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Moving a ball with constant velocity

I tried to make a box of balls where the balls move with constant velocity. The shouldn't slow down when they collide with each other. I think I have set all properties right but it didn't work and after 30s all of the balls stopped to move.

The box is set like this:

    self.physicsBody = SKPhysicsBody(edgeLoopFromRect: self.frame)
    self.physicsBody.dynamic = false
    self.physicsBody.restitution = 1
    self.physicsBody.friction = 0

The balls are set like this:

enter image description here

Is this a bug of the physics engine or am I missing something?

Miaucl
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  • I solved the problem with the `update:` method to set the velocity but this is very expensive to do it every frame for every ball. Is there a better solution? – Miaucl Jun 17 '14 at 08:24

2 Answers2

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If you want them to have a constant velocity all the time, no change at all, you're going to have to set their velocity to a fixed-length vector in SKScene update. Physics engines aren't designed to adhere strictly to the conservation of energy law ... or one could argue that some energy is dissipated through heating the device. ;)

General principle for keeping the same direction but adjusting vector length/speed to a fixed value (pseudo-code):

CGPoint velocity = somePhysicsBody.velocity;
velocity = normalized(velocity);
velocity = multiply(velocity, desiredSpeed);
somePhysicsBody.velocity = velocity;
CodeSmile
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  • In this case my `update:` method is the fastest solution. An improvement could be when I make a own `updateVelocities:` and call it ever 100ms or or whatever what provides a constant moving impression. – Miaucl Jun 17 '14 at 08:41
  • What would this be in swift? I am having a hell of a time converting it and having it work. – Siriss Jan 04 '15 at 17:03
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In addition to Steffen's (LearnCocos2D) answer, the methods mentioned in the pseudo-code can be found in this very good Ray Wenderlich tutorial. 

static inline CGPoint rwAdd(CGPoint a, CGPoint b) {
    return CGPointMake(a.x + b.x, a.y + b.y);
}

static inline CGPoint rwSub(CGPoint a, CGPoint b) {
    return CGPointMake(a.x - b.x, a.y - b.y);
}

static inline CGPoint rwMult(CGPoint a, float b) {
    return CGPointMake(a.x * b, a.y * b);
}

static inline float rwLength(CGPoint a) {
    return sqrtf(a.x * a.x + a.y * a.y);
}

// Makes a vector have a length of 1
static inline CGPoint rwNormalize(CGPoint a) {
    float length = rwLength(a);
    return CGPointMake(a.x / length, a.y / length);
}

They use CGPoint as the parameters and return value, but this can be easily converted to use CGVector.

These methods are very useful for physics calculations, and you will find use for them at many points. It is best to keep these methods in a separate header file, and use them in the code throughout your project.

ZeMoon
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