Considerations about a Physics laboratory experiment intended to students with some theoretical knowledge about oscillations are given. Students are proposed to construct a pendulum that behaves as a mathematical one, and to analyze the error due to the model assumption. Among these hypothesis, the following ones are quantitatively analyzed: vanishing friction, small amplitude, inextensible wire, punctual mass of the body, vanishing mass of the wire. It is concluded that model hypothesis are easily accomplished in practice, and that their analysis allow a better students'comprehension of the scientific way to control theoretical development validity
