Experimental mechanics Author:Robert Stawell Ball Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: LECTURE VI. THE PULLEY. Introduction.—Friction between a Rope and an Iron Bar.—The Use of the Pulley.—Large and Small Pulleys.—The Law of Friction in the P... more »ulley. —Wheels.—Knergy. INTRODUCTION. 152. The pulley forms a good introduction to the very important subject of the mechanical powers. But before entering on the discussion of the mechanical powers, it will be necessary for us to explain what is meant in mechanics by " work," or " energy," as it is more appropriately called, and we shall therefore include a short outline of this subject in the present lecture. 153. The pulley is a machine which is employed for the purpose of changing the direction of a force. We frequently wish to apply a force in a different direction from that in which it is convenient to exert it, and the pulley enables us to do so. We are not now speaking of the arrangements for increasing power in which pulleys play an important part; these will be considered in the next lecture: we refer only to change of direction. In fact, as we shall presently see, a small amount of force is lost when the single pulley is used, so that this machine cannot be called a mechanical power. 154. The occasions upon which a single pulley is used are very numerous and familiar. Let us suppose a sack of corn has to be elevated from the lower to one ef the upper stories of a building. It may of course be raised by a man who carries it, but he has to carry his owu weight in addition to that of the sack, and therefore the quantity of exertion used is greater than absolutely necessary. But supposing there be a pulley at the top of the building over which a rope passes; then, if a man attach one end of the rope to the sack and pull the other, he raises the sack without raising his own weight. The pulley has thus pro...« less