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Release two double pendulums almost exactly the same. How long do they stay twins?
Double Pendulum Twins lab · Last updated: 16 October 2026
Two double pendulums are released from almost exactly the same position. At first they swing like twins. Then, quite suddenly, they go their own wild ways. You see chaos happen and time how long the twins stay together.
What you'll discover
- A double pendulum is a pendulum hanging from the end of another pendulum.
- Two that start almost identically soon move in completely different ways.
- This sensitivity to the starting point is what scientists call chaos.
The science in plain words
A single pendulum is very predictable: it swings back and forth with a steady rhythm. Hang a second pendulum from the bottom of the first, and everything changes. Each arm pushes and pulls the other, so the motion becomes a tangle of swings, flips and loops.
The double pendulum still follows exact laws of physics. Nothing about it is random. Given its precise starting position, its future is fixed. The problem is that the motion is extremely sensitive to that start. A difference of a fraction of a degree, too small to see, grows bigger with every swing until the two pendulums look nothing alike.
This is called chaotic motion. It explains why weather can only be forecast well for a limited number of days ahead: we can never measure the air perfectly, and small errors grow over time. The same idea appears in planets' orbits over very long times, in dripping taps and in mixing fluids.
The lab makes chaos visible. You can see the twins move in step for a while, then watch the gap between them open up and grow faster and faster.
Where it fits
- India, NCERT: Chaos goes beyond the NCERT syllabus for classes 6 to 10; it builds on the simple pendulum in Class 7 Chapter 8: Measurement of Time and Motion.
- United States: Grades 9 to 12
Common questions
Why is a double pendulum chaotic?
Its two arms constantly push and pull on each other, and the motion depends so strongly on the starting position that any tiny difference grows quickly. Two releases that look identical end up swinging completely differently.
Is a double pendulum random?
No. It follows exact physical laws, so the same perfect start would always give the same motion. It only seems random because we can never set up or measure the start perfectly.
What is the butterfly effect?
It is the idea that a tiny change at the start of a chaotic system can lead to a very large difference later. The name comes from weather science, where small changes in the air can change a forecast.
Related labs
- Chaos Theory: Two waterwheels start almost exactly the same. How long until they do something completely different?
- Simple Pendulum: Time one swing. What changes it: the string, the bob, or the push?
- Probability: Each bounce is a coin toss. Why do a hundred balls always pile up into the same shape?
See all physics labs or every Taz science lab.
Try it yourself
The Double Pendulum Twins lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.