15. Rotational Equilibrium

A 40 kg, 5.0-m-long beam is supported by, but not attached to, the two posts in FIGURE P12.61. A 20 kg boy starts walking along the beam. How close can he get to the right end of the beam without it falling over?

A board 8 m in length, 20 kg in mass, and of uniform mass distribution, is supported by two scales placed underneath it. The left scale is placed 2 m from the left end of the board, and the right scale is placed on the board's right end. A small object 10 kg in mass is placed on the left end of the board. Calculate the reading on the left scale. (Use g=10 m/s².)

BONUS:Calculate the reading on the right scale.

A diving board 3.00 m long is supported at a point 1.00 m from the end, and a diver weighing 500 N stands at the free end (Fig. E11.11). The diving board is of uniform cross section and weighs 280 N. Find the force at the support point.

A 350-N, uniform, 1.50-m bar is suspended horizontally by two vertical cables at each end. Cable A can support a maximum tension of 500.0 N without breaking, and cable B can support up to 400.0 N. You want to place a small weight on this bar. (a) What is the heaviest weight you can put on without breaking either cable, and (b) where should you put this weight?

A person's center of mass is easily found by having the person lie on a reaction board. A horizontal, 2.5-m-long, 6.1 kg reaction board is supported only at the ends, with one end resting on a scale and the other on a pivot. A 60 kg woman lies on the reaction board with her feet over the pivot. The scale reads 25 kg. What is the distance from the woman's feet to her center of mass?

A shop sign weighing 215 N hangs from the end of a uniform 135-N beam as shown in Fig. 12–59. Find the tension in the supporting wire (at 35.0°), and the horizontal and vertical forces exerted by the hinge on the beam at the wall.

Two wires run from the top of a pole 2.6 m tall that supports a volleyball net. The two wires are anchored to the ground 2.0 m apart, and each is 2.0 m from the pole (Fig. 12–70). The tension in each wire is 125 N. What is the tension in the net, assumed horizontal and attached at the top of the pole?

A heavy load Mg = 62.0 kN hangs at point E of the single cantilever truss shown in Fig. 12–81. Use a torque equation for the truss as a whole to determine the tension F_T in the support cable, and then determine the force $\overrightarrow{F_A}$ on the truss at pin A. Neglect the weight of the trusses, which is small compared to the load.

(III) The truss shown in Fig. 12–82 supports a railway bridge. Determine the compressive or tension force in each strut if a 53-ton (1 ton = 10³kg) train locomotive is stopped at the midpoint between the center and one end. Ignore the masses of the rails and truss, and use only 1/2 the mass of train because there are two trusses (one on each side of the train). Assume all triangles are equilateral. [Hint: See Fig. 12–31.]

The roof over a 9.0-m x 10.0-m room in a school has a total mass of 12,400 kg. The roof is to be supported by vertical wooden “2 x 4s” (2 x4 in inches, but actually about 4.0 x 9.0 cm) equally spaced along the 10.0-m sides. How many supports are required on each side, and how far apart must they be? Consider only compression, and assume a safety factor of 12.

A heavy load M g = 62.0 kN hangs at point E of the single cantilever truss shown in Fig. 12–81. Determine the force in each member of the truss. Neglect the weight of the trusses, which is small compared to the load.

Two springs, both having stiffness constant 225 N/m, are attached to a table and to a 0.500-kg uniform thin wooden board (Fig. 12–98). The board is exactly horizontal. What are the natural lengths of each spring? [Hint: One of the springs is stretched, the other compressed, from their natural equilibrium lengths.]