Spontaneous reasoning on the propagation of visible mechanical signals

This paper analyses the difficulties encountered by students while learning the propagation of a signal on a string. Two tendencies are shown to recur: a mechanistic reasoning, and a reduction in the number of variables. For students, the medium is a passive support and the travelling ‘bump’ is a material object created and set in motion by the source. Consequently, the signal velocity depends on the source and can decrease with time. The signal cannot move before being entirely formed and its length does not depend on the medium. Students reduce the number of variables in two ways. They tend to combine different physical quantities into a single notion. They also reason on the basis of one variable at a time. Some pedagogical implications are discussed.