We consider how recent regulatory requirements on units about weight, gravity, and mass, and conventions used since the adoption of the gravitational metric system, affect present meaning and the units of weight in science and mathematics at elementary schools. Measurement units and their symbols are taught using the International System of Units (SI) in the compulsory education curriculum, in accordance with the Measurement Act of Japan. In elementary school, “weight” is used with the unit of mass both for the gravitational force on an object and the mass of an object, where the International Prototype Kilogram defines “weight” with the unit kilogram. The usage of “weight” with gram or kilogram in elementary school seems to be adequate at this stage of the education because the differentiation of concept depends on the students’ developmental stage. This usage is similar to the gravitational metric system, however, which has been replaced by SI. In lower secondary school and afterwards, according to definitions of SI, the meaning of “weight” is changed and used only for the gravitational force, while the term “mass” is introduced. There is scope for improvement regarding the discontinuity from the point of view of the step-by-step learning of knowledge. In this article, we propose two possible solutions. The first solution would be to use the term “weight” only to describe the qualitative feeling, without using units, and to introduce the quantitative academic term “mass” with the unit kilogram, without using physical formulas. Since the term “weight” has two Japanese translations, “omosa” and “jûryô”, the other solution would be to use the term “omosa” with the meaning of “mass” both in elementary school and afterwards, and “jûryô” with the meaning of “size of gravitational force”. With either proposal, coherent terms and units would be used throughout the whole duration of the compulsory education science curriculum.
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