36.9(iii)T_2 = \frac{W d}{2 \sin 36.9^{\circ}} \ldots (iii)
∴ From (ii) and (iii), we get
T1sin53.1=WT2sin36.9=WWd2T_1 \sin 53.1^{\circ} = W - T_2 \sin 36.9^{\circ} = W - \frac{W d}{2}
T1=Wsin53.1(1d2)(iv)\therefore T_1 = \frac{W}{\sin 53.1^{\circ}} \left(1 - \frac{d}{2}\right) \ldots (iv)
∴ From (i), (iii) and (iv), we get T1cos53.1=T2cos36.9T_1 \cos 53.1^{\circ} = T_2 \cos 36.9^{\circ}
or W(1d2)sin53.1cos53.1=Wdcos36.92sin36.9\frac{W \left(1 - \frac{d}{2}\right)}{\sin 53.1^{\circ}} \cos 53.1^{\circ} = \frac{W d \cos 36.9^{\circ}}{2 \sin 36.9^{\circ}}
or W(1d2)tan53.1=Wd2tan36.9\frac{W \left(1 - \frac{d}{2}\right)}{\tan 53.1^{\circ}} = \frac{W d}{2 \tan 36.9^{\circ}}
or 1d21.3319=d20.7508\frac{1 - \frac{d}{2}}{1.3319} = \frac{\frac{d}{2}}{0.7508}
or 1d2=d2×1.33190.75081 - \frac{d}{2} = \frac{d}{2} \times \frac{1.3319}{0.7508}
=d2×1.7740=0.8870d= \frac{d}{2} \times 1.7740 = 0.8870 d
or 0.5d+0.8870d=10.5 d + 0.8870 d = 1 or d=11.3870=0.721 m=72.1 cmd = \frac{1}{1.3870} = 0.721 \text{ m} = 72.1 \text{ cm}
© examsnet.com
Go to Question: