# Pro/Mechanica Units

#### Stefano

##### New member
Hello,

I am using custom system units as follows mm_g_s_

Force unit = (1/1000) kg . (1/1000)m/s^2=1.0E-6 N

So, your basic unit of force is 1.0E-6 N. (One microNewton).

If you assume g=10 m/s/s (most people use 9.807 or thereabouts - depends on your loaction on the earth), the mass that would have a weight equal to one force unit is 1.0E-6/10 = 1.0E-7 kg. (Don't interchange mass with force or weight - they are not the same thing. A kilogram.force IS a valid force unit, equal to the weight of a 1 kilogram mass subject to 1 g acceleartion - but it is NOT the same thing as a kilogram!)

Your basic stress unit will be one force unit divided by (one length unit squared) = 1.0E-6/(1/1000)^2 N/m2 = 1.0 Pa. Thus, your stress of 2E6 is in fact 2 MPa or 2 N/mm2.

(I hope I have not been too pedantic, but I have found many students have trouble with units, and the best solution is to step through the calculations slowly and steadily.)

Hope this helps.

Why don't you change your system units into mm_N_s? Then your Von Mises stress is in [N/mm