Applied Mechanics. Made Simple by George E. Drabble

By George E. Drabble

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To ordinary people, the distinction between two kilogrammes mass of apples and two kilogrammes weight of apples is academic, incomprehensible and unimportant. The only quick and practical way of evaluating a mass on Earth is by weighing it. One would hardly expect a greengrocer to apply a standard force to the apples and measure the resulting acceleration before he priced them. He very sensibly relies instead on a ready-to-hand force in the form of weight, which he compares with the weight of a standard mass.

For an engineer who is going to use the motor, neither the value of the magnetic force nor the size of the armature is of any importance: all he is concerned with is the magnitude of the twist that the shaft will provide him with, and the speed with which it will run. If he is going to use the motor to drive, say a hoist, part of his job will be to calculate what torque is required to operate the hoist. Let us consider a very elementary, almost crude, example. Fig. 21 shows a very simple type of hoist.

Here the forces are equal and opposite but, because they do not act at one point, it can be seen that they will cause the ship to turn. Furthermore, if they are placed closer together, as they are in Fig. 17 (b), this turning effect will be reduced. In fact the magnitude of the turning effect is directly proportional to the distance from the turning point. The moment of a force can thus be defined as the product of the magnitude of the force and the perpendicular distance of its line of action from the turning point.

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