I'm having trouble with challenge 2 shown on this page.

It's a Fruit Ninja clone type of game that we are creating and the challenge is...

Create a new, fast-moving type of enemy that awards the player bonus points if they hit it.

I was able to complete most of the challenge. I used the red penguin sprite from the Whack-a-Penguin project we did before, so that the player can tell the difference between the bonus penguins and the normal ones. I made the bonus penguins appear randomly, and even less frequently than the regular penguins or even bombs. I made it award 5 points instead of 1 when a player slices it. And I have made sure that the player won't lose a life if they miss one. (They are supposed to be a bonus afterall.) But I can't seem to figure out how to make them move faster than the regular penguins.

I know that it probably has something to do with this function where we create all of the sliceable enemies...

func createEnemy(forceBomb: ForceBomb = .random) {
        let enemy: SKSpriteNode

        var enemyType = Int.random(in: 0...6)

        if forceBomb == .never {
            enemyType = 1
        } else if forceBomb == .always {
            enemyType = 0
        }

        if enemyType == 0 {
            enemy = SKSpriteNode()
            enemy.zPosition = 1
            enemy.name = "bombContainer"

            let bombImage = SKSpriteNode(imageNamed: "sliceBomb")
            bombImage.name = "bomb"
            enemy.addChild(bombImage)

            if bombSoundEffect != nil {
                bombSoundEffect?.stop()
                bombSoundEffect = nil
            }

            if let path = Bundle.main.url(forResource: "sliceBombFuse", withExtension: "caf") {
                if let sound = try?  AVAudioPlayer(contentsOf: path) {
                    bombSoundEffect = sound
                    sound.play()
                }
            }

            if let emitter = SKEmitterNode(fileNamed: "sliceFuse") {
                emitter.position = CGPoint(x: bombFuseXPosition, y: bombFuseYPosition)
                enemy.addChild(emitter)
            }
        } else if enemyType == 6 {
            enemy = SKSpriteNode(imageNamed: "bonusPenguin")
            run(SKAction.playSoundFileNamed("launch.caf", waitForCompletion: false))
            enemy.name = "bonus"
        } else {
            enemy = SKSpriteNode(imageNamed: "penguin")
            run(SKAction.playSoundFileNamed("launch.caf", waitForCompletion: false))
            enemy.name = "enemy"
        }

        let randomPosition = CGPoint(x: Int.random(in: minXStartPosition...maxXStartPosition), y: yStartPosition)
        enemy.position = randomPosition

        let randomAngularVelocity = CGFloat.random(in: minAngularVelocity...maxAngularVelocity )
        let randomXVelocity: Int

        if randomPosition.x < firstQuarterOfScreen {
            randomXVelocity = Int.random(in: minOuterXVelocity...maxOuterXVelocity)
        } else if randomPosition.x < secondQuarterOfScreen {
            randomXVelocity = Int.random(in: minInnerXVelocity...maxInnerXVelocity)
        } else if randomPosition.x < thirdQuarterOfScreen {
            randomXVelocity = -Int.random(in: minInnerXVelocity...maxInnerXVelocity)
        } else {
            randomXVelocity = -Int.random(in: minOuterXVelocity...maxOuterXVelocity)
        }

        let randomYVelocity = Int.random(in: minYVelocity...maxYVelocity)

        enemy.physicsBody = SKPhysicsBody(circleOfRadius: enemyPhysicsBodyRadius)
        enemy.physicsBody?.velocity = CGVector(dx: randomXVelocity * velocityMultiplier, dy: randomYVelocity * velocityMultiplier)

        enemy.physicsBody?.angularVelocity = randomAngularVelocity
        enemy.physicsBody?.collisionBitMask = 0

        if enemy.name == "bonus" {
            //NEED HELP HERE
        }

        addChild(enemy)
        activeEnemies.append(enemy)
    }

It seems to me that there is probably just some property of enemy.physicsBody that I need to set differently for the bonus penguins to make them move faster. But I can't figure out what it is after playing around with it for a while now.

You can see near the end of that function where I have added this...

if enemy.name == "bonus" {
    //NEED HELP HERE
}

Does anybody know what I need to put in there?

I didn't include this before, but Challenge 1 of this project was to remove all the magic numbers from this function, and create constants for them instead. So, these are the constants I created.

    let numEnemyTypes = 6
    let bombFuseXPosition = 76
    let bombFuseYPosition = 64
    let minXStartPosition = 64
    let maxXStartPosition = 960
    let yStartPosition = -128
    let firstQuarterOfScreen: CGFloat = 256
    let secondQuarterOfScreen: CGFloat = 512
    let thirdQuarterOfScreen: CGFloat = 768
    let minAngularVelocity: CGFloat = -3
    let maxAngularVelocity: CGFloat = 3
    let minOuterXVelocity = 8
    let maxOuterXVelocity = 15
    let minInnerXVelocity = 3
    let maxInnerXVelocity = 5
    let minYVelocity = 24
    let maxYVelocity = 32
    let enemyPhysicsBodyRadius: CGFloat = 64
    let velocityMultiplier = 40

But I also have another questions about this same project.

1. Does anybody know the puropse of velocityMultipier here? Paul basically just tells us to write the line with no explanation. So, we calculate the randomXVelocity and randomYVelocity but then we multiply each of them by 40 for some unexplained reason.

   enemy.physicsBody?.velocity = CGVector(dx: randomXVelocity * velocityMultiplier, dy: randomYVelocity * velocityMultiplier)

   Is this just to make the random number generator more efficient? So it only has to pick a random number between 3-5, 8-15, or 24-32, rather than picking a random number between 120-200, 320-600, or 960-1280 each time?