What it shows
The time for one swing of a pendulum depends on its length, not on how heavy the bob is or how wide it swings (for small swings). A longer pendulum swings more slowly. Pendulum clocks are built on this rule.
The science
A pendulum swings because gravity pulls the bob back towards the lowest point every time it moves away, and its motion carries it through to the other side. A longer thread gives the bob a longer, gentler path, so each swing takes more time. A heavier bob is pulled harder by gravity but is also harder to get moving, and the two effects cancel, so the weight makes no difference. For small swings the width of the swing makes no difference either.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- About 1.5 metres of strong thread
- A heavy nut, a stone or a bunch of keys as the bob
- A second, heavier bob such as two nuts
- A ruler or measuring tape
- A stopwatch or a phone timer
- A table edge or a doorframe to hang from, and tape
Safety
For this project: Hang the bob where it cannot hit anyone, and keep small nuts away from young children.
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
9 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.
Tie the nut to the thread and tape the other end to the table edge so the bob hangs 1 metre below the fixing point; measure to the middle of the bob.
Pull the bob about 10 cm to one side, let go without pushing, and start the stopwatch at the same moment.
Count ten complete swings, there and back, and stop the watch; divide the time by ten to get the time for one swing.
Repeat the timing twice more and take the average, which cuts down mistakes in starting the watch.
Shorten the thread to 75 cm, then 50 cm, then 25 cm, timing ten swings at each length.
Go back to 1 metre and swap in the heavier bob, keeping the length the same; time ten swings again.
At 1 metre, pull the bob 5 cm aside and then 15 cm aside and time ten swings each time.
Make a table and a graph of swing time against length, and note that weight and small changes in swing width did not change the time.
Find the length that gives exactly one second per swing, there and back takes two seconds, which is close to 25 cm, and label it the seconds pendulum.
What you should see
Swing time rises as the length rises: roughly 2 seconds for 1 metre and 1 second for 25 cm. Changing the bob weight or the small swing width leaves the time almost unchanged.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What changes the time of a pendulum swing?
Only its length (and gravity); a longer pendulum swings more slowly.
2Does a heavier bob swing faster?
No, the weight makes no difference to the swing time.
3Why time ten swings instead of one?
Any mistake in starting and stopping the watch is shared over ten swings, so the answer is more accurate.
4Where is a pendulum used?
In old wall clocks, where its steady swing keeps the time.
Take it further
Make the length four times longer and show that the swing time only doubles, which is the square-root rule.
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.
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