Class 6 · Science and environment
What changes a pendulum's swing?
A swing in the park goes back and forth. What decides how long each swing takes: the string, the weight, or something else?
What you will learn: Test one variable at a time and find what affects the time of a swing.
Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓
Jump to: Check yourself · For grown-ups · For teachers
Safety first
Small weight, gentle swings, away from faces.
Keep it safe:
- Tape the string firmly to a heavy table so it cannot fall.
- Swing gently, away from faces and windows.
- Use only a small weight, such as coins.
You will need
- StringAbout 1 m.
- A small weightCoins taped together, or a small bag of coins.
- Tape and a table edgeTo hang it from.
- A ruler
- A clock with seconds
Time: About 45 minutes.
The story
In the park, the twins swing side by side. Tanmay is heavier, yet their swings go back and forth at almost the same pace. "Weight should make a difference!" he says. Dhruv ties a coin to a string to test it at home. What really decides how long one swing takes?
The idea
A pendulum is a weight hanging on a string that swings back and forth. One swing is there and back.
Test one variable at a time: first change the string length with the same weight, then change the weight with the same length.
Timing one swing is hard because it is quick. Time 10 swings and divide by 10: the small timing mistake is shared over 10 swings.
Keep the swings small (start it only a little to one side) and the same for every test.
Worked example
Kavya's pendulum tests
- Length test (1 coin each time): 25 cm took 10 s for 10 swings; 50 cm took 14 s; 100 cm took 20 s.
- Weight test (50 cm each time): 1 coin took 14 s; 3 coins also took 14 s.
- One swing at 50 cm: 14 ÷ 10 = 1.4 s.
In her tests, a longer string made each swing take longer. Changing the weight made no difference she could measure.
Try it
Hang the pendulum.
Tape the string to the table edge so the weight hangs freely.
Measure the length from the tape to the middle of the weight.
Start it the same way.
Pull the weight a little to one side (about a hand's width) and let go. Do not push.
Time 10 swings.
Count 10 full swings (there and back) and stop the clock.
Divide by 10 for one swing.
Change only the length.
Test 25 cm, 50 cm and 100 cm with the same weight.
Change only the weight.
Test 1 coin and 3 coins with the same length.
Conclude.
Which variable changed the time? Which did not?
What you make (paper or offline)
Your pendulum results
A table for the length test and one for the weight test.
One swing worked out from 10 swings.
A conclusion about each variable.
Make it on paper. Keep it with you. Nothing to upload.
Check yourself
Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.
How do you want to answer? You choose, and you can change at any time.
See the answer and why
- A. 2 seconds Correct. 20 ÷ 10 = 2 seconds.
- B. 200 seconds Not this one. That multiplies. Divide the time for 10 swings by 10.
- C. 10 seconds Not this one. 10 is the number of swings, not the time for one.
- D. 20 seconds Not this one. 20 seconds is for all 10 swings.
See the answer and why
- A. A small timing mistake is shared over 10 swings, so the answer is more accurate Correct. Starting and stopping the clock late matters less over a longer time.
- B. Because 10 is the length of the string Not this one. The number of swings timed is a choice, not the string length.
- C. So the string has time to stretch Not this one. The string should stay the same length. Timing many swings is about accuracy.
- D. Because a pendulum only swings 10 times Not this one. It can swing many more times than 10.
See the answer and why
- A. Each swing got quicker Not this one. The times went up, not down, as the string got longer.
- B. Each swing took longer Correct. 25 cm took 1.0 s per swing, 50 cm 1.4 s and 100 cm 2.0 s.
- C. The pendulum stopped swinging Not this one. It kept swinging; each swing just took longer.
- D. Nothing changed Not this one. The times changed from 1.0 s to 2.0 s.
See the answer and why
- A. The length and the starting pull Correct. Keep the string length and how far you pull it the same. Then only the weight changes, so the test is fair.
- B. Only the colour of the string Not this one. Colour does not matter. Length and starting pull do.
- C. Nothing: change everything at once Not this one. Changing several things means you cannot tell which one mattered.
- D. The weight Not this one. The weight is what you are testing, so it must change.
See the answer and why
- A. The test failed because the times were the same Not this one. The same result is a real result. It shows weight did not matter here.
- B. Heavier weights swing faster Not this one. Both took 14 s, so the heavier one was not faster.
- C. Heavier weights swing slower Not this one. Both took 14 s, so the heavier one was not slower.
- D. In her test, the weight made no difference she could measure Correct. The times were the same, so changing the weight did not change the time.
Think back
Which result surprised you?
What could make your timing more accurate?
Investigate further
- Predict the time for one swing with a 200 cm string, then test it with a grown-up.
- Does a bigger starting pull change the time? Keep it fair.
For grown-upsTap a heading to open it. The safety notes are at the top of the page.
Before you start
- A heavy table, string, tape, coins and a clock with seconds.
Questions that help
- "Which one thing are you changing now?"
- "Why divide by 10?"
What's really going on
For small swings, the time of a swing depends on the length, not the weight: a longer pendulum swings more slowly. This is how pendulum clocks keep time.
Very big swings take slightly longer, which is why the starting pull is kept small and the same.
Other ways to do it
- A key on a shoelace works as a pendulum.
For teachersLearning aims, classroom ideas and what to look for.
Learning aims
- Test one variable at a time.
- Improve accuracy by timing multiple swings.
Subject links
- Science: forces, motion and time.
- Mathematics: division and decimals.
These links are general. They have not been checked against a specific board syllabus.
In the classroom
- Half the class tests length and half tests weight; share and combine results.
What to look for
- Changes only one variable per test.
- Times 10 swings and divides correctly.