The four masses shown in FIGURE EX12.13 are connected by massless, rigid rods. (a) Find the coordinates of the center of mass.

The forces acting on an 85,000-kg aircraft flying at constant velocity are shown in Fig. 12–108. The engine thrust, F_T = 5.0 x 10⁵ N, acts on a line 1.6 m below the cm. Determine the drag force F_D and the distance above the cm that it acts. Assume F→_D and F→_T are horizontal. ( F→_L is the “lift” force on the wing.)

<IMAGE>

A 70 kg, 1.90 m man doing push-ups holds himself in place making 20° with the floor. His feet and arms are, respectively, 1.15 m below and 0.4 m above his center of mass. You may model him as a thin, long board, and assume his arms and feet are perpendicular to the floor. How much force does the floor apply to each of his hands? (Use g=10 m/s².) 

(II) Three children are trying to balance on a seesaw, which includes a fulcrum rock acting as a pivot at the center, and a very light board 3.2 m long (Fig. 12–60). Two playmates are already on either end. Boy A has a mass of 45 kg, and boy B a mass of 35 kg. Where should girl C, whose mass is 25 kg, place herself so as to balance the seesaw?

<IMAGE>