Disregarding friction, what is the...

Disregarding friction, what is the mechanical advantage of a twofold purchase when rove to disadvantage?

🔍 Key Concepts

• Mechanical advantage (MA) of tackles (purchase systems) and how it relates to number of parts of line supporting the moving block • Difference between rove to advantage and rove to disadvantage in block and tackle systems • How to analyze a twofold purchase (two double blocks) by counting the line segments sharing the load

💭 Think About

• Sketch or visualize the two double blocks and trace the line as it is rove. How many segments of line are actually lifting the moving block? • Ask yourself: in a system rove to disadvantage, is the hauling part attached to the moving block or the fixed block, and how does that change the effective mechanical advantage? • Compare what the MA would be for the same tackle if it were rove to advantage, then adjust for the disadvantage case.

✅ Before You Answer

• Carefully count only the line segments that directly support the moving block; do not count dead ends that are fixed • Confirm whether the hauling part is coming off the fixed block or the moving block in a rove-to-disadvantage arrangement • Verify that you are ignoring friction and sheave efficiency, using only the ideal, theoretical mechanical advantage