# Very simple question about mass, density of RBD

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I made some simulations with RBD and had some problem with density of objects.

so, first at all, I just made this simple scene and can't understand how does this sim goes.

At Gif, Left ball has 1 density, and Right one has 10000000 density.

but look, It looks very same with each ball :S

What should I add for this ??

I made house scene so I wanted to set up density of TV has high level, and some pictures on table has low level.

but It seems very same like that scene.

Help me !

Sorry for my poor English and Thanks for read

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In a physics class we dropped two balls in a vacuum. One was plumb and one was wood. They reached the floor at the same time because the force is different due to the density, but the acceleration applied is the same.

In Houdini it's the same. You can notice a big difference if you make collide the two spheres with other objects that are not static (as the ground floor) but active.

With different densities you'll see that the heavy sphere will move all other RBD objects while the sphere with low density won't move them.

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54 minutes ago, Andrea said:

In a physics class we dropped two balls in a vacuum. One was plumb and one was wood. They reached the floor at the same time because the force is different due to the density, but the acceleration applied is the same.

In Houdini it's the same. You can notice a big difference if you make collide the two spheres with other objects that are not static (as the ground floor) but active.

With different densities you'll see that the heavy sphere will move all other RBD objects while the sphere with low density won't move them.

Thanks for reply, but after this, I tested for it by moving box.

box push the balls and I expected that heavy ball will move more slower, but It didn't..

Only when I set up those density as 1 and 100000000000 I can see difference of it ..

That huge amount ..the difference just looked like plastic and plumb. It doesn't seemed 1 and 10000000000 of difference

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It's not about moving/free falling. The density it's about how much they are affected by other forces and how much their force affects other objects.

I think the easiest example where you can see the density working is by making the two spheres collide.

Another example could be as in the scene file attached.

For sure you don't need to have a huge difference like this

• 1

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Keep in my that by default you don't have most important influence - resistance of the environment (air resistance), you are playing only with vacuum environment and gravity, take Andrea's example and add drag with force scale 300 to see the effect. Heavy (brick) has less impact of drag, pushing himself through the air and falling faster than light (styrofoam).

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21 hours ago, Andrea said:

It's not about moving/free falling. The density it's about how much they are affected by other forces and how much their force affects other objects.

I think the easiest example where you can see the density working is by making the two spheres collide.

Another example could be as in the scene file attached.

For sure you don't need to have a huge difference like this

Thank you for your example, I've forgot that there are many resistance things in real world.

So, How could I match thoes things in real world ?

If there is 100kg ball in the scene, How much do I have to control its density ?

I can't estimate that issue in Houdini ,,,

Do I have to just see the simulation and control it ?

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In RBD packed object you can specify the exact mass for the object if you want.

There are also some density tables if you really need to replicate specific materials properties, but I think it's not needed for most of the cases. No one is going to notice that it's 101 kg instead of 100

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I think the main problem is heavy objects shouldn't move so much after they have hit the ground, and that is quite difficult to have any sort of control over.

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