Did you ever wonder how elevators move up and down? They use pulleys! A pulley is a simple machine, made from nothing more than a rope and a wheel. By looping the rope over the wheel, you can use a pulley to change the direction of a force. (The loop of rope on a flagpole is a simple example of a pulley.) But if you have several pulleys working together, they act as a force amplifier — or in other words, they make it easier to lift heavy things. In this project, you’ll be learning about pulley systems and how they work using Ziploc® brand sandwich bags!

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The mechanical advantage of a machine (like this one!) is a measure of how much easier the machine makes it to do something — in this case, lifting marbles. In a perfect world, this pulley system doubles your mechanical advantage, meaning the force needed to lift an object is only half what the object weighs. So if you had six marbles to lift, a perfect pulley system would only need three marbles to lift them. However, we don’t live in a perfect world. There are other forces at work, like friction, that reduce the mechanical advantage, so your pulley system will probably require the weight of about four marbles to lift six marbles.


But you can keep experimenting! Can you think of any ways you might be able to reduce the friction in your system to create a more efficient pulley? Try it out and see how close to a perfect pulley you can create!

Connections with Next Generation Science Standards (NGSS)

Engineering Design MS-ETS1-2

Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

Build a system to lift a weight using something lighter than itself. Compare a simple pulley and a compound pulley to test which one is more effective.