Integration of science and mathematics as well as with other disciplines is overarching goal of science education. In spite of its importance, teachers have concerns about mathematical difficulties that their students encounter during their teaching. One of the most common subjects that students have mathematical difficulties was reported as physics. With this regard, the present study is aimed to investigate seventh grade students' mathematical errors in a physics related subject, specifically force and motion unit. We collected data from 129 seventh grade middle school students which were chosen conveniently throughout an open-ended questionnaire. The findings revealed that the students encountered difficulties in ratio and proportion and conversion of units, topics as well as ordering numbers while answering the questions in this unit. Implication for science courses was discussed.