Time and Clocks - A Description of Ancient and Modern Methods of Measuring Time by Sir Henry H. Cunynghame

Author:Sir Henry H. Cunynghame [Cunynghame, Sir Henry H. (Henry Hardinge)]
Language: eng
Format: epub
Publisher: anboco
Published: 2017-04-12T22:00:00+00:00

Fig. 34.

Fig. 35.

Next we will knock a tintack into any point B, and tie a string on to B. Then if I pull at the string in any direction B C the board tends to twist round the screw at A. What will the strength of the twisting force be? It will depend on the strength of the pull, and on the “leverage,” or distance of the line C B from A. We might imagine the string, instead of being attached at B, to be attached at D; then, if I put P as the strength of the pull, the twisting power would be represented by P × A D. This is called the “moment” of the force P round the centre A. It would be the same as if I had simply an arm A D, and pulled upon it with the force P. It is an experimental truth, known to the old Greek philosophers, that moments, or twisting powers, are equal when in each case the result of multiplying the arm by the power acting at right angles to it is equal.

Now suppose A B is a pendulum, with a bob B of 10 lbs. weight, and suppose it has been drawn aside out of the vertical so that the bob is in the position B. Then the weight of the bob will act vertically downwards along the line B C. The moment, or twisting power, of the weight will be equal to 10 lbs. multiplied by A D, A D being a line perpendicular to B C.

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