What it shows
A seesaw balances when the turning effect (moment) on one side equals that on the other. A lighter child must sit farther from the pivot to balance a heavier child.
The science
A moment equals force multiplied by the perpendicular distance from the pivot. At balance, the weight on the left times its distance equals the weight on the right times its distance. So a 4 kg load at 0.5 m balances a 2 kg load at 1.0 m, since both give a moment of 2 units.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A wooden plank 1.5 m long
- A brick or a round log as the pivot
- Two bags of sand of 2 kg and 4 kg, to stand for the children
- A metre ruler
- A notebook
- A kitchen scale to check weights
Safety
For this project: Use sandbags and not real children for the measurements, and place the model on level ground with an adult to watch. Keep fingers clear of the plank pivot.
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.
Set the plank on the pivot at its centre so it balances when empty.
Mark every 10 cm on the plank starting from the centre.
Place the 4 kg sandbag at the 40 cm mark on the left side.
Slide the 2 kg bag along the right side until the plank is level and note the distance.
Repeat with the heavy bag at 30 cm and 50 cm and record each balanced position.
Calculate weight times distance on both sides each time.
Check that the two products are almost equal and write the percentage difference.
Draw a graph of the distance of the light bag against the distance of the heavy bag.
What you should see
For each position of the heavy bag, the light bag balances at twice the distance, and the moments on both sides match within a few 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 is a moment?
Force multiplied by its perpendicular distance from the pivot.
2What is the condition for balance?
Clockwise moment equals anticlockwise moment.
3Why does the lighter child sit farther?
A longer distance gives a bigger moment for a smaller force.
4Name another lever that works this way.
A crowbar or a pair of scissors.
Take it further
Use three bags to balance on one side and find the combined moment.
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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