Re: [sympy] Sympy.physics.mechancis: RigidBody // inertia

[Peter Stahlecker]

Dear Jason,

There is nothing wrong with *kinetic energy*, at least I could not find
anything!

I programmed a 3D ball rolling on a flat surface.
The total energy is constant, no matter where the center of mass is.

Sorry about the commotion I created!

NB:
I use solve_ivp to do the integration.
I have to set max_step = 0.01 to get a constant total energy. If I do not
do this, it is not constant.

Peter

On Sat 25. Jun 2022 at 18:15 Peter Stahlecker 
wrote:

> Dear Jason,
>
> Thanks a lot!
> It seems to be my program, rather than *kinetic energy*.
>
> The sample which did not work was my ‚Wackelstein‘, where there seem to be
> many other issues, the geometry is probably too difficult for me.
>
> I tried it on my 2D *rolling disc on an uneven street*, where the
> geometry is much simpler. There the total energy is constant also if the
> center of mass != geometric center.
> It seems to run into numeric difficulties ( a different issue), but
> outside those difficulties the total energy is constant.
>
> Better my program is wrong than *kinetic energy*!  :-)
>
> Thanks again!
>
> Peter
>
>
> On Sat 25. Jun 2022 at 14:26 Jason Moore  wrote:
>
>> Peter,
>>
>> You should be able to provide the inertia (I, P) about a point P other
>> than the mass center of the rigid body. So in your code "mass center" does
>> not have to equal "P". But, I never really do that so it could be that the
>> underlying code doesn't apply the parallel axis theorem correctly. It isn't
>> a feature that is likely used much, if at all. If you have an example that
>> shows it doesn't work, then providing that on the SymPy issue tracker would
>> be helpful so we can find and fix the bug.
>>
>> Jason
>> moorepants.info
>> +01 530-601-9791
>>
>>
>> On Thu, Jun 23, 2022 at 1:44 PM Peter Stahlecker <
>> peter.stahlec...@gmail.com> wrote:
>>
>>> I = me.inertia(A, iXX….) gives the inertia in the (normally) body  -
>>> fixed frame A
>>> Body = Me.RigidBody( ‚Body‘, mass center, frame, mass, ( I, P))
>>>
>>> My question: Does RigidBody ‚assume‘, that mass center = P ?
>>>
>>> Reason behind my question:
>>>  For regular homogenious bodies, iXX, IYY, etc are often known relative
>>> to the geometric center of the body.
>>> If the mass center is not equal to the geometric center of the body,
>>> would RigidBody know, that the inertia is relative to the geometric center?
>>>
>>> In my program, the kinetic energy comes out right, if I set mass center
>>> = geometric center, but incorrect if I do not.
>>> Therefore, I wonder, whether there is a mistake in my program, or
>>> whether RigidBody assumes that P = mass center.
>>>
>>> Thanks a lot for any help!
>>>
>>>
>>> --
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>>
>>
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>>> https://groups.google.com/d/msgid/sympy/0d3101eb-1fe7-4c24-a125-bab8c8a58145n%40googlegroups.com
>>> 
>>> .
>>>
>> --
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>> 
>> .
>>
> --
> Best regards,
>
> Peter Stahlecker
>
-- 
Best regards,

Peter Stahlecker

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Re: [sympy] Sympy.physics.mechancis: RigidBody // inertia

[Peter Stahlecker]

Dear Jason,

Thanks a lot!
It seems to be my program, rather than *kinetic energy*.

The sample which did not work was my ‚Wackelstein‘, where there seem to be
many other issues, the geometry is probably too difficult for me.

I tried it on my 2D *rolling disc on an uneven street*, where the geometry
is much simpler. There the total energy is constant also if the center of
mass != geometric center.
It seems to run into numeric difficulties ( a different issue), but outside
those difficulties the total energy is constant.

Better my program is wrong than *kinetic energy*!  :-)

Thanks again!

Peter


On Sat 25. Jun 2022 at 14:26 Jason Moore  wrote:

> Peter,
>
> You should be able to provide the inertia (I, P) about a point P other
> than the mass center of the rigid body. So in your code "mass center" does
> not have to equal "P". But, I never really do that so it could be that the
> underlying code doesn't apply the parallel axis theorem correctly. It isn't
> a feature that is likely used much, if at all. If you have an example that
> shows it doesn't work, then providing that on the SymPy issue tracker would
> be helpful so we can find and fix the bug.
>
> Jason
> moorepants.info
> +01 530-601-9791
>
>
> On Thu, Jun 23, 2022 at 1:44 PM Peter Stahlecker <
> peter.stahlec...@gmail.com> wrote:
>
>> I = me.inertia(A, iXX….) gives the inertia in the (normally) body  -
>> fixed frame A
>> Body = Me.RigidBody( ‚Body‘, mass center, frame, mass, ( I, P))
>>
>> My question: Does RigidBody ‚assume‘, that mass center = P ?
>>
>> Reason behind my question:
>>  For regular homogenious bodies, iXX, IYY, etc are often known relative
>> to the geometric center of the body.
>> If the mass center is not equal to the geometric center of the body,
>> would RigidBody know, that the inertia is relative to the geometric center?
>>
>> In my program, the kinetic energy comes out right, if I set mass center =
>> geometric center, but incorrect if I do not.
>> Therefore, I wonder, whether there is a mistake in my program, or whether
>> RigidBody assumes that P = mass center.
>>
>> Thanks a lot for any help!
>>
>>
>> --
>> You received this message because you are subscribed to the Google Groups
>> "sympy" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to sympy+unsubscr...@googlegroups.com.
>
>
>> To view this discussion on the web visit
>> https://groups.google.com/d/msgid/sympy/0d3101eb-1fe7-4c24-a125-bab8c8a58145n%40googlegroups.com
>> 
>> .
>>
> --
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> 
> .
>
-- 
Best regards,

Peter Stahlecker

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Re: [sympy] Sympy.physics.mechancis: RigidBody // inertia

[Jason Moore]

Peter,

You should be able to provide the inertia (I, P) about a point P other than
the mass center of the rigid body. So in your code "mass center" does not
have to equal "P". But, I never really do that so it could be that the
underlying code doesn't apply the parallel axis theorem correctly. It isn't
a feature that is likely used much, if at all. If you have an example that
shows it doesn't work, then providing that on the SymPy issue tracker would
be helpful so we can find and fix the bug.

Jason
moorepants.info
+01 530-601-9791


On Thu, Jun 23, 2022 at 1:44 PM Peter Stahlecker 
wrote:

> I = me.inertia(A, iXX….) gives the inertia in the (normally) body  - fixed
> frame A
> Body = Me.RigidBody( ‚Body‘, mass center, frame, mass, ( I, P))
>
> My question: Does RigidBody ‚assume‘, that mass center = P ?
>
> Reason behind my question:
>  For regular homogenious bodies, iXX, IYY, etc are often known relative to
> the geometric center of the body.
> If the mass center is not equal to the geometric center of the body, would
> RigidBody know, that the inertia is relative to the geometric center?
>
> In my program, the kinetic energy comes out right, if I set mass center =
> geometric center, but incorrect if I do not.
> Therefore, I wonder, whether there is a mistake in my program, or whether
> RigidBody assumes that P = mass center.
>
> Thanks a lot for any help!
>
>
> --
> You received this message because you are subscribed to the Google Groups
> "sympy" group.
> To unsubscribe from this group and stop receiving emails from it, send an
> email to sympy+unsubscr...@googlegroups.com.
> To view this discussion on the web visit
> https://groups.google.com/d/msgid/sympy/0d3101eb-1fe7-4c24-a125-bab8c8a58145n%40googlegroups.com
> 
> .
>

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