rigidly connected may be determined by considering the weight of each body as a parallel force applied at its center of gravity, and then finding the point of application of the resultant of these forces Suppose three parallel forces, P, Q, and S, to be applied at three points, A, B, and С, rigidly connected, as in Fig. 1.
Fig. 1
The resultant R' of P and Q will equal P+Q, and its point of application will be at a point D, determined by the proportion
Whence 
Now connect D with C, and three forces P, Q, and S are replaced by the two forces S and R'. Find in the same way the point of application E of their resultant, and this will be the center of gravity of the system.
Demonstration. - Select a board of uniform thickness and put it a scrow hook close to each corner. Weigh the board, and determine its center of gravity by calculation. Suspend from each hook a known weight, as 2, 4, and 6 pounds, for example. Let each member of the class make a drawing of the board and locate on it the position of the center of gravity of the board and the pоsition of each weight, and determine the center of gravity of the system by construction. When this has been done, put a screw eye in the upper side of the board at the point found, and lift the board and weights with a spring scale. Does the board hang horizontally? What is the weight of the system?
