Fair enough, that's a nice intuitive way to work it out for a thin-wall tube since you can assume the mass is concentrated at the radius. This clearly is an approximation, since the tube does have thickness, so to work it out accurately you have to consider the integral of mass*radius^2 and you end up with the formula I quoted after touching upon real maths! In this context the approximation is of course valid (error is <10%), however this method wont get you very far if the tube was substituted for a simple shaft.


Quote Originally Posted by Robin Hewitt View Post
The 1/8" wall tube weighs 2.35 kg

Now your numbers make sense, it must be 2700kg/m^3 and therefore aluminium tube...I thought it was steel! Hmm to be fair you did say it was aluminium originally. Whoops...