34 



RESISTANCE OF MATERIALS 



69. A horizontal beam 20 ft. long and weighing 120 Ib. rests on two supports 

 10 ft. apart. A load of 76 Ib. is hung at one end of the beam and 150 Ib. at the 

 other end. How must the beam be placed so that the pressure on the supports 



may be equal ? 



12 70. Explain how a clock hand on a 



smooth pivot can be made to show the time 

 by means of clockwork concealed in the 

 hand and carrying a weight around. 



71. A brick wall is 12 in. thick and 40 ft. 

 high. What uniform wind pressure will cause 



A. li \- IJ -J it to tip over? Weight of ordinary brick 



masonry is 125 Ib./f t. 8 



72. A masonry dam is 30 ft. high, 6 ft. 

 wide at top, and 30 ft. wide at bottom, 

 with upstream face vertical. Assuming the 

 masonry to weigh 160 lb./ft. 8 , compute the 

 moment of the weight of the dam about 

 the toe of the base. 



|2 73. In problem 72 the resultant water 



FIG. 36 pressure for a vertical strip 1 ft. wide is 



46,800 Ib. and is applied at a point 12ft. 

 above the base of the dam. Determine its stability against overturning. 



74. The casting for a gas-engine piston is a hollow cylinder of uniform thick- 

 ness, with one end closed. The external diameter is 5 in., length over all 6 in., 

 thickness of cylinder shell Jin., thickness of end IJin. Find the distance of its 

 center of gravity from the closed end. 



75. A cast-iron pulley weighs 50 Ib. and its center of gravity is 0.1 in. out of 

 center. To balance the pulley, a hole is drilled in the light side, 6 in. from the 

 center of the pulley and in line with its center of gravity, and filled with lead. 

 How much iron must be removed, the specific gravity of lead being 11.35 and of 

 iron 7.5 ? 



