We use pulleys so that we may lift an object that’s far heavier than we could lift on our own. Sometimes we use a pulley to lift an object that’s so heavy, it takes a mighty large, and multiple-wheeled pulley to lift it.
But how does a pulley lift? Why does this particular mechanical configuration allow us to lift so much weight so easily?
A pulley is just a grouping of one or more wheels over which is looped a rope (or sturdy chain, for heavier objects). Scientists call these “simple machines” because they allow humans to multiply the forces that allow us to lift a heavy weight. Using a pulley greatly multiplies the force of your physical efforts.
A pulley with one wheel allows you to reverse the direction of your lifting force by pulling down on a rope (that’s looped over the wheel), lifting your weight.
With a two-wheel pulley, you reduce the effort you exert to lift the same amount of weight. You lift the weight with half the force. This is called the mechanical advantage (ME) and your two-wheeled pulley gives you an ME of two.
The larger your ME, the less force you need to lift a weight.
The more wheels your rope can loop around, the more weight you will be able to lift. This type of pulley system often is referred to as a block-and-tackle pulley system and usually is comprised of a rope wound around two axles several times.
As you pull the rope, the lower and upper axles slowly come together, increasing your ability to lift the object.
Important note: if you’re using a pulley system and you want to use just 100 pounds of force to lift something that’s 400 pounds, you will pull four times as much rope as when you lift a 100-pound object using 100 pounds of force. But by using 100 pounds of force to lift 400 pounds, you’ll be expending as much energy as if you were lifting a 100-pound object four times as far.