It is written in legends that a single man moved an entire warship filled with soldiers by using a system of pulleys. Throughout history, pulleys and levers have made light work of lifting and moving things. Many modern machines and tools continue to use the principles of these simple machines. Giant cranes lift thousands of tons using the principles of pulleys, while simple tools such as wheelbarrows and crowbars employ the principles of the lever. Levers and pulleys provide advantages to their users.
Mechanical advantage describes the extent a simple machine multiplies the force that is exerted on it. Levers and pulleys give their users mechanical advantage. People can reduce the amount of effort that is needed to lift and move things by using levers and pulleys. Mechanical advantage is expressed by numbers, with one equaling no advantage other than changing the direction the load moves. A mechanical advantage of two means the machine doubles the person's effort. You can increase mechanical advantage in multiples of two. You can change the mechanical advantage that levers and pulleys offer by changing how they are configured.
Pulleys have wheels with grooves and ropes or cables that fit into the grooves. Pulleys are either fixed or movable, and single or multiple. Movable and multiple pulleys have a mechanical advantage of two, or more, depending on how many pulleys you use.
Levers provide a mechanical advantage by how the load is positioned on them compared with the locations of the fulcrum and the effort. You express the mechanical advantage of levers by determining the ratio of the lever's length on the effort side of the fulcrum compared to its length on the load side.
Ideal Mechanical Advantage
Engineers and physicists use formulas to calculate the ideal mechanical advantage of a pulley or lever. The results of their calculations are only theoretical because they don’t take into account real-world conditions such as temperature and humidity and how those variables affect the mechanical advantage.
Actual Mechanical Advantage
You can use formulas to express the actual mechanical advantage of a pulley or lever configuration. These formulas take into account variables such as energy lost to friction.
Fixed pulleys do not give the user any mechanical advantage, but they do give the user the ability to move the load in a direction different from the direction the force is applied. A person can use this setup to raise a load equal to their strength from the ground to the second story of a building while remaining on the ground.
When using a lever, the closer the load is to the fulcrum, the less effort it takes to move the load. However, the load does not move very far in the desired direction. If you want to move the load farther then you have to move the fulcrum farther from the load. This requires more effort to move the load.
- machines in construction image by JoLin from Fotolia.com