Several researchers attribute to students' alternative ideas about force and motion the status of an intuitive theory, whereas others consider these ideas as being less structured. This study focuses on students' understanding of horizontal lack of motion. 102 students took part in the study, conducted in the U.K. Students' knowledge was assessed with a questionnaire (pre-post and delayed). Understanding of the situation under study proved to be the most difficult. About half of the students provided answers indicating perception of the situation as a ‘lack of equilibrium' case. These answers are inconsistent with the existence of an intuitive theory of ‘force implies motion'. Analysis of interview data showed that students who changed their answer in favour of the correct answer during the interview did so by referring to a law stating that ‘no motion implies no total force'. This might indicate progress towards the construction of a theory, intuitive or scientific.