The PhET Pendulum Lab is one of the cleanest ways to teach controlled experiments in physics. Students can change one variable at a time, time the swing with built-in tools, and discover that a pendulum’s period depends on its length and on gravity, but not (for small swings) on its mass. No string, stopwatch drama or broken clamps required.
This page gives you a complete simple pendulum lab built around the free PhET simulation from the University of Colorado Boulder: learning goals, a step-by-step procedure, a data table, a graphing extension and a “mystery planet” challenge. It works for a 50-minute class, a sub day, or a review before a unit test. If you are planning a full unit, see our other classroom simulations and lab guides such as the Energy Skate Park lab.
In this guide9 sections
What students should learn
- Identify independent, dependent and controlled variables.
- Show that the period of a simple pendulum increases with length.
- Show that mass has little or no effect on the period.
- Show that small changes in amplitude barely change the period, and large amplitudes increase it slightly.
- Use the relationship T = 2π√(L/g) to estimate gravitational acceleration.
Getting to know the simulation
Open Pendulum Lab on the PhET website (it runs in a browser on Chromebooks and tablets). The simulation has several screens. The Intro screen lets students adjust length, mass, gravity and friction and use a ruler and stopwatch. The Energy screen adds an energy bar graph showing kinetic, potential and thermal energy. The Lab screen adds tools such as a period timer and a “Planet X” gravity option for inquiry challenges. Give students two minutes to explore freely before the procedure; they will ask better questions.
Teacher tip: measure ten swings
Even in a simulation, reaction time affects stopwatch measurements. Have students time 10 complete oscillations and divide by 10, or use the period timer on the Lab screen. Either way, set friction to zero for the main experiments so the swing does not die out.
Procedure
Part 1: Does mass matter?
- Set gravity to Earth, friction to none and length to a fixed value (for example 0.70 m).
- Pull the pendulum to a small angle (about 10°) and release.
- Measure the period with the lightest mass, then repeat with two heavier masses.
- Record results and write a claim: does mass change the period?
Part 2: Does amplitude matter?
- Keep length and mass constant.
- Measure the period at 10°, 20°, 40° and 60°.
- Compare. Students should see almost no change at small angles and a slightly longer period at large angles. This is a nice moment to explain why the formula is called the “small-angle approximation.”
Part 3: Does length matter?
- Keep mass and amplitude (10°) constant.
- Measure the period for at least five lengths, such as 0.20, 0.40, 0.60, 0.80 and 1.00 m.
- Record each period in the data table below.
Data table and graphing
Students fill in the measured period, then calculate T² for each length. A graph of T versus L curves; a graph of T² versus L is a straight line through the origin. That linearization is the most valuable skill in the lab.
| Length L (m) | Time for 10 swings (s) | Period T (s) | T² (s²) |
|---|---|---|---|
| 0.20 | |||
| 0.40 | |||
| 0.60 | |||
| 0.80 | |||
| 1.00 |
For reference, on Earth (g ≈ 9.8 m/s²) a 1.00 m pendulum should have a period close to 2.0 s, and a 0.25 m pendulum close to 1.0 s. If student values are far off, check that they counted full back-and-forth swings.
Finding g from the slope
Squaring the formula gives T² = (4π²/g) · L. So the slope of the T² vs. L graph equals 4π²/g, and g = 4π² ÷ slope. Students using Google Sheets or Desmos can add a linear trendline and read the slope directly. A class result within a few percent of 9.8 m/s² is a strong lab.
Challenge: the Planet X mystery
Switch gravity to Planet X (or a custom value hidden from students). Without being told g, groups must design a mini-experiment, collect data and calculate the planet’s gravitational acceleration. Compare answers across groups and discuss sources of error. This works well as an assessment because it shows whether students understood the method, not just the numbers.
Extension: energy screen
On the Energy screen, have students pause the pendulum at the top, the bottom and halfway, and sketch the energy bars each time. Then turn friction on and ask where the “missing” energy goes. It connects neatly to thermal energy and conservation of energy.
Discussion questions
- Why did we time 10 swings instead of one?
- A grandfather clock runs slow. Should the pendulum be lengthened or shortened?
- Would a pendulum clock keep the same time on the Moon? Why not?
- Which variable did you control in Part 3, and why does that matter?
Use one of these as an exit ticket to check understanding before the next lesson.
Frequently asked questions
Is the PhET Pendulum Lab free?
Yes. PhET Interactive Simulations from the University of Colorado Boulder are free to use online, and the HTML5 version runs in most modern browsers, including on Chromebooks.
Why doesn’t mass affect the period of a pendulum?
A heavier bob feels a larger gravitational force, but it also has more inertia. The two effects cancel, so the acceleration along the arc, and therefore the period, stays the same for an ideal pendulum.
What grade level is this lab for?
Parts 1–3 suit middle school physical science and introductory high school physics. The T² vs. L graph and calculating g fit high school physics, including algebra-based AP Physics 1.
How long does the lab take?
About 45–60 minutes for Parts 1–3 plus the data table. Add 20 minutes for the graphing and Planet X challenge, or assign them as homework.
Wrap-up
The PhET Pendulum Lab turns a classic experiment into a fast, low-mess investigation that teaches variables, linear graphs and real physics. Run Parts 1–3 to build the core idea, then let students prove what they learned with the Planet X challenge. Pair it with our PhET Projectile Motion lab for a strong motion unit.
