# How should the corresponding physical parameters change after the dimension changes from meters to millimeters?

Dear all,

After the dimension changes from meters to millimeters, how should physical parameters such as density, acceleration of gravity, Kn, Ks, viscousDamping, young and poisson change?

Best Regards,
Jie

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2020-06-03
2020-06-03
 Jan Stránský (honzik) said on 2020-06-03: #1

Hello,

TLTR: if you have to ask, do not change it
There is no real need and often a confusion is a by-product (or the main product)..
Also such code is more error prone (one easily forget to transform some input/output).
**I personally** strongly suggest to stay in basic SI unit system.

each quantity has a physical dimension, to transform it to another unit system, just substitute the transformation relation (1 m = 1000 mm). For the quantities mentioned:

density:
kg/m^3 = kg/(1000*mm)^3 = kg/(1000^3 * mm^3) = 1/(10^9) kg/mm^3

acceleration, gravity:
m/s^2 = (1000 mm)/s^2 = 1000 mm/s^2

Kn, Ks (assuming N/m, just "Kn" is too ambiguous):
N/m = (kg*m/s^2)/m = kg/s^2 (no change)

young:
Pa = N/m^2 = (kg*m/s^2)/m^2 = kg/(m*s^2) = kg/(1000*mm*s^2) = 1/1000 * kg/(mm*s^2)

poisson: no change (is dimensionless)

cheers
Jan

 Bruno Chareyre (bruno-chareyre) said on 2020-06-03: #2

I think both short and long versions in the first answer are perfect yet here is another possible answer (or comment):

> after the dimension changes from meters to millimeters

You cannot change from meter to millimeter in yade since yade is not in meter. My conclusion is that this question would fit in a physics forum. It doesn't contain any of the words "yade" or "dem" or "simulation". :)
B

 Bruno Chareyre (bruno-chareyre) said on 2020-06-03: #3

> yade is not in meter
To be more clear yade is not in _any_ unit. The units are in our human brains, yade crunches numbers.
B

 Jan Stránský (honzik) said on 2020-06-04: #4

@Bruno

> this question would fit in a physics forum

true, but it also fits here IMO (see below)

> The units are in our human brains, yade crunches numbers.

true again, but the numbers has some meaning. People (usually) does not run Yade (or any similar program) to just "crunch numbers", but to get some reasonable result. The physical units are part of this reasonability.

That the same numbers have infinite many other meanings (for infinite many physical units chosen and their combinations) is another story :-) usually not that important for specific simulation runs..

cheers
Jan