# Why does a rigid body with Convex Hull collision bounds not move in BGE?

I have an object whose physic is Rigid Body, and I set its collision bounds to Convex Hull (else it would collide with its own children) but now it doesn't move when I play, and doesn't even fall on the ground.
Movement is force applied to the object when I press the correct key (W, A, S, D or Space for jumping)
Rotation works fine though, I don't understand why this is happening.

An automatic turret that shoots "on sight" in the same scene is having the same problem

### Edit:

Here is the .blend file.

• The Collision Bounds describes the physics shape rather than the behavior. Physics Type (Rigid Body) describes the physics behavior, which sounds correct to me. I'm a bit confused why you physics object has children with enabled physics. That might be the source of your issues. – Monster Sep 1 '16 at 4:19
• actually it collides with a projectile that is instantiated on click event and might collide with the object (some kind of tank) and that's not expected... – joH1 Sep 1 '16 at 17:09
• You add an projectile (rigid body I guess) and this one should not collide with the tank object? Isn't that the purpose of a projectile? – Monster Sep 2 '16 at 4:31
• yes that's the point, but it collides on instantiation, with the parent tank, and that's not wanted – joH1 Sep 3 '16 at 9:09
• I suggest to emit the projectile outside of the tank e.g. shortly in front of the barrel. (You do not need to emulate real world physics). – Monster Sep 5 '16 at 6:30

## 1 Answer

Enable Physics at your material.

Hints (outside of the question):

• You named your objects and bricks - I like that. It was really helpful when looking at the scene.
• The meshes have way too much faces for a game. They do not need that many for that little detail. I strongly suggest to remove the subsurf modifier(s). You can use smooth shading instead.
• The floor creates projectiles. This was really distracting when analyzing your file. I would not expect a floor to behave this way. A separate object (e.g. called RandomProjectileSpawner) could do that job. You can add/remove it without loosing the scene's floor. You could even have more than one.
• The dimensions of the projectile's physics shapes are way too large. They collide with the physics mesh of object Ship right after creation. I suggest to enable collision bounds at the physics tab on the projectiles. A box shape should be good enough.
• Be aware the physics engine has problems with small travelling objects. It might happen that they do not collide with another face and suddenly are inside of an object, or that a collision does not get registered.

I hope it helps

• thanks, totally works now! (though some shells go right through the crate now but it doesn't truly matter ^^) – joH1 Sep 7 '16 at 18:24