How many ways are there to measure the height of a building using a smartphone? 10? 20? 50? At least 61 !, according to four researchers from the universities of Paris-Saclay and La Sapienza, in Rome, who published this finding in the September edition of the journal Physics Education.
The desire was not to break a record but “To continue to rethink the teaching of experimental physics by deconstructing the classic practical work”, indicates Frédéric Bouquet, teacher-researcher at the University of Paris-Saclay, member of the “Physics otherwise” team, specializing in new ways of popularizing or teaching physics. To this end, what could be easier for undergraduate students than using an object always in their pocket and with sometimes unexploited capacities: the smartphone?
On the roofs of the university
This is in fact a real miniature laboratory, with a microphone (sound sensor), a camera (image sensor), accelerometers (switch to landscape position), even barometers, gyroscopes, magnetometers … Several applications, such as Phyphox or Physics Toolbox, make it possible to retrieve information from these sensors to measure speeds, accelerations, light intensities, acoustics, etc. And to encourage students to survey the rooftops of their university premises.
During the sessions, the telephones are subjected to a severe test: free fall from the top of the building, swinging at the end of a rope, counting steps, night outings to better observe the optical effects … Several fields of physics are exploited: mechanics , with the fall of bodies and the law of universal gravitation (the height is proportional to the time of fall squared), or the oscillation of the pendulum, the period of which depends on the length. But also acoustics (sound propagates at a finite speed), electromagnetism (the intensity of a wave decreases like the inverse of the distance squared), or even optics (by measuring projected shadows ). The researchers even tried General Relativity, which shows that the speed of time is dependent on altitude. But the stopwatches of the phones are not precise enough … and give a height between 0 and 3 million kilometers!
The most original method goes from microscopic to macroscopic. A drop of water on the phone’s lens turns it into a microscope to measure the diameter of a hair. This hair is then illuminated at the top of the building at night by a laser pointer to observe the size of the diffraction spots on the ground, which depends on the height.
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The smartphone, a Swiss army knife for physical measurements (and pedagogy)