Centripetal Force

GoldenFrog

Hello everyone!

PinkSkink here. I'm trying to figure out how to get around the "bounding box" that surrounds objects in SketchyPhysics. I need to be able to put a small ball into an ice-cream cone shape. But whenever I try to do so, it doesn't actually go into the cone, because it is stopped by the bounding box. Any way to change this? Also, does anyone know if SketchyPhysics does centripetal force? In other words, when I spin the cone, will the ball fly out? Or do I need to try another physics simulator?

Thanks you,

--PinkSkink

Mr.K.1

Actually made something like this once, so I know it's pretty simple.
Just can't be done with SP shapes directly, those don't have holes.

Best way to go at it is with boxes in this case.

first create you cone for measurement
then create the box on the side of the cone, it should be the same size as one of the circle lines
rotate the box so it fits the cone angle
extend the box to the desired height
select the box, then the rotate tool, now hit CTRL(to copy the box) click in the center of the cone, and move the copied box to the next circle line, then type in *23 and hit Enter(to make 23 boxes, one for each cone section)
group all the boxes into one group, and you have proper collision cone

I used it as a ball lift, the swirling cone would start to rotate the ball and with it pushing on the walls it would slowly climb towards the top.
So yes, there is centripetal and centrifugal force.

GoldenFrog

Great, thanks! I actually got this to work in Blender too, but I can't seem to figure out how to get real-life units working. Does SketchyPhysics have a way to measure the speed of the cone's rotation, and convert it to m/sec or ft/sec?

Mr.K.1

Well you can display the velocity of an object by putting this code into it's Script field:
ontick{ vector = Geom::Vector3d.new( getVelocity ) logLine(vector.length.to_s) }
This takes the velocity vector and displays it's length.
Because SU is based on an inch system the output will be inches/frame, you could convert this into any other unit, but I would consider this as an abstract speed only to be compared with others in the model, I doubt the speeds in real world physics are comparable.

GoldenFrog

Well, Inches per frame is a start. Perhaps you can specify how long each frame is? Because there is an option under Plugins>SketchyPhysics>Physics Settings that allows you to change the framerate. The default framerate is 3. 3 whats? Perhaps it is a real life unit?

Thanks,

--PinkSkink

Mr.K.1

Actually to correct myself, that code will put out whatever units you have set in your model(meters, inches, centimeters,...), this being due to SU converting units with their built in classes (points, vectors,...)

Now the frame length depends entirely on your models framerate, best way to look that up is opening the Ruby console then running SP for about 500 frames and resetting, the readout should get you a fairly accurate FPS average.
Then just multiply the average fps with speed and you got a unit per second value.

The framerate setting in Physics Settings is a bit misleading because it has no direct connection to frames per second, what it defines is how many physics calculation cycles will be made before the model refreshes.
So the default 3 means all the collisions, velocities, torques, positions,... will be calculated 3x and then the models will move.
Setting it higher can serve as a fast forward feature, because all the objects will move by more iterations between frames.

GoldenFrog

@unknownuser said:

Well you can display the velocity of an object by putting this code into it's Script field:
ontick{
vector = Geom::Vector3d.new( getVelocity )
logLine(vector.length.to_s)
}

I tried selecting "OnTick" for my cone, and putting the script you wrote into the blue box that appeared below it. But it keeps saying the formula does not compile. am I doing something wrong?

Thanks,

--PinkSkink

CPhillips

Put the code in the Scripted field. Not the ontick field.

GoldenFrog

I hate to be so bothersome, but I can't seem to find this scripted field anywhere. Is it in the UI for the cone? Perhaps you could give me a screenshot?

Sorry about being so annoying!

--PinkSkink

GoldenFrog

Oh! Thanks, Jay, I need to update my Sketchyphysics, then.

--PinkSkink

GoldenFrog

Hi everyone!

I downloaded the latest version of SketchyPhysics, and everything works great now. I have only one problem. For things to work, my cone must be about a yard in diameter. I actually need it to be more like a few inches. But when I make it that small, the joints and things don't seem to work. It seems like SketchyPhysics is unable to sense certain objects when they are below a certain size. Is there a setting I can change to fix this?

Thank you,

PinkSkink

CPhillips

It is a limitation of the physics engine.

There is a setting in the plugins->Sketchy Physics settings dialog that will allow you to adjust world scale. It is still tricky tho. Best to make models big enough so that they work with the default settings.

GoldenFrog

I see. Ok, thanks.

--PinkSkink

GoldenFrog

Just a little update.

Everything is working great, including the worldscale, which i did get to work without glitching. However, the velocities needed to expel the ball seem about 10x too big. I'm wondering, in SketchyPhysics is the default acceleration due to gravity 9.8 m/sec^2(32 ft/sec^2), or is it something else?

--PinkSkink

CPhillips

That is more or less what gravity is. All other units are not in any sort of real world scale. If 10x works then its "right".

GoldenFrog

Interesting. Thanks very much!

--PinkSkink