Until 2019 the kilogram was the mass of one metal cylinder kept near Paris.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Until 2019 the kilogram was the mass of one metal cylinder kept near Paris. Now it is fixed by Planck's constant. Give two reasons why the new definition is better.
Need a reminder? The key ideaShow
Since 2019 every SI base unit is fixed by a constant of nature: the kilogram by Planck's constant , the metre by the speed of light, the second by the caesium atom.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Anyone can rebuild it. Planck's constant is the same everywhere, so any well-equipped laboratory can realise the kilogram itself. With the cylinder, every copy had to be carried to Paris to be compared.
Step 2. It cannot change. A metal cylinder can gain dirt or lose atoms when it is cleaned. Copies made in 1889 had drifted by about micrograms from the original. A constant of nature does not drift.
Answer: It can be reproduced anywhere, and it is the same for all time.
Watch out. The new kilogram has the same size as the old one. Only the way of defining it changed.
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