title: SpriteKit applyforce doesn't work
body: <p>I have <code>player</code> object on my scene and trying to apply some force on it. You can see that I'm applying force on the <code>update:</code> method on <code>MyScene.h</code> file. But, my <code>player</code> is not moving at all. What's happening?</p>
  
  <p>Below is the Player SKSpriteNode.    </p>
  
  <pre><code>#import "Player.h"
  #import "common.h"
  @implementation Player
  - (instancetype)init {
      self = [super initWithImageNamed:@"player.png"];
      self.name = @"player";
      self.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:CGSizeMake(22, 50)];
      self.physicsBody.mass = 10;
      self.physicsBody.dynamic = YES;
      self.physicsBody.allowsRotation = NO;
      self.physicsBody.affectedByGravity = YES;
      self.physicsBody.collisionBitMask = ~poopCategory &amp; groundCategory;
      self.physicsBody.categoryBitMask = playerCategory;
      self.physicsBody.contactTestBitMask = poopCategory;
      self.zPosition = 100;
  
      return self;
  }
  @end
  </code></pre>
  
  <p>Next, it's the background that I'm using. </p>
  
  <pre><code>#import "Background.h"
  #import "common.h"
  @implementation Background
  + (Background *) generateBackground {
      Background *background = [[Background alloc] initWithImageNamed:@"RPBackground.png"];
      background.physicsBody = [SKPhysicsBody bodyWithEdgeFromPoint:CGPointMake(-100, 100) toPoint:CGPointMake(400, 100)];
      background.physicsBody.categoryBitMask = groundCategory;
      background.physicsBody.collisionBitMask = poopCategory | playerCategory;
      background.physicsBody.contactTestBitMask = groundCategory;
      background.anchorPoint = CGPointMake(0, 0);
      background.position = CGPointMake(0, 0);
      background.zPosition = 0;
      background.name = @"ground";
      return background;
  }
  @end
  </code></pre>
  
  <p>Now, this is the main scene.</p>
  
  <pre><code>#import "MyScene.h"
  
  @implementation MyScene
  
  -(id)initWithSize:(CGSize)size {    
      if (self = [super initWithSize:size]) {
          /* Setup your scene here */
          Background *b = [Background generateBackground];
          [self addChild:b];
          self.physicsWorld.gravity = CGVectorMake(0, -9.8);
  
  
          Player *player = [[Player alloc]init];
          player.size = CGSizeMake(30, 60);
          player.position = CGPointMake([UIScreen mainScreen].applicationFrame.size.width/2, groundLevel+300);
  
          [self addChild:player];
          self.player = player;
      }
      return self;
  }
  
  -(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
      /* Called when a touch begins */
  
  
  }
  
  -(void)update:(CFTimeInterval)currentTime {
      /* Called before each frame is rendered */
      [self.player.physicsBody applyForce:CGVectorMake(30, 0)]; // &lt;=------------
  }
  
  @end
  </code></pre>
  
  <p>I feel like I'm making some stupid error somewhere... But I can't seem to spot it. Can anyone help? Thanks.</p>