Unit confusion 1

[trainguy]

Can anyone help with units required when using body loads?

My model is in mm, and force units are Newtons. E is in N/mm2.

My mass density is 7.85E-6 kg/mm3.

To analyze the vehicle under 1g loading due to self weight, what should my translational acceleration be? And what units?

Thanks in advance.

tg

 

[rb1957]

g = 9810 mm/sec^2

 

[rb1957]

btw, note the unit "confusion" with N/mm^2 ... N = kg.m/sec^2 ...
so (rho*V)*g = kg/mm^3*mm^3*mm/sec^2 ...

looking at it that way you're better off with m/sec^2 so you get "clean" N as weight ...

(kg/mm^3*mm^3)*m/sec^2 = kg*m/sec^2 = N

clear as mud !?

 

[trainguy]

rb,

Are you saying I should use 9.81?

I think you're right, because when I had g as 9810, my model had the erroneously input mass density of 7.85 E-9 !!

And all my runs were giving correct reactions in Newtons (I think). The errors were canceling each other out.

The issue started becoming real when I tried a problem with rotational acceleration (rad/sec^2).

No canceling out, and mass confusion ensued(pun intended...)

tg

 

[rb1957]

i can't see a problem with my 2nd post, so yes i think you can mix units carefully so long as the final dimensions are correct (N = kg*m/sec^2).

work through a problem that you can solve and Know the right answer.

 

[rb1957]

here's a link discussing this ...

http://www.endurasim.com.au/web_images/EnDuraSim-Engineering-Units.pdf

they "go on" about using consistent units, that if you model in mm then your acceleration should be in mm/sec^2 and density should be tonne/mm^3 (1000kg/mm^3) so that the force units sort themselves out ... N = (tonne/mm^3*mm^3)*mm/sec^2 = tonne*mm/sec^2 = kg*m/sec^2.

this approach is obvious Correct, a little pedantic IMHO if you're doing routine analysis where i think you can mix units (density in kg/mm^3, accel in m/sec^2). but you have to becareful ... i think you'll get into trouble if you have rotational accel, ie places where you aren't combining denisty and linear accel.

on the whole, it is better practice to use consistent units.

 

[trainguy]

rb,

Thank you for setting me straight. I was using consistent units in 95% of the work, prior to yesterday.

I'll go back to consistent units:

Length mm
g=9806 mm/s2
All masses in MT
Mass density in MT/mm3
Stress in N/mm2
Forces in N

Great link by the way...

tg