How strong is gravity?
Make a pendulum. Count its swings. Use your measurements to discover how strongly Earth pulls.
No app to install · Works offline after openingWhat if the string is longer?
Imagine you double the length of the string but keep everything else the same. Will one full back-and-forth swing take…
Measure your pendulum
Use a ruler or measuring tape. Try 50 cm first; later repeat at 100 cm. Enter the real measurement, not your guess.
Time the swings
- While holding your phone, tap Check sensor and gently tilt it. Allow Motion access if asked.
- Put the phone in a snug pouch, attach it securely, and measure pivot-to-phone centre distance.
- Tap Start countdown. You have 8 seconds to get into position. At the long beep, release from a small angle (under 15°) without pushing.
- Let it swing in one plane. Keep the screen awake. At the ending beeps, gently catch it.
Works with motion sensors on supported phones. No data is sent anywhere.
Safer choice: leave your phone on a table. Hang an eraser or small weight from a string on a fixed support. Ask a friend to operate the stopwatch.
- Measure the length from the pivot to the centre of your hanging object.
- Pull it less than 15° to one side; release without a push.
- Tap Start as the bob passes an extreme position. Count 10 complete back-and-forth swings. Stop when it returns to that same side for the 10th time.
One full oscillation: left → right → left. Not just left → right!
You measured gravity!
Where does the number come from?
A longer pendulum takes more time. The relationship is T = 2π √(L / g). Rearrange it to calculate g = 4π² L / T². Here L is in metres and T in seconds.
This assumes a short swing, a nearly point-like bob and a fixed pivot. Air resistance, sideways movement, loose attachments, length errors and stopwatch reaction time affect the result.
Longer swing. Same Earth.
Repeat the experiment at different lengths. Compare your actual readings with the green reference line for gravity on Earth. A pendulum's time period changes with length, but g should stay nearly the same.
How long is one swing?
Does gravity change?
Curves are predictions from the model, not measurements. Add a reading to see your own results as points.
| Trial | Length | Period | g | |
|---|---|---|---|---|
| No measurements yet. Your experiments will appear here. | ||||
Think like a scientist. When you make the string 4 times as long, the time for one swing is about…