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Recovering the wreck of the Golden Ray ship using a crane with a pulley


You've probably seen those amazing TV strongmen who can pull cars with their hair and drag trains with their teeth. But did you know science can make you strong too? If you need to lift huge weights, don't strain your back: use the power of science—and an amazing device called a pulley. Let's take a closer look at how they work!

Photo: A giant floating crane (top) uses huge pulleys to recover the rusting wreck of the Golden Ray (bottom) in St. Simons Sound. Photo by Michael Himes courtesy of US Coast Guard and DVIDS.

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  1. What are pulleys?
  2. Let's be clear about mass and weight!
  3. How pulleys work
  4. What's the catch?
  5. What is a block and tackle?
  6. Find out more

What are pulleys?

A pulley is simply a collection of one or more wheels over which you loop a rope (or chain) to make it easier to lift things. Here's an everyday example:

Lifting an airplane engine with a pulley and a lifting frame

Photo: A pulley mounted on a huge lifting frame to make it safer to use. Thanks to the power of pulleys, one person can lift far more than their own weight without straining any muscles, because several ropes or chains support the extra weight. Photo by Charles D. Gaddis IV courtesy of US Navy and DVIDS.

Pulleys are examples of what scientists call simple machines. That doesn't mean they're packed with engines and gears; it just means they help us multiply forces. If you want to lift a really heavy weight, there's only so much force your muscles can supply, even if you are the world's strongest man. But use a simple machine such as a pulley and you can effectively multiply the force your body produces.

Pulley sheaves and hook on a large crane

Photo: The pulley wheels and hook on a large crane. Any weight dangling from the bottom of this hook will be supported by the 16 lengths of wire rope above it. The more lengths of rope wrapped around a pulley wheel, the easier it is to lift. Photo by Jet Lowe, Historic American Engineering Record (HAER), courtesy of US Library of Congress.

Let's be clear about mass and weight!

Before we go any further, let's be very clear about the difference between weight and mass. This will help in a moment when we talk about using pulleys to lift weights (which are really masses) with a certain amount of force. In a nutshell:

If you're a person with a mass of 80kg, Earth's gravity pulls you with a force of 800 newtons (on Earth, your weight in newtons is always roughly 10 times your mass in kilograms, because Earth pulls on every kilogram of mass with a force of 10 newtons). Strictly speaking, we should weigh things in units of force (newtons), so if your mass is 80kg, your weight is really 800 newtons. But in everyday speak, we tend to confuse mass and weight and talk about weights in kilograms (or pounds) instead. By the same token, although the kilogram is a unit of mass, not force, it's okay to talk about a force equivalent to a given mass because all masses generally convert to forces in the same way. You can read more about this in our article on weights and balances.

A hand lifting an orange of mass 100kg supplies a weight of 1N (1 newton).

How much force is a newton? This orange has a mass of about 100g (0.1kg) so I need to supply 1N (one newton) of force to hold it in mid-air. Loosely speaking, we say the orange "weighs" 100g; strictly speaking, it weighs 1N.

How pulleys work

The more wheels you have, and the more times you loop the rope around them, the more you can lift.

One wheel

If you have a single wheel and a rope, a pulley helps you reverse the direction of your lifting force. So, as in the picture below, you pull the rope down to lift the weight up. If you want to lift something that weighs 100kg, you have to pull down with a force equivalent to 100kg, which is 1000N (newtons). If you want to raise the weight 1m into the air, you have to pull the loose end of the rope a total distance of 1m at the other end.

Illustration artwork showing how a pulley with one wheel and rope works.
Artwork: How pulleys work#1: With one wheel, a pulley simply reverses the direction of the force you apply. It doesn't alter the force in any other way.

Two wheels

Now if you add more wheels, and loop the rope around them, you can reduce the effort you need to lift the weight. Suppose you have two wheels and a rope looped around them, as in the figure below. The 100kg mass (1000 newton weight) is now effectively supported by two sections of the same rope (the two strands on the left) instead of just one (ignoring the loose end of the rope you're pulling with), and this means you can lift it by pulling with a force of just 500 newtons—half as much! That's why we say a pulley with two wheels, and the rope wrapped around it this way, gives a mechanical advantage (ME) of two.

Mechanical advantage is a measurement of how much a simple machine multiples a force. The bigger the mechanical advantage, the less force you need, but the greater the distance you have to use that force. The weight rises 1m, but now we have to pull the loose end of the rope twice as far (2m). How come? To make the weight rise 1m, you have to make the two sections of rope supporting it rise by 1m each. To do that, you have to pull the loose end of the rope 2m. Notice that we can also figure out the mechanical advantage by dividing the distance we have to pull the rope by the distance the weight moves.

Illustration artwork showing how a pulley with two wheels and two ropes works.
Artwork: How pulleys work#2: With two wheels, it's as though the weight is hanging from two ropes (the two strands of the same rope on the left), and a pulley halves the lifting force you need. It's like lifting the weight with two ropes instead of one. But you now have to pull the end of the rope twice as far to lift the weight the same distance.

Wait! I am confused!

Pulleys can be very confusing and you might be struggling to understand what's happening here. If it's not obvious why you can use half the force to lift just as much weight, here's a completely different way of looking at it...

Suppose you're on the roof of a building and you want to lift your 1000 newton weight. You lower a rope down, attach it to the weight, and pull up. To move the weight 1m off the ground, you have to pull the rope 1m at the top with a force of 1000 newtons.

What if you can't lift that much? You ask a friend to help. You both stand on top of the building, and loop the rope down and back up through the weight. To raise the weight 1m off the ground, you both pull it 1m at the top. That means the rope effectively shortens by 1m on either side, so, together, you shorten it by 2m. You and your friend each use only half the force, or 500 newtons, so lifting is much easier.

Explaining pulleys in terms of multiple people lifting.

What if your friend doesn't like pulling, but is quite happy to stand on the roof holding the rope for you? Now what they're doing is supporting half the weight without actually pulling it. Remember that the rope is doubled up (it goes down to the weight and back up). So, to lift the weight 1m off the ground now, you still have to "shorten" the rope by 1m on both sides. In other words, you have to pull 1m on your side and 1m on your friend's side, or 2m of rope altogether. Your friend is supporting you with 500 newtons, so you can raise the 1000 newton weight 1m by pulling the rope 2m with a force of 500 newtons.

That's broadly how a pulley works, except that you add a couple of wheels—to replace your friend and so you can pull the rope from the bottom instead of the top.

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Four wheels

Okay, what if you use four wheels held together by a long rope that loops over them, as in the picture below? You can see that the 100kg mass (1000 newton weight) is now hanging from four sections of rope (the ones on the left, ignoring the loose end of the rope you're pulling with). That means each section of rope is supporting a quarter of the total 1000 newton weight, or 250 newtons, and to raise the weight into the air, you have to pull with only a quarter of the force—also 250 newtons. To make the weight rise 1m, you have to shorten each section of the rope by 1m, so you have to pull the loose end of the rope by 4m. We say a pulley with four wheels and the rope wrapped around like this gives a mechanical advantage of four, which is twice as good as a pulley with two ropes and wheels.

Illustration artwork showing how a pulley with four wheels and four ropes works.
Artwork: How pulleys work#3: With four wheels and the rope working in four sections, a pulley cuts the lifting force you need to one quarter. But you have to pull the end of the rope four times as far.

How a pulley is like a lever

You can probably see that a pulley magnifies force in a similar way to a seesaw, which is a kind of lever. If you want to lift someone four times heavier than you on a seesaw, you need to sit four times further away from the balancing point (fulcrum) than they are. If you move your end of the lever down by 4cm, their end of the seesaw moves up only 1cm. As they rise up, they gain a certain amount of potential energy equal to their weight multiplied by the distance they move. You lose exactly the same amount of energy—equal to your weight (four times smaller) times the distance you move (four times larger). You can shift their much bigger weight because you move your end of the seesaw over a much bigger distance: the leverage of the seesaw makes it possible to produce more force by working over a bigger distance.

The same thing is happening with a pulley, except that you're pulling on a rope instead of moving the end of a seesaw. To lift something four times heavier, you can use exactly the same force but only if you pull the rope four times further. If you look at what's happening on both sides of a pulley, and multiply the force by the distance moved, you'll find it's the same. On your side, you use a small force over a large distance. On the other side, there's a much bigger weight but it's moving a smaller distance.

How a pulley works in a similar way to a lever.
Artwork: How a pulley works like a lever: Just like with a lever, a pulley can "magically" create more force—but only if you use that force over a longer distance. Why is that? Read on below!

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What's the catch?

Pulleys sound brilliant—and they are. But surely there must be a catch? If you can lift 100kg (1000 newtons) by pulling with the force-equivalent of only 25kg (250 newtons), surely you're doing only a quarter as much work and using only a quarter as much energy? And if that's true, you could build some kind of a pulley that would actually produce energy for you: put in only one unit of energy and get four units out! Sounds brilliant!

A simple homemade pulley experiment using a construction set.

Photo: Is there a hidden catch when you're using a pulley? Why not make your own simple pulley from a construction set (or just homemade materials like cotton reels and string) and test it for yourself. There's no better way to understand how pulleys work. With a simple two-wheel pulley like this, it's easy to see that you have to pull the string twice as far as the weight lifts up.

Unfortunately, such amazing things are strictly prohibited by a law of physics called the conservation of energy, which says you must always put in as much energy as you get out. So let's think about pulleys in terms of energy. If you raise a weight of 100kg (1000 newtons) a distance of 1 meter off the ground, you have to do the same amount of work whether you use a pulley or not: you have to move the same force over the same distance. If you use a pulley and reduce the force you're using by a quarter, you still have to do the same amount of work. It's just that you have to pull the end of the rope four times further to make each of the four supporting sections of rope rise by the same amount. That's the catch with a pulley. You pull with less force, but you have to pull further (and, generally speaking, use the force for longer). Far from using less energy with a pulley, you actually have to use a little bit more because of the friction where the rope rubs against the pulley wheels. But it seems and feels easier to use a pulley, and that's the important thing!

Giant red pulley wheels in a large Ransom Rapier steam crane

Photos: Pulley wheels on the arm of a large railroad maintenance crane. This one uses huge strong wire rope. Note how the wheels have deep grooves in them so the rope doesn't slide off them.

What is a block and tackle?

In engineering, the kind of pulley I've been describing here is sometimes called a block and tackle: the wheels and their mounts are the blocks and the ropes that loop around them are the tackle. In my examples, one block is fixed at the top and the other block moves up with the load. More generally, to engineers, a pulley is a wheel over which you loop a rope or a belt to connect one part of a machine to another, whether it's lifting things, transmitting power, or doing anything else. In simple science, though, we tend to use "pulley" just to mean a bunch of wheels and ropes for lifting.

Two types of pulleys compared: a block and tackle for lifting and a pulley for transmitting power
Photos: Two types of pulleys. Left: A block and tackle is a pulley-based system for lifting things, made of blocks (the wheel sections) and tackle joining them together. This one was used for lifting rocket equipment at NASA's Marshall Space Flight Center. Photo by James W. Rosenthal, Historic American Engineering Record, courtesy of US Library of Congress. Right: Pulley wheels can also be used to join different parts of a machine together. Here, a pulley wheel on a large engine is driving another pulley wheel on a machine some distance away. In this case, the pulleys are simply transmitting power. Photo: Historic American Engineering Record, courtesy of US Library of Congress.

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For younger readers

For some reason, there are loads of books about pulleys for young readers (6–10 age group). Here are just a few to start you off:

These two are more general books that put the science of forces into a wider context:

For older readers


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Text copyright © Chris Woodford 2009, 2023. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009/2023) Pulleys. Retrieved from [Accessed (Insert date here)]

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