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# The Lever Study Guide for McGraw-Hill's Firefighter Exams (page 3)

McGraw-Hill Professional
Updated on Jun 25, 2011

### Second-Class Lever

In a second-class lever the resistance force (load) is between the effort (force) and the fulcrum. The fulcrum will normally be closer to the load and therefore reduces the effort (force) required to accomplish the job at hand. An example of a second-class lever is the wheelbarrow. Its mechanical advantage is greater than 1. A second-class lever always enhances effort (force). Unlike the first-class lever, a second-class lever does not change the direction of effort (force). Pushing up (exerting force) on a second-class lever pushes up on the (load); conversely, pushing down on a second-class lever pushes down on the load. Other examples of second-class levers are a nutcracker, a bellows, a paper cutter, and a bottle opener.

Example: A wheelbarrow filled with building materials weighing 100 pounds that is 1.5 feet from the wheel would require how much force to lift it off the ground using handles that are 3 feet from the load? What is the mechanical advantage?

Force ×Effort Distance = Load × Resistance Distance

(x) × 3 ft = 100 lb × 1.5 ft Divide both sides by 3 ft to isolate the variable.

If effort is 50 lbs to move a load of 100 pounds, the MA = 2.

### Third-Class Lever

A third-class lever has the effort (force) between the fulcrum and the resistance force (load). In a third-class lever, the effort (force) required to lift the load is actually increased and, therefore, the mechanical advantage is less than 1. The trade-off, however, is an increase in speed and distance of travel of the load. An example of a third-class lever is a shovel. The worker's hands supply the effort (force) while the elbows act as a fulcrum. The load (soil, sand, coal) is moved at the end of the shovel. As in the second-class lever, the direction of effort (force) does not change. Examples of third-class levers are a pitchfork, tweezers, a hoe, tongs, and a broom.

Example: A pair of tongs is being used by fire marshals at a fire scene to pick up a one-pound circular ball to be used as evidence. Two pounds of effort force is required to lift the object. The fingers being used to squeeze the tongs are 4 inches from the object and 2 inches from the fulcrum of the tongs. What is the mechanical advantage of the tongs?

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