how to accurately model a juggling club

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how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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Re: how to accurately model a juggling club

Postby 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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