Experimental Mechanics / A Course of Lectures Delivered at the Royal College of Science for Ireland

by Robert S. Ball

The energy actually required to punch a hole of half an inch diameter through a plate eight-tenths of an inch thick is therefore less than that which would be expended in raising 1 cwt. to a height of ten feet.

556. The fly-wheel may be likened to the reservoir in [Art. 545]. The time that is actually occupied in the punching is extremely small, and the sudden expenditure of 1062 units is gradually reimbursed by the engine. If the rotating fly-wheel contain 50,000 units of energy, the abstraction of 1362 units will not perceptibly affect its velocity. There is therefore an advantage in having a heavy fly sustained at a high speed for the working of a punching machine.

LECTURE XVII.
CIRCULAR MOTION.

The Nature of Circular Motion.—Circular motion in Liquids.—The Applications of Circular Motion.—The Permanent Axes.

THE NATURE OF CIRCULAR MOTION.

557. To compel a body to swerve from motion in a straight line force must be exercised. In this chapter we shall study the comparatively simple case of a body revolving in a circle.

558. When a body moves round uniformly in a circle force must be continuously applied, and the first question for us to examine is, as to the direction of that force. We have to demonstrate the important fact, that it constantly tends towards the centre.

559. The direction of the force can be exhibited by actual experiment, and its magnitude will be at the same time clearly indicated by the extent to which a spring is stretched. The apparatus we use is shown in [Fig. 74].

The essential parts of the machine consist of two balls a, b, each 2" in diameter: these are thin hollow spheres of silvered brass. The balls are supported on arms p a, q b, which are attached to a piece of wood, p q, capable of turning on a socket at c. The arm a p is rigidly fixed to p q; the other arm, b q, is capable of turning round a pin at q. An india-rubber door-spring is shown at f; one end of this is secured to p q, the other end to the movable arm, q b. If the arm q b be turned so as to move b away from c, the spring f must be stretched.