Pulleys

Last updated: March 9, 2009.

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 pulley mounted on a huge lifting frame to make it safer to use. Thanks to the power of pulleys, one man can lift far more than his own weight without straining his muscles. Photo by R. B. Hotard courtesy of US Marine Corps and Defense Imagery.

What are pulleys?

A pulley is simply a collection of one or more wheels over which you loop a rope to make it easier to lift things. 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.

Photo: Pulleys can help you lift heavier things because several ropes or chains support the extra weight. Photo by Sheldon Rowley courtesy of US Navy and Defense Imagery.

How pulleys work

If you have a single wheel and a single 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. (Incidentally, although the kilogram is a unit of mass, not force, it's okay to talk about a force equivalent to a given mass because masses generally convert to forces in the same way.) If you want to raise the weight 1m into the air, you have to pull the rope a total distance of 1m at the other end.

Artwork: How pulleys work#1: With one wheel, a pulley simply reverses the direction of the force you apply.

Now if you add more ropes and wheels, you can reduce the effort you need to lift the weight. Suppose you have two wheels and two ropes, arranged as in the figure below. The 100kg weight is now supported by two ropes instead of one (ignoring the loose end of the rope you're pulling with), so you can lift it by pulling with a force of just 50kg—half as much! That's why we say a pulley with two wheels and ropes 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.

Artwork: How pulleys work#2: With two wheels and two ropes, a pulley halves the lifting force you need. But you have to pull the end of the rope twice as far.

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 weight is now supported by four ropes on the left (ignoring the end of the rope you're pulling with). That means each rope is supporting a quarter of the total 100kg weight, or 25kg, and to raise the weight into the air, you have to pull with only a quarter of the force—also 25kg. We say a pulley with four wheels and ropes gives a mechanical advantage of four. That's twice as good as a pulley with two ropes and wheels.

Artwork: How pulleys work#3: With four wheels and four ropes, 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.

What's the catch?

Pulleys sound brilliant—and they are. But surely there must be a catch? If you can lift 100kg by pulling with the force-equivalent of only 25kg, 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!

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 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 ropes 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 ropes rub against the pulley wheels. But it seems and feels easier to use a pulley, and that's the important thing!

Photos: Pulley equipment. Left: These small pulley wheels have hooks on them so they're easy to hang from the ceiling. Note how the wheels have grooves in them so the rope can slide over them more easily. Photo by Paula Aragon. Right: Giant pulley wheels on the arm of a large barge crane. This one uses huge strong steel hawsers instead of ropes. Photo by Don Sutherland. Both photos courtesy of Defense Imagery.