# How much weight is reduced by a pulley?

In general, the reciprocal of the number of ropes bearing the weight is the pulley load reduction, however few practical pulley systems have more than four ropes. As a result, the maximum pulley load reduction possible is one-quarter the weight of the load. Here are some examples:

A wooden block on a string has a weight of 1 kilo (2 pounds). The string is tied to two trees 10 meters (33 feet) apart. What is the load on the string?

The answer is 50 kilos (110 pounds), or approximately half the weight of the block.

What if the block were made out of iron? Then its weight would be 2 kilograms (4 pounds). The load on the string then would be 25 kilos (55 pounds), or nearly twice the weight of the block.

Why does the weight of the object affect the load on the string? Because the string can only support so much weight before it breaks! A heavy object will require a stronger string to hold it up.

Now let's say that you want to reduce the load on the string to one-quarter its original value. That means when the block was made of wood, its weight must have been 8 pounds, and when the block is made of iron, its weight must be 32 pounds.

## How much weight does a pulley system lift?

Distance is the variable that varies. When employing a pulley system, you use less force to lift a load but must pull more rope to raise the load to a specific height. With a double pulley system, you must draw twice as much rope; with a triple pulley system, you must pull three times as much rope, and so on. The equation for calculating lifting power is Force x Distance / Weight. For example, if you have a force of 50 pounds and a distance of 20 feet of 1-inch diameter rope, your power is 50 x 20 / 1,000, or 10 foot-pounds.

A pulley system allows you to use a single length of rope to lift a load that you could not otherwise lift. It is very useful in applications where space is at a premium and you need to lift a large object. Pulleys also divide up the work involved in lifting the load so that no one muscle gets too tired. A person using a pulley system can lift something that would be impossible to move by hand alone.

The force required to lift an object is called its weight. If you were to climb into every truck on the road and haul them all away, you would be able to lift all their weights. However, it is impractical to do so because it would take too long and be too expensive. Instead, you use tools that make lifting objects easier. One common tool is a lever.

## What does the number of strands have to do with the effectiveness of a pulley system?

The number of rope strands supporting the load in a multiple-pulley system essentially correlates to the system's mechanical advantage. For example, if you have two fixed and two moveable pulleys, four lengths of rope with a mechanical advantage of four sustain the weight. If you instead used eight lengths of rope with a mechanical advantage of two, the system would be equivalent to a single-pulley system and could not support as much weight.

In general, the greater the number of strands in a multiple-pulley system, the higher its mechanical advantage. However, this advantage can be achieved by using smaller pulleys or by combining movesable and fixed pulleys. For example, one combination that provides a mechanical advantage of four is a moveable upper pulley with a fixed lower pulley. Because the moveable upper pulley can be larger than the fixed lower pulley, more rope can be used to support the same amount of weight as compared to a single-pulley system.

A multiple-pulley system allows for more efficient use of rope because sections can be combined or split apart as required to match load levels. For example, if the weight on a particular section of rope becomes too heavy, the user can remove the moveable upper pulley to reduce the mechanical advantage and allow the section of rope to be re-stranded or replaced.

## Does a pulley cut the weight in half?

The trade-off with a pulley is distance. When two pulleys are used simultaneously, the amount of effort necessary is decreased in half, but twice as much rope must be pulled to elevate the item to the same target height. If you want to pull something up high, use several small ropes instead.

Pulleys can be made out of metal or wood, either single or double ended. Metal pulleys are usually forged parts that are cold-rolled and annealed after fabrication. Wood pulleys are typically square in cross-section and have flat faces that are glued together. They are often weighted at one end to make them easier to lift.

As you can see, pulleys allow you to reduce the force needed to move a load, which makes them useful for your body too. They can also be used to increase the distance that something has to be lifted, such as when climbing stairs or scaling a fence.

The best way to learn how to use a pulley is by taking a class or reading a manual about rope work. In the meantime, here are some videos that may help:

### What is the difference between a knot and a bight?

This short video from TreeHugger explains both knots and bights.

##### Wallace Dixon

Wallace Dixon is an avid collector and user of vintage technology. He has been known to take apart old radios just to see what makes them work, and he's even been known to fix them himself when they don't!

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