Levers
Inquiry activities adapted by Shawn Reintjes, The Science House, NCSU
from Vernier Activities for Middle School
Most everything that we use in our everyday lives is made of simple machines. A simple machine is defined as a device that requires only a singular force to operate. Simple machines provide us with mechanical advantage. Mechanical advantage is generally calculated as:
MA= Distance over which the force is applied/ Distance the load moves
The ideal mechanical advantage (IMA) of a machine is what the mechanical advantage of a machine is in principle, without losses due to friction, etc. The actual mechanical advantage (AMA) is the actual mechanical advantage that a machine provides, so it will always be less than the ideal mechanical advantage due to losses from friction, etc. It is calculated as:
AMA = Resistance force/ effort force
Simple machines allow us to do work. Work is equal to the force applied times the distance over which the force acts. The SI unit for work is the Newton-meter (N-m). A Newton is 1 kg-m/s2. To convert a mass into Newtons, multiply the object’s mass in kg by 9.8 m/s2. Applying this concept to work, if you lift a book that weighs 2 Newtons a distance of 3 meters you have done (2 x 3) = 6 N-m of work.
The efficiency of a simple machine can be calculated as either:
%Efficiency = (Work output/ Work input) x 100
%Efficiency = (AMA/ IMA) x 100
Ideal machines have an efficiency of 100%. Because of losses to friction, internal heat buildup, etc. the efficiency of a real machine will never be 100%
First-Class Levers:
A first-class lever is one in which the fulcrum is located between the effort (force) and the resistance (load). A first-class lever changes the direction of a force, so the effort pushes in one direction while the resistance moves in the opposite direction. When the fulcrum is closer to the resistance than the effort, we gain in force, but get less speed and distance. When the fulcrum is closer to the effort than the resistance, we get more speed and distance, but lose in force. When the fulcrum is exactly between the effort and the resistance, there is no change in force, speed, or distance, but there is a change in direction. Examples of first-class levers include the crowbar, scissors, pliers, tin snips, tack puller, and seesaw.
Second-Class Levers:
A second-class lever is one in which the resistance is between the effort and the fulcrum. A second-class lever does not change the direction of a force, so both the effort and the resistance move in the same direction. In a second-class lever the fulcrum is closer to the resistance, so there is a gain in force. Examples of second-class levers include the wheel barrow, nut cracker, crowbar, bottle opener, and the oar of a rowboat.
Third-Class Levers:
A third-class lever is one in which the effort is between the resistance and the fulcrum. A third-class lever does not change the direction of the force. In the third-class lever there is always a gain in speed and distance and a loss in force. Examples of third-class levers include the broom, shovel, sugar tongs, tweezers, and fishing pole.
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