[[alternative]]Investigation into the Mental Models and Alternative Conceptions of Gravity of Ninth-grade Students.

[[abstract]]The aim of this research is to take gravity as an example, using the method of clinical interview and the two-tier design, triangulating from different dimensions students’ understanding about force and action-at-distance, and diagnosing students’ alternative conceptions about gravity and their mental models of gravity. The sample population is 338 in total, 181 male and 157 female of Ninth-grade Students, from seven public junior high schools in Taipei City and County. Students have alternative conceptions about gravity; for instance, there exists air between the apple and the ground, so the apple can drop down, and there is an counterforce between the earth and the moon; the counterforce is equal to gravity; the moon is outside the atmosphere of the earth, so it will not be affected by gravity; because there is no air on the moon, objects or astronauts will not drop down on the surface of the moon; the earth moves around the sun because the pulling force is balanced between the earth and the other planets. In this study, there are four categories of mental models, which Ninth-grade Students develop as concepts in interpreting gravity: I. Experientialism: students think that action-at-distance works only through medium for transmission. Gravity is confined in the atmosphere, there is no gravity on the moon because no air is there. So, any object can float above the surface of the moon. The orbit of a planet is a natural phenomena, which has been formed since the universe was created. II. Atmospheric pressure: Students think that action-at-distance is transmitted through medium, but if there is atmosphere, objects will be attracted by gravity and have weight because of the atmospheric pressure. III. Generation of gravity: Students think that gravity is generated by a planet itself. And the greater the generated gravity is, the weightier an object on it will be. An object on the moon is lighter than on the earth because the moon has stored less energy, which also generates less gravity than the Earth. IV Quasi-scientific balance: Students have a comparatively better scientific concept about action-at-distance and can apply some fundamental concepts to interpret the phenomena formed under the circumstances of the Earth and the moon; when students further apply them to the orbit of a planet, they interpret it according to their concept about the balance of static force.