What it shows
The time of a swing depends on its length and not on how heavy the rider is. A small model pendulum and a real swing both give a period close to 2 times pi times the square root of length over g.
The science
A pendulum’s period T equals 2 pi times the square root of L divided by g, where L is the length from the pivot to the centre of mass. The mass cancels out of the equation because a heavier mass is pulled more by gravity but also needs a bigger force to accelerate. Only the length and the strength of gravity matter, for small swings.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A playground swing with two chains
- A stopwatch
- A measuring tape
- Sand-filled bags of 1 kg, 2 kg and 3 kg
- A helper or two
- A notebook and calculator
Safety
For this project: Use a safe swing, in good repair, with an adult in charge, and swing only small angles with a seated rider. Keep everyone else far from the swing’s path.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure from the swing bar to the seat plus a little extra for the rider’s centre of mass to get the length L.
Place the 1 kg bag on the seat and pull the swing back by a small angle, about 10 degrees, and release.
Time 10 full swings and divide by 10 to get the period.
Repeat three times and average.
Repeat with 2 kg and 3 kg bags, using the same starting angle.
Calculate the predicted period from the formula and compare.
Plot period against mass and see whether the line is flat.
Discuss why swings seem to slow when a heavier rider is on them, such as increased friction.
What you should see
The period is nearly the same for all masses, about 2 to 3 seconds, and agrees with the predicted time within about 5 per cent.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What affects a pendulum’s period?
Its length and the gravity, and not the mass.
2What is the formula?
T equals 2 pi times the square root of L over g.
3Why use a small angle?
The formula is accurate only for small angles.
4How can you pump a swing higher?
By moving your body in time with the swing so that energy is added.
Take it further
Compare standing and sitting riders and explain the change in the effective length.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
Science projects
Science Projects for Class 9 and 10
Models and experiments for high-school physics, chemistry and biology
Find out more about physics
Free, well-known places to explore the subject further. Ask a grown-up before you sign up for anything or type your name anywhere.