Child watching a swinging washer pendulum

Pendulum Experiments for Kids: 5 Tests to Run at Home

You can run a real pendulum experiment with a piece of string, a few washers and a phone timer. Tie the washers to the string, hang it from a doorframe or a table edge, and change one thing at a time: the weight, the string length or the starting angle. The surprise is what does not matter. Weight has no effect on the swing time, while length changes it a lot.

This guide walks through the setup, five tests in the order we recommend, how to time swings so your numbers are trustworthy, and how to turn the whole thing into a science fair project.

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TL;DR

  • A simple pendulum needs only string, a small weight, tape and a timer. Setup takes about ten minutes.
  • String length changes the swing time. The weight of the bob does not, which surprises most kids (and plenty of adults).
  • Time 10 full swings, divide by 10, and repeat the run at least four or five times. One swing timed by hand is mostly reaction-time error.
  • Keep the starting angle small, under about 20 degrees, or the angle itself starts to change the result.
  • The same experiment scales from a 2nd grader’s “which swings faster?” to a 7th grader’s calculation of gravity.

Table of Contents

What a pendulum is and the words your child needs

A pendulum is any weight hanging from a fixed point that swings back and forth under gravity. A playground swing is one. So is a wrecking ball, a grandfather clock’s weight and a key dangling from a string.

Four terms cover almost everything you will measure:

  • Bob: the weight at the end of the string.
  • Length: the distance from the pivot (where the string is fixed) to the center of the bob, not to the top of it.
  • Amplitude: how far you pull the bob back before letting go, usually described as an angle.
  • Period: the time for one complete cycle, out and back to where you started. One “swing” in everyday speech is half a period, so agree on what you are counting before you start.

The period of a simple pendulum depends mainly on its length and on the local strength of gravity. The textbook formula is T = 2π√(L/g), and the same university lab report that tests it also finds that bob mass has no measurable effect on the period.

What you need

Everything here is a household item or close to it:

  • About 1.5 meters (5 feet) of lightweight string or thread. Strong sewing thread or kitchen twine works.
  • Washers, coins, nuts or small bags of sand or rice for bobs. Aim for at least three different weights.
  • Tape (painter’s tape is easiest to remove).
  • A ruler or tape measure.
  • A protractor, or a printed paper protractor taped to the wall behind the pendulum.
  • A timer. A phone stopwatch is fine.
  • A pencil and paper, or a spreadsheet, for a data table.
  • A fixed spot to hang it: a doorframe, the underside of a table, a broom handle resting between two chairs.

This is the same family of materials the Exploratorium’s pendulum activity uses for K to 5 students, who test one variable at a time and watch what happens to swing speed.

Build your pendulum in five steps

  1. Make the pivot. Tape one end of the string firmly to the support. Better still, tape it over the top and then down again, so the string cannot slide as it swings. A pivot that moves ruins the data.
  2. Attach the bob. Thread washers on and knot the string through them, or tape a coin on. Keep the knot small so the weight sits at the very end.
  3. Set the length. Measure from the pivot point to the middle of the bob. Write it down in centimeters.
  4. Mark the rest position. Let it hang still and put a piece of tape on the floor or wall directly behind the bob. This is your counting line.
  5. Practice the release. Hold the bob at the angle you want, let go without pushing, and watch it swing in a flat line, not a circle. A push or a twist is the number one cause of messy results.

A good first run is a 50 cm string and three washers. Release it from about 15 degrees and count how many times it crosses the line in 10 seconds. You now have a feel for the speed before you start measuring properly.

How to time a pendulum so the numbers mean something

This is where most home experiments quietly fail. A human reaction time is a couple of tenths of a second, and a one-meter pendulum’s whole period is only about two seconds. Time a single swing and your error could be ten percent or more.

The fix is simple and comes straight from the lab report cited above:

  • Time many cycles. Start the timer when the bob passes the counting line, count 10 complete cycles, stop the timer, and divide by 10. Reaction error is spread across all ten swings instead of hitting one.
  • Repeat the run. Do each setup at least four or five times and average the results. Throw out nothing unless you know why a run went wrong (the string slipped, the bob twisted).
  • Start and stop at the same point. Choose the moment the bob passes the counting line, because it moves fastest there, or choose the turnaround point where it pauses. Pick one and stick with it.
  • Let a partner be the timer. One person releases, the other times. Swap roles halfway so any bias evens out.

Write results in a table like this:

Trial Time for 10 cycles (s) Period (s) = time ÷ 10
1
2
3
4
5
Average

Test 1: Does weight change the swing time?

Most kids predict that a heavier bob swings faster, the way a heavier ball seems to fall harder. Ask for the prediction first and write it down. It makes the result much more memorable.

Setup: Fix the string at 50 cm and the release angle at 15 degrees. Test one washer, then three, then six.

What to expect: The periods will be nearly identical. Small differences come from timing error, or from the string’s own weight, which matters more when the bob is very light.

This is the most useful “lightbulb moment” in the whole project, because it shows that a result you were sure about can be tested and overturned. The physics reason: gravity pulls harder on a heavier bob, but a heavier bob also takes more force to move, and the two effects cancel exactly. That is the same idea behind why a hammer and a feather fall together in a vacuum, which we cover in our guide to gravity experiments for kids.

Test 2: How does string length change the period?

This is the test that actually produces a pattern.

Setup: Use the same bob (three washers) and the same 15 degree release. Test lengths of 25 cm, 50 cm, 75 cm and 100 cm. Keep the table going and do five runs of 10 cycles for each length.

What to expect: Longer strings give longer periods, but not in a straight line. Doubling the length does not double the time. Using T = 2π√(L/g) with g of about 9.8 m/s², the predicted periods are:

Length Predicted period
0.25 m about 1.0 s
0.50 m about 1.4 s
1.00 m about 2.0 s

Notice that to double the period you have to make the string four times as long. A quarter-meter pendulum takes about one second per cycle, and a one-meter pendulum takes about two. Older kids can plot period against length and see a curve, then plot period squared against length and see a straight line.

Your measured values will usually come out a little higher than the formula. That is normal. Real strings stretch, the bob has size, and the pivot is never perfect.

Test 2: How does string length change the period? — overview diagram

Test 3: Does the starting angle matter?

The tidy answer in textbooks is “no”, and for small angles that is nearly right. The more honest answer is: it depends how far you pull back.

The independence of period from amplitude holds only under the small-angle approximation, typically below about 20 degrees. At larger angles the period noticeably increases with amplitude. Students often blame air resistance or sloppy timing for this, but it is a built-in property of the physics.

Setup: Use 50 cm and three washers. Release from 10, 20, 40 and 60 degrees. Mark each angle on a protractor behind the string.

What to expect: 10 and 20 degrees will look the same within your timing error. At 60 degrees the period should come out a little longer. Whether you can measure the gap depends on how carefully you time, so this test is a good check on your technique.

Test 4: Find the length of a one-second pendulum

This makes a good challenge for ages 9 and up. Use your length data to predict the string length that gives a period of exactly one second, then build it and test.

From the formula, a one-second period needs a length of roughly 25 cm. Give kids the data table from Test 2 and ask them to draw a smooth curve through the points, then read off the length for a 1 second period before building. A prediction followed by a test is the core of the scientific method, and it shows kids that graphs are tools, not decoration.

Test 5: Calculate gravity with a pendulum

Older students can turn the formula around. Rearranged, g = 4π²L ÷ T². Measure a long pendulum (a meter or more gives smaller percentage errors), time 10 cycles five times, average the period and plug in the numbers.

A result between 9.5 and 10 m/s² from a kitchen setup is a very good outcome. Larger gaps usually point to one of three problems: measuring length to the top of the bob instead of its center, timing too few cycles, or a pivot that shifts. Finding which one is worth as much as getting the “right” answer. For more force and motion activities in this vein, see our force and motion experiments.

Three common pendulum measurement errors

Which tests fit which age?

Pendulums are unusually flexible. The Exploratorium describes the activity for K to 5 learners, and the same apparatus supports quantitative projects with older students. Here is how we would pace it:

Age Best tests Goal
6 to 7 Test 1, a simplified Test 2 (short vs. long) Compare and predict. Count swings in 10 seconds.
8 to 9 Tests 1 to 3 Change one variable, record a table, average repeats.
10 to 12 Tests 2 to 5 Graph results, test a prediction, calculate g.

If you teach third or fifth grade, our lists of 3rd grade science experiments and 5th grade science experiments show how to fold this activity into standards-aligned lessons.

Common problems and quick fixes

  • The bob moves in an oval. You pushed it or released it off-axis. Hold it still for a second before releasing, and release from the side, not from above.
  • Results are all over the place. Almost always too few cycles per run. Move from 5 to 10, and repeat more times.
  • The period slowly drifts longer or shorter. The string is stretching or the tape is slipping. Re-tape the pivot and use thread that does not stretch.
  • Air resistance is blamed for everything. For dense bobs like washers, drag is small. If a very light bob (a cotton ball, say) behaves differently, that is a good side experiment, not an error.
  • The swing dies out before 10 cycles. Small bobs lose energy faster. Use a heavier bob for long runs, then notice that you are changing mass without changing the period.

Turning it into a science fair project

Judges reward a clear question, a controlled test and honest data. A pendulum does all three naturally. Science Buddies’ pendulum project is built around investigating the factors that affect the speed and duration of a pendulum’s swing, and it is a good model for structure.

A strong project looks like this:

  1. Question: “How does string length affect the period of a pendulum?” One variable, clearly stated.
  2. Hypothesis: A prediction with a reason, such as “longer strings will take longer, but not twice as long when the length doubles.”
  3. Variables: Independent (length), dependent (period), controlled (bob mass, release angle, pivot, number of cycles timed).
  4. Data: At least five lengths, five trials each, averages and a graph.
  5. Conclusion: Did the data support the hypothesis? Where did error creep in? What would you do differently?

Ideas to make a project stand out: compare a measured period against the formula and explain the gap, test whether a pendulum on a different surface or at a different height changes anything (it should not, in an ordinary house), or build a pendulum clock that beats once per second. You can try a virtual version first with the PhET Pendulum Lab simulation, which lets you change length, mass and gravity, including on other planets, in seconds.

Where real pendulums do real work

This is the part that makes kids look at buildings differently. Pendulums are used in clocks for timekeeping and in seismographs to detect earthquake tremors. Taipei 101 hangs a 726-ton pendulum to counteract wind sway and earthquake vibration. The building’s own motion sets the pendulum swinging, and the pendulum’s swing pushes back against it.

The playground swing is the same idea with a child on it. Small pushes timed to the swing’s natural rhythm build up into big motion, which is resonance. Researchers even use resonance to explain a gap in Saturn’s rings linked to the moon Mimas. Ask your child to pump a swing at the wrong moment and then the right one, and you have a resonance lesson for free.

A note on “magic” pendulums

Sooner or later a curious child will meet the crystal-on-a-chain kind of pendulum, used in dowsing or to “answer” yes-or-no questions. It makes a useful critical-thinking conversation. Those movements come from the ideomotor effect: tiny, unconscious muscle movements triggered by expectation. A paper in Neuroscience of Consciousness studied this effect and found pendulum accuracy in decision tasks was low, around 53 percent in the cited results.

Try it safely as a test: have your child hold a pendulum over a question with a known answer, then repeat with their eyes closed and a friend who does not know the answer asking. A physical pendulum measures gravity reliably. A hand-held one mostly measures the hand.

Our perspective: why we start with pendulums

We write comics for kids who say they do not like science, and the pendulum is the experiment we reach for first. It is cheap, it fits in any room, and the first result goes against what most kids expect. That moment, “wait, the heavy one is not faster?”, is worth more than any definition we could print.

We also think the timing lesson matters as much as the physics. A child who learns to time ten swings instead of one has learned that careful measurement beats a confident guess. That habit carries into chemistry, cooking and everything else.

Our approach in Gravity Gang and Fizz Force is the same: tell the story first so the question feels like it belongs to the characters, then hand the child a task that uses ordinary things from the kitchen drawer, with no screen required. The experiment answers a question the reader already wants answered.

— Brainie Comics

Keep the curiosity going

If your child enjoyed watching a washer prove a grown-up wrong, our comic book guide to physics, Gravity Gang, is built for kids aged 7 to 12. It pairs adventure stories with experiments that use household items, plus quizzes and a completion certificate. It is $59.95, and our 30-day money-back guarantee applies. For chemistry, there is Fizz Force, also $59.95. You can see both on our collection page. For more free ideas, try our nine fun physics experiments and kitchen energy experiments.

FAQ

What are some simple pendulum experiments?

The three classics each change one thing: bob weight, string length and release angle. Weight should make no difference, length should make a clear one, and angle should only matter once it is larger than about 20 degrees. Add a fourth, building a pendulum with a one-second period, once those work.

Why does the weight of the bob not change the period?

A heavier bob feels a bigger pull from gravity, but it also has more inertia, so it resists acceleration by the same factor. The two cancel, leaving length and gravity as the factors that set the period, as in this lab report. Test it yourself with one washer and six.

How many swings should I time?

Time 10 full cycles and divide by 10, then repeat the run four or five times and average. This spreads your reaction-time error over many swings, which is far more accurate than timing one swing.

What are some real-life examples of pendulums?

Pendulum clocks, playground swings and seismographs are common ones. Skyscrapers such as Taipei 101 use a giant pendulum damper to reduce swaying. A wrecking ball and a tire swing are also pendulums.

Can a pendulum predict the future?

No. Hand-held pendulums used in dowsing move because of the ideomotor effect, which is unconscious muscle motion. The published research found their accuracy on decision tasks to be low.

Sources

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