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.
A fixed pulley is fine for lifting something like a flag. But what if the load is much heavier? What if the load is the engine of a car? You could never lift it using one fixed pulley.
So, add a second pulley below the fixed one. Attach the second pulley to the load. That way, the pulley moves with the load. And the second pulley doubles your force! It’s like having two people pulling instead of one.
Adding a third pulley multiplies your force by three. A fourth pulley . . . well, you get the idea. This may seem like magic. But it’s science. The rope supports the load by looping at the bottom of the pulleys that lift it.
Cranes like these use many pulleys to lift heavy weights.
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.
Count the pieces of rope in the diagram. The number of pieces tells you how many times those pulleys increase the force. (Count only the pieces of rope that loop under a pulley. They support the load.)
Most movable pulleys are joined to fixed pulleys. Construction sites are great for watching pulleys in action.
Pulleys used together make lifting easy.
Pulleys used on sailboats are called blocks.
You know that using more pulleys will multiply your force. This means that less effort is needed for lifting.
Imagine lifting a heavy box of books. You use a fixed pulley. You take hold of the rope and pull. Four tugs later the box is on a truck.
For the next box, you add a movable pulley. Your force is doubled. Yet, you use only half the effort. It’s easier, but it takes eight tugs on the rope. That’s the trade‑off. The more pulleys you use, the easier it is to pull the rope, but the farther you have to pull it. That’s a good trade!