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| Moments (or Torque) |
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| Usually to open or close a door we apply a force on its edge. |
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| It is easier to open the door by applying force near the outer edge away from the hinges. |
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| When we apply the force the door turns on its hinges. Thus a turning effect is produced when we try to open the door. Have you ever tried to do so by applying the force near the hinge? In the first case, we are able to open the door with ease. In the second case, we have to apply much more force to cause the same turning effect. What is the reason? |
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| The turning effect produced by a force on a rigid body about a point, pivot or fulcrum is called the moment of a force or torque. It is measured by the product of the force and the perpendicular distance of the pivot from the line of action of the force. |
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| Moment of a force = Force x Perpendicular distance of the pivot from the force. |
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| The unit of moment of force is newton metre (N m). |
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| In the above example, in the first case the perpendicular distance of the line of action of the force from the hinge is much more than that in the second case. Hence, in the second case to open the door, we have to apply greater force. |
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| There are many examples around us where we use the principle of moments. A son can balance a much heavier father on a see-saw by sitting at a greater distance from the fulcrum F. |
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| Moments of weights of the father and the son about the fulcrum F are equal. |
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| You can spin a bicycle wheel easily if you spin it at the rim. If the force is applied near the hub you have to apply more force to spin the wheel. |
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| Turning effect of the force depends upon the perpendicular distance of the axle from the force. |
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| Here the distance d being less, you will have to apply more force to cause the same turning effect. |
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| The body does not turn as moment of F1and F2 about O are equal and opposite. |
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| The moment of force due to F1 = 6 N x 10 m |
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| = 60 N m |
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| This moment would cause the body to turn in the clockwise direction. |
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| The moment of force due to F2 = 15 N x 4 m |
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| = 60 N m |
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| This moment would cause the body to turn in the anticlockwise direction. |
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| Because the moments, due to both the forces, are equal and opposite, the body does not turn. |
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| 1. Calculate the resultant moment of the lamina shown in figure below. |
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| Sum of the clockwise moments |
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| = (20 x 5) + (40 x 3) + (10 x 2) |
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| = 100 + 120 + 20 |
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| = 240 N m |
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| Anticlockwise moment |
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| = 100 x 4 = 400 N m |
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| Moment due to the force of 1000 N |
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| = 0 (the distance from the line of action to the point O being zero) |
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| Resultant moment |
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| = 400 N m - 240 N m |
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| = 160 N m in the anticlockwise direction. |
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| Thus the resultant moment will cause the lamina to turn about O in the anticlockwise direction. |
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