What it shows
Earthquake waves shake the ground sideways. A simple shake table shows that wide-based, cross-braced and lighter-topped buildings stay up longer than tall, narrow, top-heavy ones, which is why real buildings in earthquake zones use bracing and a low centre of mass.
The science
When the ground moves, a building's base moves with it but the upper floors try to stay still because of inertia, so the building bends. A narrow tall building has a high centre of mass and bends more, and once it tips past its base it falls. Diagonal braces turn a bendable rectangle into stiff triangles, and a wide base means the centre of mass has to swing further before it passes the edge.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two flat boards or thick cardboard sheets
- Four rubber balls or marbles between the boards
- Four rubber bands to hold the top board loosely
- Ice-cream sticks, straws and glue or tape for towers
- Small clay lumps as weights
- A stopwatch
Safety
For this project: An adult should help with a glue gun if one is used.
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
9 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.
Place the balls between the two boards and loop the rubber bands over both so the top board can slide but not fly off.
Build three towers of the same height: one narrow with no bracing, one narrow with diagonal braces, one with a wide base.
Stick a lump of clay on the top of each tower so they all carry the same weight.
Tape the first tower to the middle of the top board.
Push the top board back and forth about two centimetres each way at a steady rhythm, counting the pushes.
Record the number of pushes before the tower falls or clearly cracks; stop at 60 if it survives.
Repeat with the other two towers using the same rhythm and distance.
Try moving the clay from the top to the bottom of the weakest tower and test again.
Record all results in a table and rank the designs.
What you should see
The unbraced narrow tower falls first, the braced tower lasts longer, and the wide-based tower usually survives the full test. Moving the weight lower makes any tower last longer.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why do tall buildings sway more?
The top is far from the base and inertia keeps it still while the base moves, so it bends further.
2How do diagonal braces help?
They turn rectangles into triangles, which cannot change shape without breaking.
3Which part of an earthquake does the table imitate?
The sideways shaking of surface waves.
4Which Indian zone is at the highest earthquake risk?
Zone V, which includes the Himalayan region and the north-east.
Take it further
Change the rhythm of shaking and find the speed at which each tower swings most; that is its resonant frequency.
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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