Pendulum Gravity — Measure g
Swing your phone on a string and, from the rhythm, it computes your local gravitational acceleration g — explained by your latitude and altitude.
Open this on your phone
This experiment measures the swing of your phone using its motion sensors — something most desktops don't have. Grab your phone and open this page there.
Motion access was blocked
Reload and tap Allow when asked for Motion & Orientation. On iOS you can re-enable it in Settings ▸ Safari ▸ Motion & Orientation Access.
Measure gravity with a pendulum
Tie your phone to a string, let it swing, and we'll time the swing to work out your local gravitational acceleration g — the same way physicists did 300 years ago, now in your pocket.
Needs a phone with motion sensors · 100% on-device
Couldn't read the sensors
Set your string length, hit Start swinging and let your phone oscillate.
Counting swings… keep it going.
Your swing trace will appear here
Longer windows = more swings counted = a steadier, more accurate reading.
Theoretical gravity for your spot
Earth isn't a perfect sphere and it spins — so gravity is a touch weaker at the equator and at altitude. We use the international WGS-84 / IGF-1980 formula to predict g for your latitude.
Tie a string firmly to your phone (a phone case lanyard or a bag works great). Use a longer string — 0.5 m or more makes the swing slow and easy to measure.
Measure the length L from the pivot point down to the middle of your phone, and type it in above. This is the single biggest factor in accuracy.
Hang it from a fixed point. Pull it to one side by a small angle (under ~15°) so the swing stays a clean pendulum.
Tap Start swinging, then let go. Keep your hand off the string while it records — we count complete back-and-forth swings.
When the window ends, we compute g = 4π²L ⁄ T² from the average period and compare it to the textbook value.
Honest accuracy note. Phone sensors, string stretch, air resistance and large swing angles all add error — expect to land within roughly ±2–5% of the true value (about 9.81 m/s²). Longer strings, smaller angles and longer recording windows tighten the result. This is a real, working physics demonstration, not a laboratory instrument.
🔒 Everything runs in your browser — no sensor data ever leaves your phone. Your steadiest reading so far: .
Why you’ll love Pendulum Gravity — Measure g
Instant & free
No signup, no paywall, no limits — Pendulum Gravity — Measure g works the moment the page loads.
Completely private
Everything runs in your browser. Nothing you enter is ever uploaded or stored on a server.
Works on any device
Fully responsive and touch-friendly — use it on your phone, tablet or desktop.
Uses your real device
Taps into your actual camera, mic or motion sensors — right in the browser.
How to use it
Open it
No download and no login — the tool is ready right at the top of this page.
Use it
Enter your details or start interacting. Everything updates live as you go.
Get your result
Copy, download or share your result in a single tap. That’s it.
Frequently asked questions
Pendulum Gravity — Measure g is 100% free — no signup, no watermarks and no usage limits. It’s one of 200+ free tools we build and give away.
No. All sensor data is processed live on your device and never leaves it or gets stored.
Completely. Pendulum Gravity — Measure g runs entirely in your browser (client-side). Nothing you type, upload or generate is sent to our servers, so your data never leaves your device.
No. It works instantly in any modern web browser — Chrome, Safari, Edge or Firefox — on desktop and mobile.
Yes. The tool is fully responsive and touch-friendly, so it works great on phones and tablets as well as computers.
Global Maple System — a software & AI studio. We built Pendulum Gravity — Measure g (and 200+ other free tools) to show what we can do. Need something custom built? We’d love to help.
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