Earthquake: a tall tower and base isolation
A crank mechanism shakes the platform. A narrow tower falls, a wide house stands, and the same tower on soft springs and dampers survives the shaking.
Press “Run”. Bodies move by the laws of physics: gravity, friction, elasticity.
What the experiment shows
A building tips over if the ground acceleration is too large for its shape: tall narrow buildings are riskier than low wide ones. A soft mount under the building does not pass quick jolts on to it.
How to run it
- Press “Run”: the crank starts to shake the platform.
- Compare the fate of the three buildings: tower, house and tower on springs.
- Select the crank motor and reduce its speed to 2 rad/s — now the tower may survive.
Questions to discuss
- Why does the tower on springs sway but not fall?
- What else do engineers do so buildings survive earthquakes?
The science
The platform shakes with an amplitude of 14 cm at 8 rad/s, so its acceleration reaches nearly 9 m/s². A tower 40 wide and 140 tall tips when the acceleration exceeds g·(width/height) ≈ 2.8 m/s². A building on soft mounts has a natural frequency of 3 rad/s — far below the shaking frequency — so only a small part of the jolt gets through (vibration isolation).
Try it at home
Put a tall narrow box and a low wide box on a tray and shake the tray from side to side: the tall one falls first.
What to change in the editor
- Make the isolator springs stiffer (k = 400) — the isolation disappears.
- Make the tower wider.