What it shows
When one ball hits a row of balls, the one at the far end swings out and the others stay still. This shows that momentum and energy are conserved in an elastic collision.
The science
In an elastic collision, both momentum and kinetic energy stay the same. When one ball strikes the row, it stops and passes its momentum through the others, and the last ball moves off with the same speed. If two balls swing in, two swing out. Friction and the small loss of energy to sound mean the cradle gradually comes to rest.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Five identical steel balls or large marbles
- Strong thread
- Ice-cream sticks to make a frame
- Strong glue or tape
- A ruler
- Small beads and clay to hold the thread
Safety
For this project: An adult must cut and glue the frame. Keep marbles away from small children.
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.
Build a rectangular frame from sticks, about 15 cm high and 15 cm wide.
Tie each marble with two threads in a V shape so that it swings in a line only.
Fix the threads to the top bar so all the marbles hang in a row, just touching.
Pull one marble to the side and release it.
Observe that one marble leaves at the far end.
Try pulling two marbles, then three.
Count how many swings it takes to stop.
Explain the results using momentum conservation.
What you should see
The number of balls that swing out equals the number let in, as predicted by conservation laws.
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 conserved in an elastic collision?
Momentum and kinetic energy.
2Why does only the last ball move?
Momentum passes through the row.
3Why does the cradle stop?
Energy is lost to sound and friction.
4Why are two threads used for each ball?
To make it swing in a straight line.
Take it further
Replace one marble with a heavier one and see what happens.
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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