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Journal Thread #1 / Prompt 4 - One Little Problem

Step into the shoes of a lead metrologist and plan how you would persuade your country's leaders to make the switch to using universal constants as standards.

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Title of Activity: Journal Thread #1 / Prompt 4 - One Little Problem

Brief Description: Students prepare a short argument for why it is important to redefine the kilogram using constants.

Target Grade Level: Grades 8-12

Discipline or Course: Physical Science

Estimated Time Required: 20 minutes

Individual / Partner / Group Work: Individual

Key Vocabulary:

  • Constant
  • Standard
  • Accuracy

Teacher Prep:

There are two prompts urging students to justify the effort to redefine the kilogram. The first question asks how the metal artifact might change. Students might first imagine it getting smaller through micro-abrasion (touch or even air.) The cylinder might also increase in mass, as chemicals or particulates in the air adhere or bond to its surface. Students should discuss both possibilities.

The second question in the prompt asks “why.” It could be answered by citing either scientific or economic (engineering) problems that could be answered with more accurate measurements. A complete answer would include both current issues (like nano-processes) and future research.

Possible Rubric

Area Novice Average Advanced
Constructing Explanations No reasonable hypotheses for how the cylinder might change. Hypothesis for how the cylinder might gain mass or lose mass, but not both. Hypotheses for how the cylinder might increase or decrease mass.
Communicating Information No discussion of communication for precision Mention of communication for precision without specific guidelines Specific guidelines for communication to improve precision are included.
Identification of arguments based on evidence No mention of problems that imprecision in measurement might cause or questions that a more accurate standard might answer Mention of problems that imprecision in measurement might cause or questions that a more accurate standard might answer but not both. Mention of problems that imprecision in measurement might cause and questions that a more accurate standard might answer


Guarded and locked with 3 different keys, hermetically sealed in a glass bell jar, “Le Grande Kilogram” seems perfectly protected. Yet that cesium-iridium cylinder can change over time.

The process of redefining the kilogram was long and complex. It was also controversial. Metrologists from all over the world debated whether it should be done and how.

Imagine you are justifying your country’s expenses for the work of its metrologists redefining the kilogram in terms of a universal constant. The budget-masters ask: “How could it change?” and “Why would such a tiny difference matter?”

Prepare a short argument for your congressional budget committee justifying your participation in a process to redefine the weight of the world.