What it shows
A balance compares two masses by checking that they pull equally on both sides of a pivot. You build one from a hanger and use it with a reference weight to measure the mass of kitchen items.
The science
A balance stays level when the turning effect, or moment, on each side is the same. If the cups hang at equal distances from the pivot, the masses must be equal for it to balance. Using known masses lets you read the unknown mass of what you put in the other cup.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A wire or plastic coat hanger
- Two identical paper cups or small bowls
- Strong string
- A hook or a nail to hang it on
- Coins or a 50 g weight as reference
- A marker, ruler and tape
- Sugar, salt and rice
Safety
For this project: Hang the balance from a safe place and keep the cup contents low. An adult must help with nails. Wire ends can be sharp, so tape them.
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
7 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.
Hang the coat hanger from a hook by its middle so that it can swing freely.
Tie a cup to each end of the hanger with equal lengths of string, and tape them in place.
Check that the hanger is level when both cups are empty, adjusting the tape if needed.
Put a 50 g reference weight in the left cup, or a known number of coins.
Add sugar to the right cup slowly until the hanger levels, and mark the amount in a measuring spoon.
Repeat with salt and rice, noting how many spoons each takes to balance 50 g.
Compare the volume of 50 g of each and rank them by density.
What you should see
The balance levels when the right cup holds about the same mass as the reference. Sugar, salt and rice take different volumes to reach 50 g, which shows differences in density.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1When does a balance stay level?
When the moments on both sides are equal.
2Why must the cups hang at equal distance?
So equal masses give equal moments.
3Which item took the smallest volume for 50 g?
The densest one, usually salt.
4What is mass?
The amount of matter in an object.
Take it further
Make a ruler seesaw balance and test the principle of moments with unequal distances.
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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