What it shows
A beam stays balanced when the clockwise moment equals the anticlockwise moment. The board lets students test the rule with known weights.
The science
The moment of a force about a point is the force times the perpendicular distance from that point. A beam is in balance when the total clockwise moment equals the total anticlockwise moment. So a small weight far from the pivot can balance a larger weight near it. This is how see-saws, spanners and a crowbar work.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 30 cm ruler or a wooden scale
- A thread to hang it
- Paper clips as hooks
- Small identical coins or nuts as weights
- A stand made from a cardboard box
- Pencil and notebook
Safety
For this project: Keep small coins and nuts away from small children. An adult should help set up the stand.
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.
Tie the thread at the 15 cm mark of the ruler so it balances level.
Hang the thread from a stand so the ruler can swing freely.
Hang 4 coins at 5 cm left of the pivot with a paper clip hook.
Add coins on the right side at a distance until the ruler is level again.
Record the weights and distances for each side.
Multiply weight by distance for both sides and compare.
Repeat with five different arrangements.
Plot a graph of weight against distance for balance and see the curve.
What you should see
The two moments are equal within a small error in every trial.
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 of a force?
Force times perpendicular distance from the pivot.
2When is a beam balanced?
When clockwise and anticlockwise moments are equal.
3Why can a light child balance a heavy one on a seesaw?
By sitting farther from the pivot.
4Name a tool that uses moments.
A spanner or a crowbar.
Take it further
Use unequal arms and find the mass of an unknown object using the law.
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
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