You are using tackle number 3 as shown in illustration D029DG below to lift a weight of 120 lbs. If you include 10 percent of the weight for each sheave for friction, what is the pull on the hauling part required to lift the weight?
• Count the number of parts of line supporting the moving (lower) block in tackle number 3 to find the theoretical mechanical advantage (TMA). • Treat friction as an extra percentage of the load for each sheave; here it is 10% of the weight per sheave in the system. • Use the relationship Pull = Effective Load ÷ Mechanical Advantage after you adjust the weight for friction.
• In tackle number 3, exactly how many segments of rope are directly holding up the weight (attached to the moving block)? What does that give you for theoretical mechanical advantage? • How many sheaves does the line pass over in tackle number 3? If each sheave adds 10% of the 120 lb load, what total percentage of extra load do you need to account for? • Once you have the effective (increased) load due to friction and the mechanical advantage, what pull on the hauling part would balance that effective load? Compare this pull to the choices given.
• Be sure you are counting only the parts of line supporting the moving block, not the free hauling end. • Verify your total number of sheaves by carefully tracing the path of the line in illustration 3. • Double-check that you applied the 10% per sheave to the 120 lb weight, then divided that adjusted load by the correct mechanical advantage.
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