Density units in Examples

[HelenN]

Hi all,
I can not find information about the units in the tutorial.
1) I'm interested in two examples: co27 (2D foot model) and mi28 (3D finger model).
For co27 the density of the bones (rigid body) is 1000, while for mi28 it is 1.
I found in sec. 4.6.2 General Specification of Multiphasic Materials the table with the units, but
there is no mass or density, and I think it is for different kind of problem.

2) In the example co36 for two spheres, the force is 0.2. My question: is it 0.2N?

Thanks in advance,
Elena

[ateshian]

Hi Elena,

You need to pick a self-consistent set of units. For example,

If length is in mm, force is in N and time is in s, this means that acceleration is in mm/s^2. It follows that mass = force/acceleration = N*s^2/mm = tonne (1000 kg). So density must be in units of tonne/mm^3.

If length is in m, force in N and time in s, then mass is in kg and density is in kg/m^3.

In the examples you mention, there is no fool-proof way of knowing what the units are (e.g., length could be in m, mm, inches, ft, etc.), unless the units are specifically mentioned in the comments.

Best,

Gerard

[HelenN]

Thank you, Gerard. It is now more clear. I hope that the force in the example mi28 is in N. But therefore, the density
1 Tonne/mm^3 seems exaggerated because the density of osmium is 20 g/cm^3 = 20 x 10^(-9) Tonne/ mm^3.