Nowadays, it is well stablished the effectiveness of flip teaching in different areas. Video is widely used as a pre-class learning material and it is progressively replacing text-document materials. The students, 70 in total, are enrolled in the laboratory practice of the subject of Physics (Bachelor's Degree: Industrial Electronics and Automation Engineering) at the Universitat Politècnica de València. Student's perception data were collected from the results of a survey. Students were asked about the usefulness of the videos and the in-class work, paying special attention to the roll of the instructor in relation to the flip teaching methodology and the pre-class material. The students were divided into two groups. In one of them, the videos were recorded by the same instructor present at the lab session, whereas the other group, the videos used were recorded by instructors different from the instructor present at the lab. The results indicate that students feel that the videos are very useful, rather than the written documents. They attribute a high value to the instructor in the lab session as they feel more confident with their work and they can ask for help when needed. They value positively the possibility of work autonomously. Small differences have been found in the opinion of both groups being the higher in the question that concerns the preference of the videos respect to the text.

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