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.
- 📱 Device Tool
- ✓ Free
- ✓ No sign-up
- ✓ Private — runs in your browser
- ✓ Any device
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
How to use it
- 01
Open it
No download and no login — the tool is ready right at the top of this page.
- 02
Use it
Enter your details or start interacting. Everything updates live as you go.
- 03
Get your result
Copy, download or share your result in a single tap. That’s it.
About Pendulum Gravity — Measure g
Pendulum Gravity lets you measure your local gravitational acceleration using nothing but your phone and a string. Swing your phone like a pendulum, and the tool calculates g from the oscillation frequency. It's free, runs entirely in your browser, and needs no signup or special permissions—just the built-in motion sensors on your device. The result shows you how gravity actually varies across Earth due to latitude and altitude.
Gravitational acceleration isn't uniform. It's stronger near the poles and weaker near the equator, and it decreases with altitude. A simple pendulum experiment reveals these differences. By timing how fast your phone swings, you're measuring the same gravitational field that keeps us grounded. This local g calculator turns your smartphone into a working gravimeter, letting you see real physics in action without leaving your device.
Use this pendulum experiment for education, curiosity, or verification. High school and college students learn how period and length relate to g; geographers see how latitude shapes gravity; engineers understand why precision matters at different locations. Your data stays on your phone—nothing is uploaded or tracked. It's a hands-on way to measure gravity and understand why your weight (and physics) changes around the world.
Popular uses
- ✦ High school physics students demonstrating the relationship between pendulum period and gravitational acceleration
- ✦ Geography classes showing how gravity varies with latitude and altitude across different locations
- ✦ Educators explaining why objects weigh slightly different amounts in different parts of the world
- ✦ Physics enthusiasts measuring and comparing local g values at home, at altitude, or while traveling
- ✦ STEM outreach events using a phone-based gravimeter to engage students with hands-on measurement
Frequently asked questions
How do you measure gravity with a pendulum?
Why does gravity change between locations?
What's a normal value for gravitational acceleration?
How accurate is measuring gravity with a phone?
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