What it shows
The digits of a number stay in order while the decimal point slides. Moving the point one place to the right multiplies by 10 (3.45 becomes 34.5) and one place to the left divides by 10 (3.45 becomes 0.345).
The science
Decimal places continue the place value pattern to the right of the ones: tenths are ten times smaller than ones and hundredths ten times smaller than tenths. Multiplying by 10 makes every digit worth ten times more, which is the same as each digit moving one place left, or the point moving one place right. The digits themselves never change.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A strip of thick cardboard 40 cm by 10 cm
- Six digit cards that slot into pockets
- A bright red bead or button on a thread as the sliding point
- Place labels: hundreds, tens, ones, tenths, hundredths, thousandths
- A ruler and glue
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
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.
Draw six equal boxes along the cardboard strip and glue a small pocket under each box for a digit card.
Write the place labels above the boxes in order from hundreds on the left to thousandths on the right.
Pierce a slot along the bottom edge and thread the red bead through so it can slide between any two boxes to act as the decimal point.
Slot in the digits 3, 4 and 5 in the ones, tenths and hundredths pockets, with the bead after the 3, to show 3.45.
Slide the bead one box to the right. Read the new number: 34.5, which is 3.45 times 10.
Slide it back two boxes to the left to show 0.345, which is 3.45 divided by 10. Put a 0 card in the ones box so the number is easy to read.
Write the expanded form under each position: 3.45 = 3 + 4/10 + 5/100.
Test with money: show Rs 3.45 and explain that the 4 is 40 paise and the 5 is 5 paise.
What you should see
The board shows that multiplying or dividing by 10 does not change the digits, only their place: 3.45, 34.5, 345 and 0.345 all use the same three digit cards.
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 3.45 multiplied by 100?
345. The point moves two places to the right.
2What is the value of the 5 in 3.45?
Five hundredths, or 5 paise if the number is rupees.
3Why do we write 0.345 and not .345?
The zero in the ones place makes the point easy to see and shows there are no whole ones.
4Which is bigger, 0.5 or 0.45?
0.5, because 5 tenths is more than 4 tenths and 5 hundredths.
Take it further
Add a kilometre, metre, centimetre and millimetre scale under the boxes to show that 1.25 km is 1,250 m.
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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