The Science of Pulleys
A pulley is a wheel that you loop a rope over. The key scientific concept for pulleys is that they redirect force. For example, instead of PUSHING a load UP, you can attach it to a rope, run the rope through a pulley above you and then PULL DOWN on the rope to move it up. All pulley systems re-direct force. Some pulley systems make work easier, allowing a worker to use less force to move the object.
A demonstration of pulley action:
- Tie a rope on something heavy (e.g. a basket of books, a sack of flour, or a gallon jug of water).
- Have a child lift the object off the ground by pulling up on the rope. They may only be able to lift it six inches or so off the ground – partially because it’s heavy, partially because their little arms can only reach so high.
- Then have them pick up the object itself and try to lift it high up above their head with their arms. It’s higher now, but wow it feels heavy to push it up.
- Then, put the rope through a pulley up high and then they pull down. It feels* so much easier to pull down to lift the load, and it can go as high as the pulley can be mounted.
In step #4, we said it feels* easier to pull down on a rope that’s looped around a pulley. This is just a body mechanics sensation – it’s easier to pull down because you can add your body weight in to the motion. But it’s not a scientific measurement of actual total “work” needed. A single fixed pulley does not change the amount of force you need to lift a load, it just redirects your force.
On a movable pulley (see below), the work is distributed over two parts of the rope, and the rope is supporting part of the weight. So using a movable pulley takes half as much work to pull the load up, but you have to pull twice as much length of rope. A compound pulley system (see below) distributes the work over multiple pulleys. The more pulleys in your system, the less force you need to use. (And the more rope you have to move.) The block and tackle has multiple loops of rope on multiple pulleys. The more we divide up the work, the less total force is needed. That’s why block and tackles have been used to lift heavy loads for centuries. This Explain that Stuff article does a nice job of explaining the basics of force and pulleys at an adult level. The Wikipedia article on pulleys has good descriptions of block and tackles.
If you want to buy a basic pulley set for your kids, I think it’s a great investment in a fun toy they can use in a variety of ways for a number of years, and you might find other handy uses for it. We’ve used wooden pulleys from Haba, but they’re no longer available. Luckily, there are lots of fine pulleys. Utility pulleys like these work just fine, and I quite like these Rocari pulleys, which we use on the playground with 1/4 or 3/8 inch nylon rope. (The M20 Rocari and a 3/8 inch SGT Knot rope combo is rated to lift up to 165 pounds… we usually just lift up backpacks or buckets of pinecones.) I like the nylon rope because it’s soft and easy on kids’ hands. I usually attach my pulleys to things with either cord or pipe cleaners, but you could use carabiners or quick links.
You need a lot of rope… 20 feet of rope is just barely enough to use a fixed pulley to lift something from the ground to a slide platform or second story balcony. I’d probably go 25 feet if you’re buying new. If you want to be able to use multiple pulleys, like a block and tackle system, you’ll need a lot more.
If you want a set of small pulleys for classroom use, I’ve considered but haven’t purchased the American Educational 7-1607 Pulley Kit. They have very shallow grooves, and we found that the pulleys from a Delta Science Modules kit that had shallow grooves are just not useful for kids to play with and explore because the string jumps off the track too easily.
How to get pulley action without a pulley
You can get pulley-like action just by wrapping a rope over any smooth round object and pulling down. Some pulley substitutes you can jerry rig:
- Put a rope over a doorknob
- Lay a broom over two desks with a gap between them so the broom handle bridges the gap. put the rope over the broom handle
- Throw a rope over the side of a slide platform (or around a railing on an open staircase)
- A rolling pin pulley: http://cabinetofcuriositiesva.com/2014/08/21/a-lesson-on-simple-machines-simply-worked/)
- An empty spool from cord or ribbon: https://gosciencegirls.com/pulley-loungeroom-stairs-physics-fun-kids/.
- Carabiners: www.youtube.com/watch?v=oGgCaFTcbHk
- A spool for thread and a wire hanger: www.ehow.com/how_5896241_make-pulley-children.html
- Sewing machine bobbins can be used for a fun miniature pulley system
Types of Pulley Systems
Here is an image showing the four main types of pulleys, and here’s a PDF mini-poster you can print of all 6 types of pulley systems I describe.
Setting Up Pulley Explorations
Safety Note: pulleys should be used under adult supervision, because anything involving ropes involved a possible strangulation risk. That said:
Kids LOVE playing with pulleys! They can spend hours engaged in a simple task like lifting toys up to the slide platform, sending them down the slide, and lifting them up again. (Child development theorists call this schema of play “transportation.”) And this hands-on experience is so effective in teaching about the physics of simple machines – even the parents learn as they play along!
You can set up a variety of pulley systems for kids to play with and explore… we had them all set up in one class session, but you could also do one each day over a series of sessions. Or just set one style up on the playground or in the backyard, and leave it there for months of fun! (See photos of our pulleys at the bottom of the post, and lots more details on how to set them up in my Pulley Play for Kids post.)
A fixed pulley. Requires one pulley, and a high place to anchor it. You anchor the pulley (fasten it to the high place), tie a rope onto a bucket, run the rope up over the pulley and back down to the ground. A child standing on the ground pulls down on the loose end of the rope (shown with the green arrow), and it lifts the basket high up to the pulley. This is the quickest / easiest system to set up.
- This activity is way more fun if the high anchor is next to somewhere a child can safely stand, like on a slide platform, on a stair landing, in a tree house or the top of a ladder. One person stands on the ground, loads a stuffed animal or other soft toy into the bucket, and sends it up. The person on the platform can send it back down in the basket, or can toss it down or send it down the slide.
Movable pulley. Requires one pulley, a high place to anchor one end of the rope, and the ability to stand and pull from higher up than the pulley is mounted. Tie a rope up high, run it down through a pulley that has the load hanging on it, then bring the end of the rope up high. Pull the rope up from above the anchor point to bring the load up.
- For the fixed pulley, you need two kids – one at the bottom to pull the rope, and one at the top to catch the load. With the movable pulley, one kid up top can do both jobs. They run/climb down, load the basket, run/climb up to the top, and pull the load up.
Compound pulley. Requires two pulleys – it’s basically a combination of a moveable and a fixed pulley. Tie the rope up high, then run the rope down through pulley #1 that has a load attached to it, then up through pulley #2, then down to the ground. You pull down to lift the load up.
For the simple version, anchor a pulley. Tie the rope to the bottom of it. Run the rope down around pulley #2, then up around pulley #1. Pull down to lift up.
A basket on a track. The fixed pulley lifts things vertically – straight up from the ground. This is a variant on the fixed pulley, where you can get vertical and horizontal movement, as the basket travels up a slanted track, shown in brown. Start by either running the track cord through the handle on the bucket, or through a pulley (shown in pink) that is attached to the bucket – the pulley allows it to run more smoothly. Tie both ends of the cord to anchors to form your track. Make sure the cord is taut, and the basket can slide easily up and down the cord. Then anchor your pulley (red) near the top end of the track cord. Tie one end of the yellow rope to the basket handle, throw the rope over the pulley, and leave it hanging down. When you pull down on the yellow cord, the basket slides up the brown track.
The clothesline. Requires two pulleys. Attach pulleys to anchor points. Take a rope and wrap it around both pulleys, then tie it, making one big loop. Clip things on with clothespins, or tie a basket or other container onto it. Then you pull on either the top or the bottom cord to bring the basket from one end to the other. This is easy for even two year olds to work, and still plenty of fun for elementary aged kids. A vertical clothesline can work like a flagpole to raise things high.
Math extension for Pulley Exploration
Put weights into each of the baskets. Dominoes? Glass pebbles? Books? What object you choose depends on how strong your baskets, your rope, and your child are. Count how many of those objects you can lift with each type of pulley. Which pulley system helps you do the most work?
Challenge – Build a Flagpole
We make a flagpole using a ruler, binder clip, paper fasteners and bobbins. Here’s the full flagpole tutorial and a video preview.
We have a flag making station, where children can design and decorate their own paper flags with markers. We had the children decorate paper flags with markers. We put out flag design tips and posters showing the flags of all 50 states, and most countries, some corporate logos to serve as inspiration for the activity. (Looking for a short primer on flag design? Check out Good Flag, Bad Flag) In the past we used a half sheet or full sheet of paper for this, to give them plenty of room to work in, but for their ruler flagpoles, we make smaller flags.
A Pulley Kit
We’ve had them assemble a kit with miniature pulleys to take home. The supplies we put out are: bobbins, mini clothespins and dixie cups (and materials to decorate the clothespins and cups, yarn, a ruler, scissors and ziplock bags. We also set up a sample pulley system between two paint jugs so they know what they’re assembling. They use the ruler to measure 20 feet of yarn or string, then put into their bag: 2 bobbin pulleys, the string, one clip, and one cup.
Water Table / Sensory Bin
One year, we built a wishing well from Duplos, thread, and a plastic cup. (picture 1) Note: this requires some specialty Duplo pieces which we got from our Duplo Simple Machines pack. A pulley lifts the bucket up out of the water. Another year, we built a bucket elevator, aka rope pump (picture 2). These were used in the Hanging Gardens of Babylon in 450 BC, and in ancient China. We tied a small plastic “shot” glass to the cord to be the bucket. It almost worked. Another year, we set up fixed pulleys at the sensory table. (picture 3) You MIGHT be able to use this idea at the sensory table: glue metal cans onto a piece of cardboard to form a series of “pulleys” that children can explore: https://www.123homeschool4me.com/teach-your-child-to-engineer-their-own_21/
Building a Well
We made a well using a tall skinny water bottle, a bucket made from a thimble with a glued on handle, and a pulley and crank made with cardboard and a plastic lid.
Hand cranked winch from paper towel tubes: Here’s a fun craft to make with recyclables: build a winch system: http://littlebinsforlittlehands.com/
For a pulleys-only class, you can do a demonstration exactly as described near the top of this post to show the mechanics of pulleys, then demo how each system that you have set up works, and what the advantages are of each system. So, we demo a fixed, a movable, and a compound. Plus a block and tackle:
We combine this topic with inclined planes, so we have set up a ramp (inclined plane) using a long plank propped up on a step ladder. At the bottom of the plank, we set a basket with a stuffed elephant inside. We set up a block and tackle pulley to it. Kids take the rope and start backing across the room to lift the elephant. We teach the hand-over hand method of pulling the rope.
Then, while I read the day’s book (Raising a Bag of Toys: Pulley vs. Inclined Plane), we acted out the story with our slide, which illustrated the different advantages of pulleys and inclined planes to raise a toy up to the slide platform.
Books, Songs, and Videos
Song: I didn’t really find a song that was good for this age group to sing… but for the entertainment and amusement of adults or older kids, check out “The Sick Note / Why Paddy’s Not at Work Today”: www.youtube.com/watch?v=iA5RGI3zn20
Books: Here’s a full post on all the kids’ books about simple machines: https://inventorsoftomorrow.com/2017/03/01/books-about-simple-machines/
Videos about pulleys: www.youtube.com/watch?v=9T7tGosXM58 is a 6 minute segment from an episode of Bill Nye the Science Guy and www.youtube.com/watch?v=LiarGb_LK10 is a 1.5 minute video showing kids the basics of pulleys.
For a great collection of pulley activities for kids, including recommended books, click here: http://iijuan12.hubpages.com/hub/gears-and-pulleys-simple-machines-lesson-plan. Here are a couple fun posts about how parents added pulley play to their backyards for hours or years of fun for their kids: https://innovationkidslab.com/backyard-pulley-engineering-kids/; http://happyhooligans.ca/a-bucket-and-a-rope/; and http://littlebinsforlittlehands.com/homemade-outdoor-pulley-play-idea-simple-machines/
Photos of Pulleys in Action
Here are pictures from class. There are more details about how to build and use these in my post on Pulley Play for Kids. In the photos, you’ll see: Fixed. The pulley is tied onto the slide platform. Child at the bottom loads a stuffed animal and pulls it up to their buddy. Compound: We anchored one end of the rope to a nail on the wall, then ran it through the bucket handle for a movable pulley then up over a spool for the fixed pulley. Block and Tackle: Here’s our block and tackle system, set up on a ramp. Basket on a Track – 2 pictures The brown cord is the track. It goes through a pulley attached to the green basket. We tied one end of the yellow rope around the basket, and the other went up to the “pulley” at the top and back down.
Build Your Own
The peg board offered a build-your-own pulley system, where kids could re-arrange the pulleys and the ropes to test multiple paths. In the photo, the cord is fixed on the left, then runs over a red fixed pulley, down to a silver movable pulley tied to a pouch full of glass weights, then up and over a red fixed pulley then tied to a second pouch. Kids could load pouch #2 with stones until it was heavy enough to pull pouch #1 up. This was a great idea but harder in practice as the cord was very prone to slipping off the pulleys due to their narrow grooves. With adult assistance, it was a fun activity.
Set up Note
This class session takes longer to set up than any other class we teach all year (we usually take 90 minutes to set up for a class, and this day takes a solid two hours). But it is so worth it! The play value and learning value are supreme. You could choose to do just one or two pulleys though and still have a great fun learning experience.
Note: All the activities described in my posts are from Family Inventor’s Lab, a parent-child cooperative class in Bellevue, WA. We are a play-based, STEM focused class for preschool through early elementary (kids age 3 – 7). We do a wide variety of fun, hands-on activities to learn about Science, Tools, Engineering, Nature, and Art. We also sing songs and read stories. Most of our activities are cheap, easy, and use everyday materials that most families would have in their homes (or their recycle bins!), so that our activities are appropriate for classroom teachers, parents who homeschool, or after school programs.