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I Mass standard unit definition

  1. Nov 2, 2016 #1
    The standard unit of mass is defined to be "equal to the mass" of a cyliner of platinum and iridium in france. I have always wondered what this means? Why are all sources saying mass is defined in terms of mass? What were the steps taken to decide this unit?

    I know that base quantities have to be defined some way at least, but in particular I am not able to get my head around this definition, I understand the ones for distance and other units. I know from mechanics tuahgt in school that mass is a measure of an objects inertia (and inertia is an inherent property in all objects). So, just like we cannot define words using the same word it confuses me when people say the standard unit of mass is the mass of the cylinder in France. What does the bolded part actually refer to?
  2. jcsd
  3. Nov 2, 2016 #2


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    Staff: Mentor

    The concept of mass is not defined in terms of mass, but the unit by which we measure against is.
  4. Nov 2, 2016 #3
    The video in the link does a pretty good job at explaining how the base si unit of mass (kg) is currently defined, and the issues that come with the current method

  5. Nov 4, 2016 #4
    To back up what drakkith said, we don't need an artifact to define what we mean by length or distance. However not so very long ago the UNIT of length, the meter, was defined as the length of a particular bar of platinum-iridium kept in the same place as the standard kilogram.

    Science hates this kind of definition of units by artifact because the artifact changes over time. Material erodes away by handling, or the properties change by chemical reaction with the atmosphere. Minimizing chemical reactions is the reason for the platinum by the way. The standards organizations have been working to define the base units in terms of immutable physical quantities for the entire history of science. The kilogram is the last unit based on an artifact, and that will change very soon. The change is already well along in the process.
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