A non-uniform bar of weight W is suspended at rest by two strings of negligible weight as shown in Fig.7.39. 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.
Question
A non-uniform bar of weight W is suspended at rest by two strings of negligible weight as shown in Fig.7.39. 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
To solve this problem, we need to use the equilibrium conditions. The bar is at rest, so the sum of the forces in the vertical and horizontal directions must be zero.
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Resolve the tension in the strings into vertical and horizontal components.
For the left string (angle 36.9°), the vertical component of tension (T1v) is T1cos(36.9°) and the horizontal component of tension (T1h) is T1sin(36.9°).
For the right string (angle 53.1°), the vertical component of tension (T2v) is T2cos(53.1°) and the horizontal component of tension (T2h) is T2sin(53.1°).
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Apply the equilibrium conditions.
For vertical equilibrium, the sum of the vertical components of tension must equal the weight of the bar. So, T1v + T2v = W.
For horizontal equilibrium, the sum of the horizontal components of tension must be zero. So, T1h = T2h.
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From the horizontal equilibrium condition, we can express T1 in terms of T2 (or vice versa).
T1sin(36.9°) = T2sin(53.1°) => T1 = T2*sin(53.1°)/sin(36.9°)
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Substitute this expression for T1 into the vertical equilibrium condition to solve for T2.
T2*sin(53.1°)/sin(36.9°)cos(36.9°) + T2cos(53.1°) = W => T2 = W / (sin(53.1°)*cos(36.9°)/sin(36.9°) + cos(53.1°))
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Now that we have T2, we can find T1 using the expression from step 3.
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The position of the center of gravity can be found by considering the moments about the left end of the bar. The sum of the moments must be zero for the bar to be in equilibrium.
The moment due to T1 is zero because it acts at the point of rotation. The moment due to T2 is T2d (where d is the distance we want to find), and the moment due to the weight of the bar is WL/2 (where L is the length of the bar).
So, T2d = WL/2 => d = WL / (2T2)
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Substitute the values given in the problem (W = weight of the bar, L = 2 m) into this equation to find d.
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