G07: Physics Forces & Motion: Pulleys

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!

Pulleys are simple devices with many uses, and as an engineering expert, I’ve often incorporated them into mechanical designs. They’re used in machinery to transmit mechanical power from electric motors and engines to devices like blowers and pumps. Another common usage for pulleys is to aid in lifting. There are two types of pulleys for this purpose, simple or compound. We’ll start our discussion off by looking at the simple type today.

when it comes to lifting massive objects one simple pulley isn’t going to be enough to get the job done. Even the improved simple pulley, which we introduced last week, is often not enough, a situation which I’ve run across in my career as an engineering expert. To get past the limitations of the simple pulley and improved simple pulley, ancient Greeks went on to devise the compound pulley, which we’ll introduce today.

The compound pulley illustrated below contains three rope sections with three representative tension forces, F1, F2, and F3. Together, these three forces work to offset the weight, W, of a suspended urn weighing 40 lbs. Weight itself is a downward pulling force due to the effects of gravity.

The theoretical mechanical advantage of a system is the ratio of the force that performs the useful work to the force applied, assuming there is no friction in the system.

A pulley is one of the original simple machines. The pulley was possibly created by Archimedes in B.C. times. Its original use was to lift buckets of water. The term sheave can also be used to refer to a pulley.