If this lever is in equilibrium with an effort force of 18 ft. lb. at the blue arrow and a resistance force of 9 ft. lb. at the green box, what is its mechanical advantage?

Mechanical advantage (MA) is the ratio by which effort force relates to resistance force. If both forces are known, calculating MA is simply a matter of dividing resistance force by effort force:

\(M A=F_{r} \div F_{e}=9 f t . \div 18 f t .=0.5\)

NOTE

In this case, the mechanical advantage is less than one meaning that each unit of effort force results in just 0.5 units of resistance force. However, a third class lever like this isn't designed to multiply force like a first class lever. A third class lever is designed to multiply distance and speed at the resistance at a sacrifice of force at the resistance. Different lever styles have different purposes and multiply forces in different ways.