The Lever Study Guide for McGraw-Hill's Firefighter Exams (page 2)
A lever is a simple machine consisting of a bar or rigid object that is free to turn about a fixed point called the fulcrum. The fulcrum is known as the pivot point. A lever will apply an effort (force) at a different point from the resistance force (load). Levers are classified into three classes—first class, second class, and third class—based on the position of the effort (force), the resistance force (load), and the fulcrum.
In a first-class lever, the fulcrum is between the effort (force) and the resistance force (load). Common examples of first-class levers are the crowbar, the claw hammer (when being used to remove nails), pliers, tin snips, a car jack, and a seesaw. With a first-class lever, the direction of the force always changes. An example of this is when a downward effort (force) on the lever causes an upward movement of the resistance force (load).
If the fulcrum of a first-class lever is located an equal distance from the effort (force) being applied and the resistance force (load), there is no mechanical advantage—the MA is 1. The closer the fulcrum is to the load, the less effort (force) will be needed to lift the load and the mechanical advantage will be greater than 1. Remember the formula for mechanical advantage is:
Example: In the illustration shown, the load and mechanical advantage are known, and the effort needed can be calculated:
The trade off, however, is that the effort (force) will have to move (down) a greater distance and the load will move (rise up) a smaller distance.
Conversely, if the fulcrum is moved closer to the effort (force) being applied, greater effort (force) will be required to lift the resistance force (load) but the effort (force) will have to move a shorter distance and the resistance force (load) will move (rise up) a greater distance.
Note: Another mechanical principle formula that can be used to calculate the value of lever systems is the following:
Force × Effort Distance = Load × Resistance Distance
Example: A 300-pound weight is placed on the end of a plank 3 feet from a fulcrum. How much effort (force) would a firefighter have to exert on the opposite end of the plank at 9 feet from the fulcrum to obtain equilibrium?
Force Effort Distance = Load × Resistance Distance
(x) × 9 feet = 300 pounds×3 feet Divide both sides by 9 to isolate the variable.
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.
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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