July 24, 2002 – “You should put the scale on a hard surface rather than a carpeted carpet before you step on it!” »Who has not heard this wise advice from a benevolent friend who seeks to reduce the number of kilos inscribed on the dial? This myth has now come true thanks to a physicist at the University of Cambridge who challenged one of his students to analyze and explain this difference in weight.
The student in question therefore got down to the task with his teacher by purchasing a series of standard bathroom scales in order to compare measurements on different surfaces. The two researchers then discovered that on thick carpet, the weight displayed increased on average by 10% compared to a hard surface.
They then measured the movement of the internal mechanism when the scale was subjected to weight on different surfaces. Inside the scale is a metal plate which rests on four support points pointing upwards and thus acts as a lever. The plate is connected to a metal rod and a spring on the front of the device. The heavier the weight, the more the spring deforms, causing the dial hand to move. However, it would seem that on a hard surface, the base of the scale bends, thus reducing the distance between each point of support and the place where the load is carried. On carpeted rugs, the base sinks, preventing it from bending. At the same load, the difference between the fulcrum of the base and the pressure point of the foot is therefore greater on a soft surface, which manifests itself in a heavier weight.1
No need to dwell on the fact that you might weigh more than the scale indicates since manufacturers calibrate their scales on a hard surface. And if you are still in doubt, you can remedy the contingencies by choosing digital display scales that have a slightly different mechanism and are therefore less prone to the problem.
Élisabeth Mercader – PasseportSanté.net
According to New Scientist, June 30, 2002
1. New Scientist. People weigh less on a hard surface. [Consulté le 22 juillet 2002]. http://www.newscientist.com/news/news.jsp?id=ns99992462
