# d13

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1. ## Rollercoaster

Hi Everyone!! I'm trying to build a simple interactive rollercoaster to demonstrate some physics concepts. I came across this demo, which looks like a good start: https://www.babylonjs-playground.com/#92EYG#21 To turn this into a rollercoaster, a force would need to be applied to the box as it's climbing the first hill, and then gravity/potential energy would have take over. Does anyone know what steps I should take to start implementing this? Thanks! d13.
2. ## Move DOM objects position relative to 3D Objects

... or you're required to meet the W3C Web Content Accessibility Guidelines (https://www.w3.org/WAI/intro/wcag)
3. ## Getting the list of border vertices

This is a fascinating problem!!! Does no one here have any ideas on how to solve it?
4. ## How to point particle system away from mesh?

@aWeirdo Awesome, that works!! I hadn't considered updating the direction... and normalizing it! I also got it working by using `halleysComet.lookAt(sun)` in the update function (`halleysComet` is the emitter), and by initializing the `cometTail` particle system with this direction: cometTail.direction1 = new BABYLON.Vector3(3, 2, 16); cometTail.direction2 = new BABYLON.Vector3(-3, -2, 16); Now it always points away from the sun. Thanks so much everyone!!!!
5. ## How to point particle system away from mesh?

`lookAt` not a function .... oh... but it does work if I use lookAt on the emitter!
6. ## How to point particle system away from mesh?

This effect works pretty well, I just have to find out how to orient the tail away from the sun: cometTail.direction1 = new BABYLON.Vector3(-4, -8, 3); cometTail.direction2 = new BABYLON.Vector3(4, -8, -3);
7. ## How to point particle system away from mesh?

Here's a playground with a simplified version of the problem: http://www.babylonjs-playground.com/#1UGDQC#20 (I can reproduce the bunching-up effect if I use `ps.direction1 = -vectorSunToMoon`)
8. ## How to point particle system away from mesh?

Thanks @Amarth2Estel! Obtaining the vector between the sun and valley's comet works! But I'm not sure how to apply that vector to the comet tail emitter let vectorSunToHalley = halleysComet.position.subtract(sun.position); cometTail.direction1 = vectorSunToHalley Does that seem like it should work? In my program it has the result of bunching the particles up into a cube:
9. ## How to point particle system away from mesh?

Hi Everyone! I'm working on a solar system model and am currently trying to create an accurate-ish simulation of Halley's Comet. The comet tail aways points away from then sun. So, I have a two part question for any experienced users out there: 1. How can I obtain the vector between 2 meshes (in this case Halley's Comet and the Sun)? 2. Which properties do I need to set in the ParticleSystem object to make the particles emit in the direction of that vector?
10. ## solved Line Visibility

@Wingnut Thank you Mr, Wingnut sir, `isVisible` worked perfectly!!!
11. ## solved Line Visibility

Does anyone know if it's possible to set the visibility of lines created with `MeshBuilder.CreateLines`? (The reason I ask is I need to toggle lines on and off.)
12. ## Drawing an ellipse from 3 points

Yes That's correct! Pluto's weirdness is one of the reasons it was demoted as a planet.
13. ## Drawing an ellipse from 3 points

I'm scaling down by a factor of 100,000,000, but use a slider to scale up to 1:1. No, I haven't Celest System
14. ## Drawing an ellipse from 3 points

Arrgh, those calculations turned out not to be accurate enough It looks like I'm back to the drawing board again. I'll need to do one of these two things: 1. Calculate the ellipse dynamically by extrapolating from a few points of captured planet position data. 2. Figure out how to calculate an accurate ellipse using this data: orbit : { base : {a : 17.83414429 * ns.AU , e : 0.967142908, i: 162.262691, M : 360 * (438393600 / (75.1 * ns.YEAR * ns.DAY)), w : 111.332485, o : 58.420081}, day : {a : 0 , e : 0, i: 0, M : (360 / (75.1 * 365.25) ), w : 0, o : 0} } ... Can anyone make sense out of that?
15. ## Drawing an ellipse from 3 points

@Pryme8 Yes, I've been using it to move the planets and it works great - I just have no idea how to use that data to display pre-renderd orbit lines.