All your science in here


Opportunity for a yotta yotta watts sadly missed


Light engineering clearly explained.


Interstellar cannonball



What a film.



we might get heavier mince pies…


Is there really a need for a totally separate definition of the kg when it is intrinsically linked to the metre?


I’m guessing (and it really is guessing on my part), that it’s easier to define it in these terms in order to get reliable replication.


That is the main reason, I believe, including the aspect of constancy over time.

The mass definition now involves fixing the value of Planck’s constant, which has the dimensions of m^2 kg/s. So extracting the kg from that does depend on us being sure of the metre and the second. But the metre and the second on their own don’t tell us enough to determine mass. In the system we’ve chosen mass is a fundamental unit, and if we’d chosen a different set of fundamental units then at least one of them would have had to have covered the properties of mass.



I stupidly thought that 1kg was the weight of 1 litre of H2O :roll_eyes:


But surely the kg is the mass of a litre of water at standard temperature and pressure, with the litre being accurately defined from the unit of length (metre) which is now defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 second.


The definition in terms of a litre of water was abandoned in the 19th century (actually having just checked it seems the first metal block kilogramme was produced in 1799, so the 18th century, but I don’t know if it was then defined as the standard or whether it was just a convenient secondary reference). I think it was replaced by a definition based on a standard metal block, being a predecessor of the one which has just been retired.



mass, not weight.



And the mass of that ‘metal block’ was derived from?

A litre of water perchance?

I don’t know, I’m asking :smile:


Something about number of atoms iirc, but VB will tell all.


I recall a mole being based around the number of atoms (6.02 x 10^23), but not weights. There was a scheme afoot to replace the Paris weight with a different weight, but that still had some of the issues that the Paris weight has.


I thought in JB’s case it was the distance from his arse to his elbow :grin:


Yes, it was derived from that. Just like the metre was originally derived from one ten millionth of the distance between the earth’s pole and its equator. And then from the length of a standard metal bar. And now from the wavelength of light from a particular source.

Each time they change the definition they go to a great deal of trouble to make sure that the new definition is the same size as its predecessor. But once the change is made the previous definition becomes completely irrelevant and invalid as the standard.



Ok, I get that.

In that case, why change it?