A square threaded screw is used to lift a load $W$ by applying a force $F$. Efficiency of square threaded screw is expressed as

1. The ratio of work done by $W$ per revolution to work done by $F$ per revolution
2. $\frac{W}{F}$
3. $\frac{F}{W}$
4. The ratio of work done by $F$ per revolution to work done by $W$ per revolution

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