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(c) Figure 5.12 shows the same force F being applied to a metal rod at positions A, B and C on separate occasions to lift a large stone. metal rod large stone State the position, A, B or C, in which force F exerts the largest moment about the small stone. Give a reason for your answer. (position B) F F (position C) small stone F (position A) ▲ Figure 5.12 Position B. At position B, the perpendicular distance of the line of action of the force to the pivot (small stone) is the longest, resulting in the largest moment about the pivot. (d) Figure 5.13 shows an angler holding a stiff fishing rod (of negligible mass) in a horizontal position to support a fish of mass 3 kg. Calculate the force F needed to keep the rod in the horizontal position. (Take g = 10 N kg−1) 30 cm 90 cm F supporting hand lifting hand 3 kg Taking moments about the supporting hand, F × 0.3 = (3 × 10) × (0.3 + 0.9) F = 120 N ▲ Figure 5.13 Data-Based Question 2. Cranes such as the one in Figure 5.14 are used to move heavy loads across large distances. The maximum load a crane can carry depends on its working radius, i.e. the maximum length of the jib at a specific angle. A crane operator can refer to the data in Table 5.1 to find out the maximum load a crane can lift for a particular working radius at an angle of 30°. working radius load jib 30° ▲ Figure 5.14 ▼ Table 5.1 Working radius/m 14 20 28 36 Maximum load/N 900 630 450 350 (a) Using the data in Table 5.1, deduce the maximum moment provided by the crane. Maximum moment = F × d = 630 × 20 × cos 30° = 10 910 N m [Note: Each working radius and corresponding maximum load in Table 5.1 should give the same maximum moment.] © 2013 Marshall Cavendish International (Singapore) Private Limited (S)PMWB_05.indd 43 Turning Effect of Forces 43 8/27/12 9:05 AM