What it shows
When one marble rolls into a line of touching marbles, one marble shoots off the far end; roll two in and two leave. The push, or momentum, passes right through the line, as in a Newton’s cradle.
The science
A moving marble carries momentum, which depends on its mass and its speed. When it hits the first marble in the line, it stops almost dead and its momentum is passed from marble to marble through the line in a fraction of a second, until the last marble, which has nothing in front of it, rolls away with nearly the same speed. The marbles in the middle barely move because each one receives a push from behind and immediately gives it to the one in front. Momentum cannot be lost in a collision, only passed on, which is the rule behind this trick.
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.
- Six to ten marbles of the same size
- A grooved wooden ruler, or two rulers taped side by side with a gap between them
- Books to prop one end of the ruler as a small ramp
- A smooth table
- A larger or heavier marble for the extension
Safety
For this project: Keep marbles away from children under five who may put them in their mouths, and pick up any that roll on the floor.
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.
Lay the grooved ruler flat on the table and place five marbles touching each other in the middle of the groove.
Roll one marble gently along the groove into the end of the line and watch one marble leave the other end.
Repeat with a harder push and see that the last marble leaves faster but still only one leaves.
Roll two marbles together into the line and watch two leave the far end.
Try three and check that three leave; make a table of marbles in against marbles out.
Leave a small gap between two marbles in the line and see whether the trick still works.
Roll the heavier marble into the line and describe what is different.
Raise one end of the ruler slightly on a book so the marble always starts from the same height, for fair tests.
What you should see
The same number of marbles always leaves the far end as hit the near end, and the middle marbles hardly move. A harder push sends the leaving marble off faster.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does only one marble leave when one hits?
The momentum of one marble is passed through the line and only one marble’s worth of momentum comes out.
2Why do the middle marbles not move?
Each one gets a push from behind and immediately passes it on to the marble in front.
3What is momentum?
The amount of motion an object has, found by multiplying its mass by its speed.
4What is a Newton’s cradle?
A row of hanging steel balls that shows the same passing on of momentum.
Take it further
Build a hanging Newton’s cradle from five marbles glued to threads and a cardboard frame.
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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