A pulley is a simple machine that can change the direction of a force to make it easier to lift or move large objects. Thousands of years ago, early engineers used pulleys to do many useful tasks and build structures like obelisks, pyramids and wells. Today, modern engineers use many different types of pulleys in a wide variety of real-world applications.
Know / Want to Know / Learn (KWL) Chart: Create a classroom KWL chart with the title, “Simple Machines: Pulleys.” Draw three columns for students to indicate what they already know about pulleys and what they want to learn about them. Fill out the K and W sections during the lesson as facts and questions emerge, and then fill out the L section at the end of the lesson to summarize student responses.
What are the three types of pulleys?
A fixed pulley is a pulley that is attached to a beam or other secure surface and stays in place. The rope that is attached to the pulley is also fixed to the beam or other secure surface. The other type of pulley is a movable pulley. A movable pulley has a free axle and is used to transform forces. When stationary, the total force on the axle balances the total force provided by the tension in the rope.
Why do pulleys have mechanical advantages?
A pulley can create more force than a simple lever, but only if the force is applied over a longer distance. This is why it can help people lift much heavier weights than they would be able to without it.
Suppose you were trying to lift something four times heavier than it is. You can do this by pulling the loose end of the rope four times further than you would need to with a simple lever. The same is true with a pulley, but the advantage is even greater if the load you’re lifting is supported on four strands of rope instead of one.
The same effect can happen with a single wheel and rope system, but the mechanical advantage increases to 1. If there are two wheels and one rope attached to each, the ideal mechanical advantage is 2. This means that you need to apply half the amount of effort to lift the load with the movable pulley system.
What is a compound pulley?
A compound pulley is a type of pulley that has a fixed and a moveable pulley in the same system. This type of system is usually found in cranes that lift heavy items because the movable pulley helps the crane do the work.
What are the mechanical advantages of a single pulley?
A single pulley has a mechanical advantage of 1. It changes the direction of the force so it is easier to lift or pull a load. In a movable pulley, the mechanical advantage is 2 because two rope segments pull up on the load, making it easier to lift or pull the load.