SpriteKit make two force on a body with joint - objective-c

Question

The problem: I want balloons rise up in the air. They can collide with one another and they can rotate. With a gravitiy in upright direction that works fine.
But now I want to connect a basket to each balloon. Doing so will result in the basket "flying" up like the balloon. But in real life the basket should be "heavier" than the balloon, so it would always point downwards to earth.
How would I achieve that ?

Answer 1

One way to simulate a balloon with a basket is to set gravity to be in the downward direction to pull the basket toward the bottom of the scene and apply a buoyant force to the balloon to create lift. Here's an example of how to do that:
Define the constants
#define kRandMax 0x7fffffff
#define kNumBalloons 5
Create the balloons and baskets
self.physicsWorld.gravity = CGVectorMake(0, -3);
for (int i=0;i<kNumBalloons;i++) {
// Create a balloon near the bottom of the scene with a random x position
SKShapeNode *balloon = [SKShapeNode shapeNodeWithCircleOfRadius:16];
balloon.fillColor = [SKColor redColor];
balloon.name = #"balloon";
balloon.physicsBody.allowsRotation = NO;
CGFloat rand1 = arc4random_uniform(kRandMax) / (double)kRandMax;
// A value in (-view.frame.size.width/2, view.frame.size.width/2)
CGFloat xOffset = (rand1 - 0.5) * view.frame.size.width / 2;
balloon.position = CGPointMake (CGRectGetMidX(view.frame)+xOffset,
CGRectGetMinY(view.frame)+50);
balloon.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:16];
[self addChild:balloon];
// Create the basket
SKSpriteNode *basket = [SKSpriteNode spriteNodeWithColor:[SKColor brownColor] size:CGSizeMake(8, 8)];
basket.position = CGPointMake(balloon.position.x, balloon.position.y-24);
basket.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:basket.size.width/2];
[self addChild:basket];
// Connect the balloon and basket with an SKPhysicsJointPin
SKPhysicsJointPin *joint = [SKPhysicsJointPin jointWithBodyA:balloon.physicsBody
bodyB:basket.physicsBody anchor:balloon.position];
[self.physicsWorld addJoint:joint];
}
In the update method, apply a force to each balloon to create lift
-(void)update:(CFTimeInterval)currentTime {
// Apply buoyant force to all balloons in the scene
[self enumerateChildNodesWithName:#"//balloon" usingBlock:^(SKNode *node, BOOL *stop){
// Adjust force as needed
[node.physicsBody applyForce:CGVectorMake(0, 17.2)];
}];
}

Answer 2

Look into SKPhysicsBody documentations (can be found here).
Look into Defining a Body’s Physical Properties section.
Every physics body has properties (such as mass) that you want to play with, in order to get a more 'realistic' behaviour.
If you want a less general answer,
Post some code examples.
Good luck mate.

Answer 3

I have now a solution like this:
I create a forcefield going up
func createForceField(strength:Float) {
// add force field
for i in 1...9 {
let field = SKFieldNode.electricField()
field.physicsBody?.categoryBitMask = PhysikcsCategory.ForceField
field.position = CGPointMake(frame.size.width*0.1*CGFloat(i), -1000)
field.strength = strength
addChild(field)
}
}
Before starting the game I turn on gravity and forcefield
override init(size: CGSize) {
super.init(size: size)
// Set-Up forcefield
createForceField(0.1)
self.physicsWorld.gravity = CGVectorMake(0, -0.09)
startGame()
}
here I create the balloons with a rope and tag, the balloon is charged so it will react to the forcefield
func createBalloon(vokabel:Vokabel) {
let x = self.frame.width*0.2
let y = self.frame.height*0.2
// Balloon
var balloon = BalloonNode()
balloon.size = CGSizeMake(balloon.size.width*0.7, balloon.size.height*0.7)
balloon.position = CGPointMake(x-6,y)
balloon.name = "SPRITE"
balloon.vokabel = vokabel
balloon.score = 50
balloonLayer.addChild(balloon)
// Special Start
var specialStar = SKSpriteNode(imageNamed: "Star")
specialStar.size = CGSizeMake(balloon.size.width*0.7, balloon.size.height*0.7)
specialStar.hidden = true
specialStar.name = "STAR"
balloon.addChild(specialStar)
// Rope
let rope = SKSpriteNode(color: UIColor.blackColor(), size: CGSize(width: 2, height: 15))
rope.position = CGPointMake(x, y-balloon.size.height/2-rope.size.height/2)
rope.alpha = 0.2
balloonLayer.addChild(rope)
// Tag
var labelText = getLabelText(vokabel)
let tag = createTagWithText(labelText)
tag.position = CGPointMake(x, y-balloon.size.height/2-rope.size.height)
balloonLayer.addChild(tag)
self.physicsWorld.gravity=CGVectorMake(0.0, -0.3)
// physical bodies
balloon.physicsBody = SKPhysicsBody(rectangleOfSize: balloon.size)
rope.physicsBody = SKPhysicsBody(rectangleOfSize: rope.size)
tag.physicsBody = SKPhysicsBody(rectangleOfSize: tag.size)
// physical forces
balloon.physicsBody?.allowsRotation = false
balloon.physicsBody?.charge = 0.5
tag.physicsBody?.allowsRotation = false
tag.physicsBody?.linearDamping = 1.0
rope.physicsBody?.linearDamping = 1.0
// anchor Points
let anchorBalloonRope = CGPointMake(x, y-balloon.size.height/2)
let anchorRopeTag = CGPointMake(x, y-balloon.size.height/2-rope.size.height)
// create joints
let joint = SKPhysicsJointPin.jointWithBodyA(balloon.physicsBody, bodyB: rope.physicsBody, anchor: anchorBalloonRope)
let joint_rope_tag = SKPhysicsJointPin.jointWithBodyA(rope.physicsBody, bodyB: tag.physicsBody, anchor: anchorRopeTag)
// add joints to physic
self.physicsWorld.addJoint(joint)
self.physicsWorld.addJoint(joint_rope_tag)
// give it a horizontal push
balloon.physicsBody?.applyImpulse(CGVectorMake(10.0, 0.0))
// BalloonNode ( is just a layer, has no special position so it is the same coordinate system as the gamelayer)
let node = BalloonNode()
node.vokabel = vokabel
node.score = SCORE_STANDARD
node.name = "BALLOON"
node.rope = rope
node.tag = tag
}