Pulleys on the Move

The pulley at the top of a flagpole stays in one place. It’s attached, or fixed, to the pole, so it’s called a fixed pulley. Fixed pulleys are also used to raise sails on some sailboats. The pulley is attached to the top of the mast. All a sailor has to do to raise the sail is pull down on the rope.

A fixed pulley is fine for lifting something that doesn’t weigh very much, like a bucket with lunch inside or a flag. But what if the load is much heavier? For example, mechanics sometimes have to lift a heavy engine out of a car. One person could never lift it using one fixed pulley.

So, what’s the answer? More pulleys! You can add a second pulley below the fixed one. Notice that this second pulley attaches to the load. That way, the pulley moves with the load.

Adding the movable pulley does something special: It doubles your force! It’s like having two people pulling instead of one. But why stop there? Adding a third pulley multiplies your force by three. A fourth pulley . . . well, you get the idea.

Pulley multiplication may seem like magic, but it’s science through and through. The rope supports the load by looping beneath the pulleys that lift it. This support helps the lifting.

Cranes like these use many pulleys to lift heavy weights.

Count the segments of rope in the diagram. The number of segments tells you how many times that combination of pulleys multiplies the force. Count only the segments of rope that loop under a pulley, not the segment that you pull on. The segment you pull on doesn’t support the load.



A fixed pulley changes the direction of force.

As you add more pulleys, you pull the rope a greater distance but use less force to move the load.

Most pulleys are a combination of movable pulleys connected to one or more fixed pulleys. Construction sites are terrific places to see pulleys in action. So are loading docks, where cranes lift and lower cargo on and off ships.

You can find other examples just about anywhere something needs to be raised and lowered. That includes the blinds in your home or school. Window washers use pulleys to raise and lower their platform high above city streets. And what if a hiker on a mountain is stranded and needs to be rescued by a helicopter crew? Pulleys lift the hiker to safety.

Pulleys used together make lifting easy.

Pulleys used on sailboats are called blocks.



You know that the more pulleys you use, the more they multiply your force. Another way to look at it: The more pulleys you use, the less effort you need to lift the same load.

Imagine using a fixed pulley to lift a heavy box of books onto the back of a truck. You grab a good hold on the rope with two hands and pull. It’s a strain, but four tugs later the box is on the truck.

For the next box of books, you add a movable pulley connected to the box. Your force is doubled, which means you pull the rope with only half the effort. It’s a lot easier but you have to give eight tugs on the rope. Do you see the pattern?

Each time you add a pulley, you save on effort but you increase the distance you have to pull the rope. That’s the thing about simple machines: There are always trade‑offs. The more pulleys you use, the easier it is to pull the rope, but the farther you have to pull it. You trade distance for effort. You also save yourself from getting a sore back, so it’s a good trade!