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| Principle of Moments |
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| If a body is in equilibrium under the action of a number of forces, then the algebraic sum of the moments of the forces about any point is equal to zero. |
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| In other words, the sum of the clockwise moments equals sum of the anticlockwise moments when the body is in equilibrium. |
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| Clockwise moments equal to anticlockwise moments |
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| In the figure above, |
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| Sum of the anticlockwise moments = Sum of the clockwise moments |
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| i.e., (50 x 40) + (100 x 20) + (60 x 10) = (30 x 20) + (100 x 40) |
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| A metre scale is supported at the centre. It is balanced by two weights A and B as shown in figure below, find the distance of B from the pivot. |
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| Clockwise moment and anticlockwise moment about 50 cm divisions are equal. |
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| 20 x d = 40 x 20 |
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Therefore,  |
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| Hence the 20 N force of B is acting from 90 cm mark. |
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| The illustration in figure below shows a uniform metre rule weighing 30 N pivoted on a wedge placed under the 40 cm mark and carrying a weight of 70 N hanging from the 10 cm mark. The ruler is balanced horizontally by a weight W hanging from the 100 cm mark. Calculate the value of the weight W. |
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| W x (100 - 40) + 30 (50 - 40) = 70 x (40 - 10) |
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 60 x W + 30 x 10 = 70 x 30 |
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 60 W = 2100 - 300 |
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Therefore,  |
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| The illustration in figure below represents a metre scale balancing on a knife edge at 20 cm mark when a weight of 60 N is suspended from 10 cm mark. Calculate the weight of the ruler. |
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| Weight of the ruler is acting from the centre of gravity of the ruler (i.e., the mid point 50 cm) |
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 W x 30 = 60 x 10 |
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Therefore,  |
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