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 and pulley above you and PULL DOWN 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-style 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, string the rope up over a doorknob or over a broom handle (lay the broom handle over a gap between two desks) and have them sit on the floor and pull down on the rope. It feels easier* to pull the rope down to lift the object than it did to push the object up with their hands. But they can only lift the object as high as where the pulley action is happening (e.g. height of the doorknob).
- Then throw a rope over the side of a slide platform (or around a railing on an open staircase: http://handsonaswegrow.com/simple-machines-for-kids-the-pulley/) and have the child haul something up as high as they can. Look how high you can lift something heavy when you use this tool!
Where to find a pulley:
Notice that none of those examples above use an actual “pulley” from a store – they used a broom handle or doorknob, a slide platform or a stair railing. You’re getting pulley action just by wrapping a rope over an object and pulling down.
You can also rig a “pulley” substitute with other objects you may have:
- A rolling pin pulley: http://cabinetofcuriositiesva.com/2014/08/21/a-lesson-on-simple-machines-simply-worked/)
- A spool that cord or ribbon was once wound around: 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
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 use cord or pipe cleaners to attach my pulleys to things, 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 Pulleys Discovery Kit and 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 much.
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.)
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.
- This is a fun three kid activity. One to load the basket at the bottom of the track. One below the pulley to pull down on the rope to raise the basket, and one up high by the pulley to unload the basket.
- If the basket is heavy enough, then when the child lets go of the rope, the basket will slide back down to the bottom of the cord. It takes a lot of tinkering to get that to work just right. If it’s not heavy enough, the kid on the ground may need to pull the basket back to the bottom by hand.
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.
- Challenge: The rope must be perfectly taut for this to work! If it sags, it falls off of the pulleys. (And if it’s really tight, it’s hard for kids to get it moving.) So, use a rope / cord that won’t stretch out over time, and if possible anchor one end on something you can move back to make the rope tighter. If you’re anchoring it between two immovable objects (like two trees), expect to spend quite a while (10 minutes?) on tinkering as you set it up to get it just right.
- To really engage kids in this activity, make it a delivery system! Put one end at the writing center, and the other end in the block center and let kids write messages, clip them on, and send them to a friend.
Safety note: try not to set up clotheslines and basket-on-a-track system in high traffic areas where kids will run into them and “clothesline” themselves around the neck.
See photos at the bottom of this post.
In step #4 of our initial demonstration, we said it feels easier* to pull down on a rope that’s looped around something for pulley-like action. 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. That demo used something like a fixed pulley system, and with a fixed pulley (or basket on a track or clothesline pulley), you need to use the same amount of force, the pulley just redirects your force. Those systems do not change the amount of force required to move a load.
On a movable pulley, 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 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.
Math activity: 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?
Water table: One year, we built a wishing well from Duplos, thread, and a plastic cup. 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). 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.
Challenge Activity – Build a Flagpole System. We put out straws, binder clips, spools, bobbins, string, and clay, and encouraged children to try making a flag pole. We also offered a couple samples we had assembled. This project was over the head of most of our preschool aged kids, but our older kids and parents had fun with the challenge.
Art project: Flagpoles use pulley systems to raise the flag. We had the children decorate paper flags with markers. Next to this station, we put posters showing the flags of all 50 states, and most countries, to serve as inspiration for the activity. (Looking for a short primer on flag design? Check out Good Flag, Bad Flag)
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 works, and what the advantages are of each system.
We combine this topic with inclined planes, so here’s our circle time plan: We 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 tied a rope on the basket, ran it through a pulley attached to the step ladder, and back down. (A fixed pulley.) We had the kids haul on the rope to pull the elephants up the slide. It was HARD work! They had to learn how to brace their bodies to pull hard (a good large motor skill to learn!) Some kids would just grab on the rope and pull by backing up further and further. Some understood how to pull hand over hand, so they could stand in one place and alternate hands to pull with. (Pulling hand over hand is also a great large motor skill to learn.) They were putting so much force into their pulling that an adult had to brace the ladder so it wouldn’t tip over.
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.
Before closing circle, my co-teacher re-rigged the pulley system so it was a block and tackle attached to the basket / elephant. After the story, we had them all try pulling the elephant up again. It was SO MUCH EASIER for the kids to do. (And no need for an adult to brace the ladder.) This was a fabulous hands-on experience that really helped them appreciate the power of the block and tackle.
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. I apologize for their quality. It’s hard to get a good picture of pulleys and ropes! (Click on any image to see a larger image.)
Fixed. The pulley is tied onto the slide platform. Child at the bottom loads a stuffed animal and pulls it up to their buddy.
Here’s one compound “pulley” system. We anchored one end of the rope to a nail on the wall, then ran it through the bucket handle (we cheated a bit by using the handle as the movable “pulley”) then up over a spool for the fixed pulley.
For this next compound system, the rope is tied onto PVC frame on the left then down through a movable pulley, then up through a fixed pulley in the center, back down to another movable pulley at the bucket, then back up to a pulley the top rod. If kids pull where the sign is, it lifts the very heavy bucket up high.
We had a very heavy bag of sand one year and a bucket of clay another. (Goal is for it to be heavy enough that our 5 year olds could barely lift it off the ground on their own.) When we put it in the bucket on the system, it was easy-peasy for them to lift it all the way up. (We learned we had to tell them to then carefully lower it to the ground, not just let go and let it drop.)
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.
Block and Tackle:
Here’s our block and tackle system, set up on a ramp.
Basket on a Track. 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. (Note that we’re not actually using a pulley at the top – we just looped it up and over the round railing on the slide platform.
Clothesline. Anchored on the slide platform and to a hook on the wall.
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 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.