affectedbygravity in SceneKit? - ios8

Question

I want to make a floating ball.
In spritekit I do physicsbody.affectedbygravity=NO;
But what have I do in scenekit?
I tried to set mass to zero, but dynamicbody turns to static: it not moved by gravity, nor by collision. My code is
// add ball
SCNNode *ball = [SCNNode node];
ball.geometry = [SCNSphere sphereWithRadius:5];
ball.geometry.firstMaterial.locksAmbientWithDiffuse = YES;
ball.geometry.firstMaterial.diffuse.contents = #"ball.jpg";
ball.geometry.firstMaterial.diffuse.contentsTransform = SCNMatrix4MakeScale(2, 1, 1);
ball.geometry.firstMaterial.diffuse.wrapS = SCNWrapModeMirror;
ball.physicsBody = [SCNPhysicsBody dynamicBody];
ball.physicsBody.restitution = 0.9;
//ball.physicsBody.mass=0; //here makes a ball statical and not moving
ball.physicsBody.allowsResting = YES;
[[scene rootNode] addChildNode:ball];
UPD this not helps too - ball falls slowly.
- (void)renderer:(id<SCNSceneRenderer>)aRenderer didSimulatePhysicsAtTime:(NSTimeInterval)time{
[ball.physicsBody applyForce:SCNVector3Make(0, 9.8, 0) atPosition:SCNVector3Make(0,0,0) impulse:NO]; //this
ball.physicsBody.velocity=SCNVector3Make(0,0,0); //or this
}

Answer 1

You can turn off gravity by setting scene.physicsWorld.gravity to zero.
However If you want some objects to be affected and some not you need to add a [SCNPhysicsField linearGravityField] and configure the categoryBitMask to make it affect the nodes you want.

Answer 2

I found partial solution and some fun bug.
This makes ball stay unmovable until collision and "float" after that
// .... the same code above in creation
ball.physicsBody.mass=1;
[[scene rootNode] addChildNode:ball];
ball.physicsBody.mass=0; //important right after creating!!! or it become bodyless after changing mass to 1
and release it if contact. If you apply up force equal to gravity, it will be floating, slowly falling down. Speed of falling may be damped, of cause.
// physics contact delegate
- (void)physicsWorld:(SCNPhysicsWorld *)world didBeginContact:(SCNPhysicsContact *)contact{
if (contact.nodeA.physicsBody.type == SCNPhysicsBodyTypeDynamic && contact.nodeA.physicsBody.mass==0)
contact.nodeA.physicsBody.mass=1;
else
if (contact.nodeB.physicsBody.type == SCNPhysicsBodyTypeDynamic && contact.nodeB.physicsBody.mass==0)
contact.nodeB.physicsBody.mass=1;
}