Your mission
A string. A metal washer. That is a pendulum, and it holds a secret that surprised scientists 400 years ago.
Work through each tab in the menu. Read how it works, then show your thinking. In the Swing lab, you will make predictions and test them.
Earn all five badges to unlock the build plans. Then you get to make a real pendulum and time it.
My badges
Teacher note
- For each pair: about 1 meter of string, 3 metal washers that are all the same size, tape, a pencil, and a ruler or meter stick.
- For the class: stopwatches or timers. Hardware store washers work well, and one small box is plenty.
- The tab called The surprise stays locked until the lab missions are done, so students cannot read the answer before they predict.
- The build plans unlock after all five badges are earned. Progress is saved on this Chromebook only.
- The Swing lab is a simple model. In real life, very big swings take a tiny bit longer, so have students keep the pull back small (about a hand width).
- Real timing is messy. Timing 10 swings and dividing by 10 gives much better results than timing 1 swing.
Meet the pendulum
It is one of the simplest machines in science. You have ridden one.
Read: three parts
A pendulum is a weight that hangs from one fixed spot and swings freely back and forth. A playground swing is a pendulum. So is the swinging arm inside a tall clock.
Every pendulum has three parts. The pivot is the fixed spot at the top. The string hangs down from the pivot. The weight at the bottom is called the bob.
One full swing means the bob goes all the way over and all the way back to where it started. The time one full swing takes is called the period.
One swing is too quick to time well. So scientists time several swings in a row and then divide. In the lab on this page, the timer counts 5 full swings.
Picture it
Why does it keep swinging?
Nobody pushes it. Two ideas explain the whole thing.
Read: gravity and inertia
Pull the bob to one side and you also lift it a little. Let go, and gravity pulls it back down. The string will not let it fall straight down, so it follows a curved path called an arc.
The bob speeds up all the way to the bottom. That is where it moves fastest.
So why does it not stop there? A moving object keeps moving until something stops it. Scientists call this inertia. The bob sails right through the bottom and climbs the other side.
On the way up, gravity pulls against it and slows it down. For an instant it stops at the top. Then it swings back, and the whole thing repeats.
Each swing is a tiny bit smaller than the last. Air resistance and rubbing at the pivot steal a little of the motion every time.
Try it: move the bob
The energy swap
A pendulum trades one kind of energy for another, over and over.
Read: stored energy and motion energy
Lifting the bob to the side stores energy in it. Stored energy is called potential energy. The higher the bob, the more it has.
As the bob swings down, the stored energy changes into kinetic energy, the energy of motion. At the bottom, the bob is at its lowest and its fastest. Almost all of its energy is kinetic.
As it climbs the other side, the kinetic energy changes back into potential energy. Back and forth, the energy keeps swapping forms.
A pendulum can never swing higher than the spot where you let it go, unless you give it a push. It cannot end up with more energy than it started with.
Big idea: Patterns. A pendulum repeats the same swap again and again. A pattern that repeats this steadily can be used to measure time.
Try it: watch the swap
Swing lab
Three things can change on a pendulum: the weight, the pull back, and the string length. Which ones change the swing time? Predict first, then test.
Missions
A fair test changes only one thing at a time. Finish all four missions to earn the Fair Tester badge.
Set it up
The timer stops after 5 full swings. Then it divides by 5 to find the time for one swing.
Side to side over time
The dashed line is your last test. If both lines cross the middle at the same moments, the swing time did not change.
My test results
| String | Washers | Pull back | 5 swings | 1 swing |
|---|
Your tests will show up here, newest on top.
This lab is a model. A real pendulum will not match these numbers exactly, but the patterns are the same.
The surprise
You ran the tests. Here is why they turned out the way they did.
Locked until you finish the lab
No peeking. Finish all the lab missions first, then come back to find out why.
Read: only one thing matters
You tested three things: the weight, the pull back, and the string length. Only one of them changed the swing time. It was the length.
Why not the weight? Gravity pulls harder on a heavier bob. But a heavier bob is also harder to get moving. Those two things cancel out exactly, so heavy and light bobs swing in step.
Why not the pull back? A bigger swing has farther to go. But it also starts on a steeper part of the arc, so it moves faster. For small swings, those two things cancel out as well.
Why the length? A longer string makes a bigger, flatter arc. The bob has farther to travel and a gentler slope to travel on. Each swing takes more time.
This is why pendulums make great clocks. The swing time stays the same even as the swings get smaller. The scientist Galileo noticed this steady timing more than 400 years ago. To make a pendulum clock run slower, you make the pendulum longer.
A number pattern
| String length | One swing |
|---|---|
| 25 cm | 1.0 second |
| 50 cm | 1.4 seconds |
| 100 cm | 2.0 seconds |
Look at the first and last rows. To double the swing time, the string has to be four times as long.
A kid and a grown up on matching playground swings will swing in step, as long as nobody pumps their legs.
Build it
The build plans are locked
Earn every badge to open them. You still need:
You earned it. Time to make a real pendulum and see if it agrees with the lab.
How to build your pendulum
You need: string, 3 metal washers, tape, a pencil, a ruler, and a timer. Work with a partner.
- Tie one washer to the end of the string. This is your bob.
- Tape a pencil to the top of a desk so half of it sticks out past the edge. This is your pivot.
- Tie or tape the string to the pencil. Measure 25 cm from the pencil to the middle of the washer.
- Pull the bob to the side about one hand width. Keep the string tight.
- Let go. Do not push. Your partner starts the timer at the same moment.
- Count 10 full swings, over and back. Stop the timer on swing number 10.
- Divide your time by 10. That is the time for one swing.
Stay safe: swing your pendulum away from faces, and keep the swings small.
The plan
Measure from the pencil to the middle of the washer.
Test like a scientist
Change only one thing at a time. Time 10 swings, three times, for each setup. Then divide the average by 10 to get one swing.
| What I changed | 10 swings, try 1 (s) | Try 2 (s) | Try 3 (s) | Average |
|---|
Ideas to test: add a second and third washer, pull back a little farther, or make the string 50 cm long. Can you make a pendulum that swings exactly once per second?