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In Blender, i have a particle system with random size of particle element. How can i simulate so the bigger particle will fall faster than smaller particles. The idea is to have different 'drag' effect to each particles where the bigger the particle, the smaller the 'drag' will affect it or in simple word : the bigger particle will easily fight air resistence so it will fall faster. Is it any parameter to set to handle this task ?

So far i can think only to create multiple particle system with different 'drag' value then combine those particles systems.

edit : additional notes... i'm not trying to simulate the ideal physical world, but real world. in ideal world v(speed) is not affected by mass, but by acceleration (a) or in this case gravity (g). v=v0+gt. and since the g is constant and no v0 (initial velocity) then the V will be the same -> in ideal world bigger and smaller fall the same, the only different is the energy need to stop them. So i want to simulate the real world where the bigger mass will 'easily' fight air resistance as its force (f=ma) is bigger so the air don't have enough energy to stop them. simple example : the real world like when bullet hits a wall and when debris is falling , the big chunk will fall faster (air can't hold it) while the smaller debris just need to float before hits the ground. The story will be different if there's no air.

Also what i mean by bigger particle is bigger in term of mass not just the volume. Bigger volume does not neccessarily have bigger mass. As in my example : bigger chunck of wall hit debris. Also i'm just curious if this can be done in blender simulation, i know for my case i can solve it with other way, but i'm just want to explore how far this blender sim can go.

Thanks

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  • $\begingroup$ I'm not exactly sure what your end goal is but you might be able to use Soft Bodies. You can apply different masses to them. $\endgroup$ – Dontwalk Jan 10 '18 at 19:33
  • $\begingroup$ @Dontwalk certainly it could work, but OP is talking about particles so we can loosely assume that he has in mind a lot of them. In this case using soft bodies would be highly impractical resources wise. His idea about using multiple particles system (you can assign to them different mass and also a drag) seems the best way to do this. My answer contains information about one particle system. $\endgroup$ – cgslav Jan 10 '18 at 21:24
  • $\begingroup$ @LukeD - I CLEARLY said I did not know what his "end goal" was. I was just offering up a "possible" alternative NOT in the "Answer" section. If it had to be particle system based I too would use multiple particle systems. $\endgroup$ – Dontwalk Jan 10 '18 at 21:29
  • $\begingroup$ @Dontwalk I also clearly stated that we can "assume" taking in consideration before mentioned particle system and I didn't said that your alternative is invalid, just that it is impractical. $\endgroup$ – cgslav Jan 10 '18 at 21:31
  • $\begingroup$ @LukeD - I NEVER assume anything. Just trying to help out... In addition maybe he or another reader might not have known that rigid bodies could have different masses and could use that approach in another situation. Knowledge is power... $\endgroup$ – Dontwalk Jan 10 '18 at 21:43
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If you don't want a real world physics involved You can do that easily with Particle Texture.

  1. Let assume that you want the particles to move downwards (-Z) emitted from a Plane.
  2. Set the particle system Field Weights - > Gravity to 0
  3. Set the direction of the particles to Normal : -3
  4. Optionally, set the all the particles to be emitted from the Start frame
  5. Set the size of the particles to some value. You don't need to randomize the size with the Random option at this point.
  6. With the Emitter object selected go to Texture tab in Properties editor. Make sure you're in Particles Textures .
  7. Create a new Texture.
  8. Make the new texture to be of Type: Clouds and for Basis chose Cell Noise. Let Coordinates be Generated.
  9. And finally in Influence deselect all and turn on Size in General and Velocity in Physics.

In this way the color of the texture will control the size and velocity of the particle by it's initial location. Lighter values will increase the size and velocity. Darker values will do the opposite.

Particle Texture setup

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This will be kinda 'nay answer'. I'm also not physic expert or amateur, not even physics noob so don't take my word for it.

I've read those:

First of all your statement is invalid.

Bigger particles will not fall faster and certainly can't fight air resistance better than smaller one. That's because bigger ones have bigger size thus air resistance is greater - assuming they have the same mass.

They could fall faster though with presumption that they have significantly greater mass than small ones. Bowling ball vs. ping pong ball case.

Why we can't do this with Blender Particles?

As you can see in below example smaller ones are falling significantly faster than the bigger ones. This is because by default Blender keep the same mass for every particle so the difference is only due to particle size. Bigger size = more air resistance.

We have option Multiply mass with size, but after checking it only thing you can see is that this difference is just smaller. This is because mass will not be significantly different between particles. Really quick calculation from my example:

Mass * Size

0.01*0.3 = 0.003

0.01*0.2 = 0.002

0.01*0.1 = 0.001

All of this is because we can't assign different mass per particle (size/group) in Blender other than multiplying mass with size.

gifsettings

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  • $\begingroup$ Thanks, but i'm not trying to simulate the ideal world. Pls check my first post , i just edit to add some notes. since i can't paste it here, it's too long. $\endgroup$ – andio Jan 11 '18 at 5:55
  • $\begingroup$ @andio real world case is described when I wrote about Multiply mass with size. Sadly the difference between mass vs. air resistance is to low to simulate effect you want to achieve. $\endgroup$ – cgslav Jan 11 '18 at 7:21
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I am not sure how many particles you want to use but an option would be to add a Rigid Body World and use Rigid Bodies.

enter image description here

You can adjust the mass of the Rigid Bodies separately.

enter image description here

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