What it shows
A heavy ball on a stretched cloth makes a dip, and small marbles rolled nearby curve round it and spiral in. This is a simple analogy for how a massive object like the Sun pulls on planets and why orbits slowly lose energy to friction.
The science
The dip in the cloth is a model, not a picture of real space. Gravity is a pull between masses, and it makes a small object travel in a curve around a big one. A marble launched at the right speed can circle for a while because its sideways motion balances the pull inwards; friction on the cloth slows it so it spirals in, unlike planets in vacuum.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A stretchy cloth such as an old dupatta or Lycra, about 1 m square
- A large hoop, round basin rim or cardboard frame to hold it
- A heavy ball or a stone wrapped in cloth
- Several marbles of different sizes
- Clips or strong tape
- A phone for slow-motion video
Safety
For this project: Place the frame on a table or the floor and keep marbles away from small children. Watch your toes if the heavy ball falls.
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.
Stretch the cloth tight over the frame and clip it all round so it is flat like a drum.
Place the heavy ball in the middle and see the dip it makes.
Roll a marble straight at the dip and note that its path bends.
Roll the marble from the side so that it passes the ball and curves around it.
Try a fast marble, a slow marble and a heavy marble and record how many circles each makes.
Take a slow-motion video of the best circling roll.
Replace the heavy ball with a lighter one and compare the dip and the paths.
Write down where the model fits real orbits and where it does not.
What you should see
Marbles circle the heavy ball in curved paths and spiral in slowly. A heavier centre makes a deeper dip and a stronger bend.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What force keeps a planet in orbit?
The pull of gravity from the Sun.
2Why do the marbles spiral in?
Friction with the cloth slows them; planets in empty space have almost none.
3What does a heavier central object do to the cloth?
It makes a deeper dip, showing a stronger pull.
4What is a weakness of this model?
Real gravity works in three dimensions, not on a flat sheet.
Take it further
Put two balls of different weights on the cloth to model a double star and see how marbles behave.
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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