# Physically accurate falling behaviour

Hi,
In this example I applied a mass of 100 to the red sphere (head) and a mass of 10 to th body and joined them together using fixed joint. When falling, I assume a physically correct behavior would be the object will tilt and the red sphere will hit the ground first. Is it possible to achieve this?

Hello @qazs! Iâ€™ve shared your post with the dev team for further assistance.

You will probably need to simulate air resistance somehow if you want to achieve this - though I am uncertain if even that might get the behavior you are seeking. I would do a real world test, if you can make a similar object, and observe the behavior.

Absent air resistance or other forces, all objects fall at the same speed regardless of mass.
(See, for example: If You Drop A Feather And A Metal Cube In A Vacuum Chamber Will They Hit At The Same Time? - YouTube)

This could also be hacked by adding rotational velocity as well.

How do you add a joint? I have looked at the samples and not figured that out.

Select two rigid bodies, then Create->Physics->Joint

I did that, but they do not move together. If I move one the other stays where it is. I would think a joint would be connected.

I must not understand the Joint part.

OK, I got further, but now I get an error: Cannot create a joint between static bodies.

It worked fine with 1 then I made 20 copies and I got this error.

Looks like the bodies you are trying to connect are not dynamic bodies. The joint connects either two bodies where at least one has to be a dynamic body, or it is a joint to the world where only one body is provided. Then the body must be a dynamic body.

If you crate two dynamic bodies and connect them with a fixed joint then moving one of the bodies will make sure that the bodies stay connected. Like I am trying to do that in the attached video.

I donâ€™t understand the term dynamic body. I know how to add physics - rigid body with colliders preset. Is that what you mean? I just tried what you did and it doesnâ€™t work for me.

Edit: I just figured out it wasnâ€™t working because I wasnâ€™t running the simulation. So the joint is not active during design mode?
That is what was throwing me off I think.

Thank you

Ah yes, simulation must be running (play needs to be pressed). Without play physics is not active.

1 Like

I wonder how to set or simulate air resistance force in Isaac gym or isaac sim ?