This is scientific American 60 second science. I'm Christopher Intagiyata.

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.

Sylvain Karestupon, 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.

Kerestupon and his students wanted to get to the bottom of why 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.

Here's what they found.

Under normal rolling conditions, the forward motion of the suitcase is perpendicular

to the wheels 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 wheels coming down, the same thing

happens and now the suitcase is really rocking. And the first reaction that you would have maybe is to slow down, but actually if you slow down

you will experience very big rocking oscillations.

The way back to a smooth ride he says is to keep up your speed or move even faster once rocking begins.

The studies in the proceedings of the Royal Society A.

There are other solutions to suitcase instability, like doing a better packing job.

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