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This is Scientific American's 60-second science. I'm Christopher in Taliatta.

If you've ever pulled a rolling suitcase through the airport,

chances are you've also been annoyed by this common occurrence.

The suitcase starts rocking back and forth on each wheel,

and finally tips over.

It's a very classic phenomena in physics.

Sovain Koreschupon, a physicist at the University of Paris, Didero.

So if you perturb the system, it goes very easily unstable.

This is shared by many, many phenomena in nature.

Koreshtapon and his students wanted to get to the bottom of wide luggage tips.

So they built what looks like the skeleton of a wheeled suitcase and

filmed it rolling on a treadmill. And then they modeled what they saw mathematically. And here's

what they found. Under normal rolling conditions, the forward motion of the suitcase is perpendicular

to the wheel's axis of rotation. All good. But when you hit a bump or jerk the handle,

one wheel lifts up.

All of a sudden, the pulling motion is no longer perpendicular to the still rolling wheels rotation, and the wheel corrects that by drifting towards the center. But by that time, the other

wheel is coming down. The same thing happens, and now the suitcase is really rocking.

...