Discussion
A square threaded screw is used to lift a load W by applying a force F. Efficiency of square threaded screw is expressed as
Answer: A
Efficiency is the ratio of work done by W per revolution to work done by F per revolution.
Efficiency is always useful work out over work in. In one turn of the screw the effort F moves through the circumference at its point of application and does the input work; the load W is raised by one lead, and the work done on it is the output. So \(\eta = W \times \text{lead} / (F \times 2\pi R)\), which is less than 1 because of thread friction.
W/F alone is the mechanical advantage, not the efficiency, and ignores the distances moved. F/W is its inverse. The last option inverts the efficiency and would always exceed 1.
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