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SCIENTISTS have come up with a solution for expats heading home for the summer or locals searching for cooler climes who find their two-wheeled suitcases spinning out of control as they rush to their fights at Bahrain International Airport.
Speeding up rather than slowing down can solve the problem, they say, having investigated the conundrum of everyday physics. Alternatively, passengers should pivot the handle of the suitcase as close to the ground as possible.
This swaying motion is ‘a bit funny and counterintuitive’, says study coauthor Sylvain Courrech du Pont. It actually gets smaller when the suitcase rolls faster. Lowering the angle of the suitcase can get the rocking to stop altogether, he says, when the bag moves alarmingly from side-to-side and threatens to overturn.
French scientists studied a model suitcase on a treadmill to see what goes wrong. They developed equations to explain why two-wheeled trolleys have a tendency to rock from one wheel to the other.
In cases of unstable bags - after having gone over a bump, for example - they found luggage rocks from side-to-side until it falls over, or it reaches a regular side-to-side swing. “Thus, one should accelerate rather than decelerate to attenuate the amplitude of oscillations,” the researchers say. “A non-experienced suitcase puller would not react this way. The outcome should not be as dramatic for a suitcase, but it could be troublesome for a trailer towed by a vehicle.”
This leads on to important practical implications of the research, which is published in the journal Royal Society Proceedings A, especially as the car and technology industries rush towards automated transport solutions.
Understanding the physics of this system could be useful for more than designing more stable suitcases, because it also applies to other two-wheel carriers such as car-pulled trailers. “In the near future, maybe we will have a car without a driver,” added Courrech. “It would be a good thing if the car knows how to stop this kind of motion.
“The suitcase is a fun way to tackle the problem but the study would be the same for any trolley with two wheels or blades. So it will be the same for a caravan or maybe also for airplanes.”
In technical terms, the mechanical instability is mainly due to the fact that there is a coupling between the translational motion and the rotational motion of the suitcase.
It comes about because the two wheels are fixed together on a rod.
In April, scientists solved another problem of everyday physics - why shoe laces come undone. They found the force of a foot striking the ground stretches and then relaxes the knot, while a second force caused by the leg swinging acts on the ends of the laces, like an invisible hand.
This too will likely have practical applications for structures.
