What do elevators, flagpoles, trains, planes and automobiles have in common? They all use a pulley system. A pulley is a wheel with a groove that holds a rope, cable or belt, and is used to help lift an object or change the direction of a force. Too often, we take for granted everyday items that use pulleys as part of their engineered design.
History of pulleys
An advancement on the technology of the wheel, the pulley allowed great weights to be lifted with little force. The first use of a pulley can be traced back to Archimedes, the ancient Greek mathematician, physicist, engineer, astronomer, and philosopher (287 BC – 212 BC).
It was Plutarch – no, not Plutarch Heavensbee from Mockingjay – but the ancient Greek philosopher who recorded that Archimedes designed a block and tackle pulley to move an entire warship, laden with men. This pulley was known as the Claw of Archimedes and nicknamed the “iron hand.” The Claw was an ancient crane with a grappling hook that was able to lift enemy ships out of the water, causing the ships to capsize or be suddenly dropped.
In fact, STEMists will be surprised to learn that the Greeks used pulley systems to place actors on stage. Using pulleys on stage continued during the 1500’s in England, when William Shakespeare used a variety of special effects at the Globe Theatre, including flying actors using a pulley system—a technique which is still used in today’s theaters.
Types of Pulleys
Fixed – A fixed (class 1) pulley has a fixed axle, anchored in place, which redirects the force in a rope, called a belt, when it goes full circle. A fixed pulley has a mechanical advantage of one—it is useful because when you pull down, you can use your body’s own weight to add to the push.
Movable – A movable (class 2) pulley has an axle in a movable block. A moveable pulley supports an object with two ropes, placing the pulley in the middle. Since the pulley is being supported by two ropes, the amount of force you need to move an object is cut in half. It has a mechanical advantage of two.
Compound – A compound pulley is a combination of a fixed and movable pulley that forms a block and tackle, which can have several pulleys mounted on the fixed and moving axles, thereby increasing the amount of force. The block and tackle has been a key tool for raising boat sails and cargo for centuries.
How It Works
The design of a wheel on an axle supports movement and changes direction of a rope/belt or cable along the circumference of the wheel. Pulleys are used to lift loads, apply forces and to transmit power. In simple terms, your STEMists will understand the concept of an object being too heavy for one person to lift, even for the strongest of men. For example, if you want to lift a brick that weighs 20 kilograms, you have to pull down with a force equivalent to 20 kilograms. If you want to raise the brick two meters into the air, you have to pull the loose end of the rope a total distance of two meters at the other end. Therefore, with the use of a pulley, your STEMists can effectively multiply the force their body produces.
Be sure to order “Pull Your Weight” – a pulley-themed Groovy Lab in a Box to get your STEMists excited to investigate pulleys, cranes and Newton’s Third Law.