Scientists in Paris have figured out what causes rolling suitcases to wobble then overturn. Their counterintuitive solution to the problem: speed up.
How it happens: Physicists studied suitcase instability by creating a model of a two-wheeled suitcase rolling on a treadmill. They found that a suitcase unsurprisingly begins to rock when it hits a small bump or something knocks into it. When one wheel is lifted off the ground, the tilted suitcase shifts to realign the wheels but when the wheel hits the ground again, the other one lifts and the energy from movement is transferred, rocking the suitcase the other direction. The motion continues or the suitcase falls over so long as the energy from rocking is greater than the dampening that occurs when the wheel touches the ground again.
How to make it stop: Go faster. It won't stop the oscillations entirely but it will decrease the amount of energy transferred from pulling to rocking. "Thus, one should rather accelerate than decelerate to attenuate the amplitude of oscillations. A non-experienced suitcase puller would not react this way," the authors wrote.
What's new: "People used to assume it was a resonance and that when you are walking, tiny movements in your hands are transferred to the suitcase and amplified," says Paris Diderot University physicist Sylvain Courrech du Pont. "By we, I mean the one guy who studied this."
Beyond suitcases: The rocking motion could inform the design of towed trailers, especially autonomous ones that will need to know how to self-correct the movement.