IOS 8精灵套件与顶部和底部碰撞而不是两侧碰撞？

我做了这是一个简单的效果,一个球弹跳的屏幕它只适用于顶部和底部边缘idk为什么,此外,当我在initwithsize方法中的iOS 7精灵工具包模板中尝试此代码时,它的工作原理
是一个bug还是什么？
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@H_502_10@

@H_502_10@

#import "GameScene.h"

@implementation GameScene



-(void)didMoveToView:(SKView *)view {
    self.BACkgroundColor = [SKColor whiteColor];
    SKSpriteNode *ball =[SKSpriteNode spriteNodeWithImagenamed:@"ball"];
    ball.position =CGPointMake(self.frame.size.width/2,self.frame.size.height/2);
    ball.physicsBody = [SKPhysicsBody bodyWithCircLeofRadius:ball.frame.size.width/2];
    self.physicsBody = [SKPhysicsBody bodyWithEdgeLoopFromRect:self.frame];
    self.physicsWorld.gravity = CGVectorMake(0,0);
    CGVector impulse =     CGVectorMake(20,20);
    [self addChild:ball];

    [ball.physicsBody applyImpulse:  impulse];

    ball.physicsBody.restitution = 1;
    ball.physicsBody.friction = 0;
    ball.physicsBody.linearDamping = 0;




  /* 
   SKSpriteNode *wall =[SKSpriteNode spriteNodeWithImagenamed:@"wall"];

   SKSpriteNode *bar =[SKSpriteNode spriteNodeWithImagenamed:@"bar"];

   wall.position= CGPointMake(self.frame.size.width/2,103);
    bar.position = CGPointMake(self.frame.size.width/2,(wall.frame.size.height));
    [self addChild:wall];
    [self addChild:bar];


    */

}


-(void)update:(CFTimeInterval)currentTime {
    /* Called before each frame is rendered */
}

@end