If you've ever pulled a rolling suitcase through the airport, chances are that 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 phenomenon in physics," said Sylvain Courrech du Pont, a physicist at the University of Paris Diderot. "So if you disturb the system it goes very easily unstable. This is shared by many, many phenomena in nature."
Courrech du Pont and his students wanted to get to the bottom of why luggage tips. So they built what looks like the structure of a wheeled suitcase, and then filmed it rolling on a treadmill (a machine for running). 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(垂直的） to the wheels' axis of rotation（旋转轴）. All is good. But when you hit a bump or drag the handle fiercely and abruptly, one wheel lifts up. All of a sudden the pulling motion is no longer perpendicular to the still-rolling wheel's rotation axis, and the wheel corrects that by accelerating towards the center. But by that time the other wheel's coming down, the same thing happens, and now the suitcase is really rocking.
"And the first reaction you will have maybe is to slow down. But actually if you slow down you'll experience very big rocking. Therefore, the way back to a smooth ride is to keep up your speed," he says, "or move even faster, once rocking begins.
There are other solutions to suitcase instability—like doing a better packing job. "If you put heavy contents close to the axis of symmetry, then it will be stable, too."
And Courrech du Pont has one more piece of advice to avoid rocking in the first place: "If you go slow enough, it will always be stable." Keep that in mind, next time you're running to your gate.
【小题1】What can we infer from the first two paragraphs?| A．Everyone has been upset by a rolling suitcase at the airport. |
| B．Physical phenomenon can be found in our daily life. |
| C．The disturbance of the system results from the instability of the suitcase. |
| D．The phenomenon of suitcase rolling is rare at the airport. |
| A．It hits a bump |
| B．One wheel lifts up. |
| C．One wheel falls down. |
| D．It is not perpendicular to the axis. |
| A．tip it over |
| B．accelerate its moving. |
| C．slow down its moving |
| D．drag its handle with great strength. |
| A．one | B．two |
| C．three | D．four |
