Hands-On Physics! Build a DIY Movable Pulley to Master the “Work” Principle and Wow Your Students!
I’m Ken Kuwako, your Science Trainer. Life is one big experiment!
Hello, fellow science teachers! Today, I want to share a brilliant DIY experiment that will make your students go “Aha!” using simple materials you can find at any hardware store. Have you ever explained that “a movable pulley cuts the required force in half,” only to be met with blank stares? It’s not surprising. The tiny pulleys found in most educational catalogs aren’t very impressive. Even with a 100g or 200g weight, students can barely feel the difference in effort. Plus, buying enough high-end kits for an entire class is often budget-breaking.
But don’t worry! I’ve found a way to build large-scale movable pulleys for about $2 each that let students feel a genuine “Whoa, that’s light!” sensation. This hands-on tool is guaranteed to make your physics lessons more engaging and deepen their understanding of mechanics.
The Science Recipe
The secret ingredient for this experiment is an unexpected item from the hardware store: sliding door rollers! You can usually find a pair for just a few dollars, making them incredibly cost-effective.
What You’ll Need
Sliding door or “Fusuma” rollers (roughly $3 for a pair at most hardware stores).
In addition, we used twine, a plastic bottle, sand, and water.
How to Build It
① Identify the top and bottom of the roller.
This side should face up.
② Create the movable pulley.
For the weight, we filled a plastic bottle with sand and water so that the total mass (including the pulley) was exactly 1kg (approx. 10N). This is heavy enough for students to really feel the resistance—or lack thereof!
Let’s assemble the pulley.
Thread the twine through the hole on the side of the roller housing,
And tie it securely.
Tie the other end to the neck of the plastic bottle.
Attach the other side so the pulley stays horizontal.
Finally, loop a long piece of twine over the wheel. Secure one end to a stand and hold the other. You’re ready to go!
Let the experiment begin!
First, the baseline. Let’s lift the bottle directly using a spring scale without the pulley. Our weight is about 1kg, which translates to a force of roughly 10N.
As you can see on the scale, it points exactly to 10N.
Now, let’s use our DIY movable pulley.
The result? The scale now reads exactly 5N.
When you look closely, this “half the force” result makes perfect sense. The load is being shared between two points: you (the spring scale) and the “ceiling” (the fixed end of the string). In this setup, I’ve used a stand, but you can have one student hold one end while another pulls the other. This lets them physically feel how they are splitting the load.
By preparing enough of these DIY pulleys so every student can try it, their grasp of the material will skyrocket.
Through this experiment, you can help students truly internalize the Principle of Work: that a movable pulley trades distance for force—cutting the effort in half but requiring you to pull twice as much string.
For more detailed measurements and data on the Principle of Work, check out these posts:
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