Cyclopedia of architecture, carpentry, and building : a general reference work . s; I angle between E and 70-pound force =_- 40° Force Polygon. If lines representing the magnitudesand directions of any number of forces be drawn continuous andso that the arrowheads on the lines point the same way around onthe series of lines, the figure so formed is called the force poI//(/onfor the forces. Thus ABCD (Fig. 8) is a force polygon for the80-, 90-, and 100-pound forces of Fig. 5, for AB, BC, and CD rep-resent the magnitudes and directions of those forces respectively,and the arrowheads point
Cyclopedia of architecture, carpentry, and building : a general reference work . s; I angle between E and 70-pound force =_- 40° Force Polygon. If lines representing the magnitudesand directions of any number of forces be drawn continuous andso that the arrowheads on the lines point the same way around onthe series of lines, the figure so formed is called the force poI//(/onfor the forces. Thus ABCD (Fig. 8) is a force polygon for the80-, 90-, and 100-pound forces of Fig. 5, for AB, BC, and CD rep-resent the magnitudes and directions of those forces respectively,and the arrowheads point in the same way around, from A to D. 145 12 STATICS A number of force polygons can be drawn for any system offorces, no two alike. Thus A^ B^ C^ D^ and A, B, C^ D^ are otherforce polygons for the same three forces, 80, 90, and 100 that Ag Bg Cg Dg is not a force polygon for the three forcesalthough the lines represent the three forces in magnitude anddirection. The reason why it is not a force polygon is that thearrowheads do not all point the same way A force polygon is not necessarily a closed figure. If a forcepolygon closes for a system of concurrent forces, then evidentlythe resultant equals zero. EXAMPLE FOR PRACTICE. Draw to the same scale as many different force polygons asyou can for the 100-, 120- and 160-pound forces of Fig. 5. Bearin mind that the arrowheads on a force polygon point the sameway around. 12. Composition of More Than Two Concurrent Forces. Thegraphical is much the simpler method; therefore the algebraic onewill not be explained. The following is a rule for performing thecomposition graphically: (1). Draw a force polygon for the given forces. (2). Join the two ends of the polygon and place an arrow-head on the joining line pointing from the beginning to the endof the polygon. That line then represents the magnitude anddirection of the resultant. (3). Draw a line through the point of concurrence of thegiven forces parallel to the
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