Machinery's Reference Series Volume 1-10 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1910 Excerpt: ...resistance F on the lower end of the block is a force acting in a direction opposite to P. On the ice, the force F is smaller than the force P, but on the rougher surface it becomes exactly equal to it; for, if F should be smaller than P, instead of equal to it, the block would not overturn, but would move to the left as It did when resting upon the ice. Similarly, whenever rotary motion of any body occurs, there must be at least two equal and opposite forces, not in the same straight line. This principle is universal. In Fig. 13, for example, the bolt must react with a force equal and opposite to that applied to the handle of the wrench. There is a pig. 14 n?. is reaction at the shaft and bearing of a gear wheel or pulley, which la equal and opposite to the force applied by the driving gear or belt. The Principle of Moments When two or more forces act upon a rigid body and tend to turn it about an axis, then, for equilibrium to exist, the sum of the moments of the forces which tend to turn the body in one direction must be equal to the sum of the moments of those which tend to turn it in the opposite direction about the same axis. In Fig. 15, a lever 30 inches long is pivoted at the fulcrum O. At the right, and 10 inches from 0 is a weight, B, of 12 pounds, tending to turn the bar in a right-hand direction about its fulcrum 0. At the left end, 12 inches from 0. the weight A of 4 pounds tends to turn the bar in a left-hand direction, while weight C, at the other end, 18 inches from 0, has a like effect, through the use of the string and pulley P. Taking moments about 0, which is the center of rotation, we have: Moment of B = 10 X 12 = 120 inch-pounds. Opposed to this are the moments of A and C: Moment of A== 4 X 12 = 48 inch-pounds. Moment of &= 4 X 18 ...