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Pendulum Lab: What Changes the Swing Time?
Virtual Science Teachers
Pendulum Lab
What changes the swing time?
Set a pendulum's string length, bob mass, and release angle, then run a trial and time its swing. Record your data in a table and a chart, and discover the one thing that really controls how fast it swings. (The answer surprises most people!)
โฑ Pendulum Lab
Run a controlled experiment! First pick the one variable you want to test. The other two lock as your constants. Then change your test variable, click Run Trial, and watch the chart. Because only one thing can change at a time, the graph always tells the truth.
Trials0
Latest period-
StatusReady
โฑ 0.00 sswings: 0
15ยฐ
100 cm
Set your conditions and run a trial to time the swing.
๐ Period vs Length
Chart could not load. The table beside it still works.
๐ Data
Pick a variable to test.
#
Length cm
Period s
Run a trial to record data.
Periods use the simple-pendulum formula 2ฯโ(length รท gravity) with a real large-angle correction, plus a little measurement scatter and gentle air-resistance damping so the data feels realistic. โฑ
How to Use the Lab
This is a real experiment: you're trying to find out which variable controls a pendulum's period (the time for one full back-and-forth swing).
Steps
Pick what to test: use Variable to test to choose String length, Bob mass, or Release angle. That one stays adjustable; the other two become your constants.
Set your constants: choose values for the other two controls before you start. They lock in once you run your first trial.
Run a trial: click Run Trial. The pendulum swings, the lab times it, and the period is recorded in your table and on the chart.
Vary your test variable: change just that one control and run again. The chart plots period against it automatically, so the graph always matches what you changed.
Repeat to average (optional): run the same setting more than once. You'll see each reading in the table, and the chart plots their average, which smooths out the measurement scatter.
Switch experiments: change Variable to test any time. Each variable keeps its own saved trials, constants, and chart, so you can build all three experiments and switch between them. The picker shows a count next to any variable that has data.
Start over:Reset clears only the experiment you're viewing; the others stay saved.
Be a scientist
Changing only one variable while holding the others fixed is called a controlled experiment. This lab keeps you honest by locking the other two for you.
The note under the pendulum reads your trials and tells you what your data shows.
Learn the Science
A pendulum's period is the time it takes to swing over and back once. You might guess that a heavier bob or a bigger push changes it, but here's what actually happens:
๐
Length: this is the one that matters. A longer string makes a slower swing (a longer period). But it's not a straight line: to double the period you need four times the length, because period grows with the square root of length.
โ๏ธ
Mass: no effect (surprising!). A heavy bob and a light bob on the same string swing in exactly the same time. Gravity speeds up all masses equally, so the extra weight doesn't slow the timing.
๐
Release angle: almost no effect. For small swings the period barely changes, because a wider swing just makes the bob move faster to cover the extra distance. Push to a big angle like 45ยฐ and the period grows by only a few percent, a real effect but a small one.
๐งฎ The rule: period = 2 ร ฯ ร โ(length รท gravity). Mass doesn't appear at all, and the release angle only shows up as a small correction at wide swings, so length (with gravity) is what really sets the period.
๐ฌ Try a controlled experiment
Set Variable to test to Length, then run a trial at each length from 25 to 200 cm. The chart plots period against length, and you'll see a gentle curve. Then switch the test variable to Mass (your length data stays saved) and run trials at different masses. This time the line is flat. That flat line is the discovery.
โฑ Why real data is a little messy
Notice that repeated trials don't give exactly the same number; they wobble by about a percent. That is measurement scatter from imperfect timing, just like real lab data, which is why scientists repeat a measurement and average. If you run the same setting more than once here, the chart automatically plots the average of those readings.
Air resistance also nibbles at the swing: watch the pendulum's arc slowly shrink as it swings. Friction mainly shrinks the size of the swing, not its timing, so the period stays nearly the same even as the swing gets smaller.
Note: this lab includes the real large-angle correction, so very wide swings give a slightly longer period. Across the 5ยฐ to 45ยฐ range offered here, that effect stays small (under about 4%).