what's gravity got to do with it.?

Replies to This Discussion

Permalink Reply by djordje on March 21, 2013 at 4:13pm

Hi Roderick,

You are using Rhino 5, so I could not open your .3dm file.
But from what I see on photo and by opening your .gh file - the catenary does begin and end in both intersection points of the circle and the line. You can see that clearly in your screenshot above, at the far right.

Gravity is essential for catenary curves, as they basically get their shape from their self-weight.
Convenient ability of the Grasshopper's Catenary component is ability to edit the gravity vector. That means that it does not always need to be pointed downwards (vector: 0,0,-1). But it can also point upwards (like in your case) or even any other direction. And the shape of the catenary curve will follow that vector direction.

Permalink Reply by djordje on March 21, 2013 at 4:21pm

Check the attached file.
Maybe that will help you to understand the gravity vector better.

Attachments:

• gravityquestion2.gh, 7 KB

Permalink Reply by roderick read on March 21, 2013 at 4:24pm

Thanks Djordje, Yes I can confirm the start and end points of the catenary are exactly on the intersection points I specified....

The problem however is noticable when you highlight the last endpoint in the definition...

when I set the force defining vector sliders to the position shown...and only at this point...

the end points swap over

end of catenary is on left not right..... the catenary reverses direction momentarily.

it switches back to normal as soon as you move x even the smallest way either side.

Is this how it's meant to behave or is it error?

Permalink Reply by djordje on March 21, 2013 at 4:41pm

I see now, sorry.
Yes it might be some sort of a bug.

Do what Jesus told you.

Permalink Reply by David Rutten on March 21, 2013 at 5:00pm

Do what Jesus told you.

Or you'll make Jesus cry.

--

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Jesus Galvez on March 21, 2013 at 5:20pm

lol. I don't know, crying about it... but I might lose some sleep, so after fooling around with the equations (to no end...), I'm going to bed before I end up going down the rabbits hole. 

Cheers!

Permalink Reply by djordje on March 21, 2013 at 5:24pm

He was referring to the guy on the cross.
:)

Permalink Reply by Jesus Galvez on March 21, 2013 at 4:25pm

Hi Roderick,

Maybe it's a bug in the component's code. Try using the mathematical expression instead of the component. I think the parameter Gravity simply acts like a coordinate system transformation (rotation).

Permalink Reply by roderick read on March 22, 2013 at 7:48am

oh blimey Jesus, I havn't used maths that strenuously in ages... sent my head into a fog. in the end.

I think you are onto something suggesting the catenary component interpretation of reference frame may be odd.

A more detailed description of the issue may help shed light on the issue in the component.

When Z is negative .. as per normal gravity there is no problem. the start and end don't move no matter where x and y are in relation to each other.

When z,x&y are all positive and x>y the start is on one side. (right)

When z,x&y are all positive and x<y the start is on the other side. (left)

When z is positive and x&y are both negative, with x more negative than y, the start is on one side.(right)

When z is positive and x&y are both negative, with x less negative than y, the start is on the other side. (left)

When z is positive, x is positive and y is negative the start is on the left.

here's the weird one

When z is positive and x is negative and y positive.... it totally goes bonkers depending on the ratio of x and y.....

So it looks like some sort of trig quadrant framing analysis is a bit off...

makes a nice catenary looking curve however... but I could really do with these end points sitting still... any ideas?

Permalink Reply by djordje on March 22, 2013 at 9:28am

You can use either first or the second equation. This the example with the second one:

But with regular grasshopper component you control your catenary with end points fixed. Now it seems the "a" factor scales the whole curve, and changes the position of the end points along with it, too.

Attachments:

• gravityquestion3.gh, 4 KB

Permalink Reply by roderick read on March 22, 2013 at 4:01pm

Thanks Djordje,

That's cool to play with. I'm not sure how to get into the guts of the original grasshopper catenary component so as I could try alter my own version of it... any pointers what I should start to look at to edit it?

Permalink Reply by Jesus Galvez on March 22, 2013 at 5:20pm

Well the equation of the catenary is quite simply, from wikipedia:

y = a * cosh (x/a)

The question is what is "a" and figuring how it relates with the direction of gravity. 

My guess is that a is a parameter that has something to do with the relation between the horizontal and vertical tension at the anchor points. But I still don't know where to introduce gravity. Maybe here. 

Attachments:

• gravityquestion 1.gh, 9 KB

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