Читать книгу Experimental Mechanics. A Course of Lectures Delivered at the Royal College of Science for Ireland онлайн

65 страница из 68

106. I try to balance an iron ring upon the end of a stick h, ssss1, but I cannot easily succeed in doing so. This is because its centre of gravity s is above the point of support; but if I place the stick at f, the ring is in stable equilibrium, for now the centre of gravity is below the point of support.

ssss1

107. There are other curious consequences which follow from the properties of the centre of gravity, and we shall conclude by illustrating one of the most remarkable, which is at the same time of the utmost importance in machinery.

Fig. 30.

108. It is generally necessary that a machine should work as steadily as possible, and that undue vibration and shaking of the framework should be avoided: this is particularly the case when any parts of the machine rotate with great velocity, as, if these be heavy, inconvenient vibration will be produced when the proper adjustments are not made. The connection between this and the centre of gravity will be understood by reference to the apparatus represented in the accompanying figure (ssss1). We have here an arrangement consisting of a large cog wheel c working into a small one b, whereby, when the handle h is turned, a velocity of rotation can be given to the iron disk d, which weighs 14 lbs, and is 18" in diameter. This disk being uniform, and being attached to the axis at its centre, it follows that its centre of gravity is also the centre of rotation. The wheels are attached to a stand, which, though massive, is still unconnected with the floor. By turning the handle I can rotate the disk very rapidly, even as much as twelve times in a second. Still the stand remains quite steady, and even the shutter bell attached to it at e is silent.