A place to discuss everything related to Newton Dynamics.
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by MrPrezident » Tue Oct 16, 2018 5:33 pm
I'm trying to somewhat accurately model the motion of a juggling club using the MSPhysics SketchUp plugin. Juggling clubs are actually partly hollow like this:
https://s.hswstatic.com/gif/juggling-19.jpg. This obviously has an effect on the center of gravity and moment of inertia. Is there some way to handle this? Also, is there a way to specify different friction coefficients etc on different parts of the club?
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MrPrezident
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by Julio Jerez » Tue Oct 16, 2018 6:06 pm
you can model that very accurately with newton, but you need to put some work, there is not a single function that will do it.
here are some ideas.
1 you need to make the collision shape out of a compound.
2 you can make three or four sections.
3 to calculate the inertia on a thin hollow object, you can make two compound one large than the other and both with the same center of mass.
the the inertia of the hollow shape is simply the difference of the tow inertia.
then you set the inertia using the direct funtionality
and that should do it.
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Julio Jerez
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by Julio Jerez » Tue Oct 16, 2018 6:13 pm
there are tricks that can do the same job without the complications.
basically the reason they make the shape hollow is that they want the object to resist rotation when they spin it in the air. kind of like ballums, they have all the mass distribution on the surface. based on that, you do not even have to make two shape, a simple one will do, them you read the inertia and make it larger without changing the mass. that will yield the exact same result.
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by MrPrezident » Wed Oct 17, 2018 6:28 pm
Ok. I see. What about changing friction coefficients on different parts of the club. The knob and end caps are rubber, whereas the rest of the club is plastic. In my simulation, the club slides in one direction on its side (where the friction is mostly the plastic parts), and then it slides against the end cap before launching in the air.
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MrPrezident
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by Julio Jerez » Wed Oct 17, 2018 6:53 pm
Newton does not have per shape matrial only per bodies pairs,
but you can overload the contact callback and there for each contact you can override the friction. base of the sub shape or the contact location.
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by MrPrezident » Wed Oct 17, 2018 7:40 pm
Would that work if the club is sliding on a surface where both the plastic and the rubber parts of the club are contacting the surface at the same time?
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by MrPrezident » Wed Oct 17, 2018 7:42 pm
Also, I'm confused on the terminology. Isn't a compound shape made up of multiple bodies?
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by Sweenie » Thu Oct 18, 2018 2:15 am
No, a compound is a single body but with a hull combining two or more collision shapes.
Technically a compound could contain one collider but that would be meaningless.
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