- #1
binbagsss
- 1,254
- 11
I am trying to attain the parallel axis theorem from the displaced axes therom.
I have the displaced axes thorem stated in this form:
[itex]\hat{I}[/itex]=[itex]\hat{I}[/itex]com+M[itex]\hat{A}[/itex]
-Where Rc is the position of the centre of mass position-
-Where [itex]\hat{}[/itex]is the inertia tensor of a rigid body wrt to rotations about the origin, [itex]\hat{}[/itex]com with respect to rotations about its COM.
-[itex]\hat{}[/itex]can be represented as a matrix, the elements of which are determined by the elements of the COM position vector:
Aαβ=Rc^2δαβ - RcαRcβ
And the parallel axes theorem stated as :
I=Icom+Md^2 **
- Where d is the distance of parallel axis from the axis of roation passing through the COM.
I think I am having a hardtime linking these two statements due to my lack of understanding of index notation.
-First of all, [itex]\hat{I}[/itex] is a 3x3 matrix right? So [itex]\hat{A}[/itex] must be a 3x3 matrix?
-I am told α=1,2,3 = x,y,z. I assume then that β=1,2,3=x’,y’z’?; as we are linking different coordinate systems as we are liking inertia tensors representing axes of rotation passing through different origins.
-From the first term of Aαβ, I conclude that α=β or this term would be equal to zero. Looking at **, there is clearly no negative , so this must be the case.
-Most importantly, when we deduce this corollary – parallel axis theorem – from the displaced axes theorem, have we gone from [itex]\hat{A}[/itex]which is 3x3, to a 1x1 matrix? So in this case, I am looking to attain A – in matrix form – as a 1x1 matrix.
-Trying to attain this expression –looking at what I need to get to – it appears that RcαRcβ =0. I’m not sure why this would be – probably again due to my lack of understanding – would it be something along the lines of generally looking at 6 axes in total in the displaced axes theorem – 3 in each coordinate system – but only 2 axes in the parallel axes theorem – 1 in each coordinate system?
Many thanks to anyone who can shed some light on this, really appreciate it !
I have the displaced axes thorem stated in this form:
[itex]\hat{I}[/itex]=[itex]\hat{I}[/itex]com+M[itex]\hat{A}[/itex]
-Where Rc is the position of the centre of mass position-
-Where [itex]\hat{}[/itex]is the inertia tensor of a rigid body wrt to rotations about the origin, [itex]\hat{}[/itex]com with respect to rotations about its COM.
-[itex]\hat{}[/itex]can be represented as a matrix, the elements of which are determined by the elements of the COM position vector:
Aαβ=Rc^2δαβ - RcαRcβ
And the parallel axes theorem stated as :
I=Icom+Md^2 **
- Where d is the distance of parallel axis from the axis of roation passing through the COM.
I think I am having a hardtime linking these two statements due to my lack of understanding of index notation.
-First of all, [itex]\hat{I}[/itex] is a 3x3 matrix right? So [itex]\hat{A}[/itex] must be a 3x3 matrix?
-I am told α=1,2,3 = x,y,z. I assume then that β=1,2,3=x’,y’z’?; as we are linking different coordinate systems as we are liking inertia tensors representing axes of rotation passing through different origins.
-From the first term of Aαβ, I conclude that α=β or this term would be equal to zero. Looking at **, there is clearly no negative , so this must be the case.
-Most importantly, when we deduce this corollary – parallel axis theorem – from the displaced axes theorem, have we gone from [itex]\hat{A}[/itex]which is 3x3, to a 1x1 matrix? So in this case, I am looking to attain A – in matrix form – as a 1x1 matrix.
-Trying to attain this expression –looking at what I need to get to – it appears that RcαRcβ =0. I’m not sure why this would be – probably again due to my lack of understanding – would it be something along the lines of generally looking at 6 axes in total in the displaced axes theorem – 3 in each coordinate system – but only 2 axes in the parallel axes theorem – 1 in each coordinate system?
Many thanks to anyone who can shed some light on this, really appreciate it !
Last edited: