What it shows
Every object has one point where its weight seems to act, the centre of gravity. A shape hung from a pin comes to rest with this point directly below the pin, which lets you find it with a plumb line.
The science
The centre of gravity of a flat object is the point where it can be balanced. When the object hangs freely from a pin, gravity pulls on it until the centre of gravity lies right below the point of support. A plumb line hanging from the same pin points straight down through the centre of gravity. Hanging the shape from two different points gives two lines that cross at the centre of gravity.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Thick cardboard sheets
- Scissors
- A pushpin or a thin nail
- A cork or eraser to hold the pin
- A thread with a nut as a plumb line
- A pencil and ruler
- A tray
Safety
For this project: An adult must supervise the use of scissors and pins and keep the pin covered when not in use.
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.
Cut an irregular shape from the cardboard, such as a map of India, a star or a blob.
Make three small holes near the edge of the shape in different places.
Push the pin through one hole into the cork so that the shape hangs freely and swings.
Hang the plumb line from the same pin, in front of the cardboard. Wait until both are still.
Draw a pencil line on the cardboard along the thread.
Repeat with the second and third holes and draw the lines. The three lines should meet at one point.
Balance the shape on the tip of a pencil at that point and see it stay level.
Find the centre of gravity of a rectangle, a triangle and a ring-shaped piece and compare with where you expect it.
What you should see
The three lines cross in one point and the shape balances there on a pencil tip. The point for a rectangle is the middle and that of the ring is in the hole.
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 the centre of gravity?
The point where the whole weight of the object seems to act.
2Why do the lines from different holes meet at one point?
The centre of gravity is a fixed point in the shape and always hangs below the pin.
3Can the centre of gravity be outside the object?
Yes, for a ring or a bent shape, it can lie in empty space.
4Why do tall buses with a low load not tip easily?
A low centre of gravity makes them more stable.
Take it further
Use the method to find the balance point of a banana-shaped piece and explain why a leaning tower does not fall as long as its centre of gravity is above its base.
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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