ios - Character falling through ground -

i'm using sprite kit part of basic game. game there guy who, depending on tap, moves location. currently, player not affected gravity (for sake of playibility) i'm trying implement gravity. however, when turn gravity on player, i'm having trouble character falling through ground. i've created node ground , have disabled ground physics (ground.physicsbody.dynamic = no) character still flies right on through. i'm using built in sprite-kit gravity feature, , if physics disabled, theoretically character should stop @ ground. i'm @ loss why problem still exists , appreciate on matter.

code in myscene.m file:

// //  myscene.m //  dodgeman // //  created cormac chester on 3/8/14. //  copyright (c) 2014 testman industries. rights reserved. //  #import "myscene.h" #import "endgamescene.h"  static const uint32_t redballcategory =  0x1 << 0; static const uint32_t playercategory =  0x1 << 1;  @implementation myscene  -(id)initwithsize:(cgsize)size {     if (self = [super initwithsize:size])     {         //sets data storage         self.storedata = [nsuserdefaults standarduserdefaults];          //sets gravity         self.physicsworld.gravity = cgvectormake(0,-2);         self.physicsworld.contactdelegate = self;          //set background         self.backgroundcolor = [skcolor colorwithred:0.53 green:0.81 blue:0.92 alpha:1.0];          //set ground         skspritenode *ground = [skspritenode spritenodewithimagenamed:@"ground"];         ground.position = cgpointmake(cgrectgetmidx(self.frame), 34);         ground.xscale = 0.5;         ground.yscale = 0.5;         ground.physicsbody = [skphysicsbody bodywithrectangleofsize:ground.size];         ground.physicsbody.dynamic = no;         ground.physicsbody.collisionbitmask = 1;         ground.physicsbody.usesprecisecollisiondetection = yes;          //player         self.posx = 50;         self.posy = 88;         self.playersprite = [skspritenode spritenodewithimagenamed:@"character"];         self.playersprite.position = cgpointmake(self.posx, self.posy);          //set player physics         self.playersprite.physicsbody = [skphysicsbody bodywithrectangleofsize:self.playersprite.size];         self.playersprite.physicsbody.dynamic = no;         self.playersprite.physicsbody.categorybitmask = playercategory;         self.playersprite.physicsbody.contacttestbitmask = redballcategory;         self.playersprite.physicsbody.collisionbitmask = 0;         self.playersprite.physicsbody.usesprecisecollisiondetection = yes;          //score label         self.scorelabel = [sklabelnode labelnodewithfontnamed:@"arial-boldmt"];         self.scorelabel.text = @"0";         self.scorelabel.fontsize = 40;         self.scorelabel.fontcolor = [skcolor blackcolor];         self.scorelabel.position = cgpointmake(50, 260);          //pause button         self.pausebutton = [skspritenode spritenodewithimagenamed:@"pausebutton"];         self.pausebutton.position = cgpointmake(self.frame.size.width / 2, self.frame.size.height - 40);         self.pausebutton.name = @"pausebutton";          //pause label         nsstring *pausemessage;         pausemessage = @"game paused";         self.pauselabel = [sklabelnode labelnodewithfontnamed:@"arial-boldmt"];         self.pauselabel.text = pausemessage;         self.pauselabel.fontsize = 40;         self.pauselabel.fontcolor = [skcolor blackcolor];         self.pauselabel.position = cgpointmake(self.size.width/2, self.size.height/2);          //set score         self.score = 0;          //add nodes         [self addchild:ground];         [self addchild:self.playersprite];         [self addchild:self.scorelabel];         [self addchild:self.pausebutton];      }     return self; }  //add red ball -(void)addball {     skspritenode *redball = [skspritenode spritenodewithimagenamed:@"locationindicator"];     int miny = redball.size.height / 2;     int maxy = self.frame.size.height - redball.size.height / 2;     int rangey = maxy - miny;     int actualy = (arc4random() % rangey) + miny;      //initiates red ball offscreen     if (actualy >= 75)     {         //prevents balls spawning in ground         redball.position = cgpointmake(self.frame.size.width + redball.size.width/2, actualy);         [self addchild:redball];     }     redball.physicsbody = [skphysicsbody bodywithcircleofradius:redball.size.width/2];     redball.physicsbody.dynamic = yes;     redball.physicsbody.categorybitmask = redballcategory;     redball.physicsbody.contacttestbitmask = playercategory;     redball.physicsbody.collisionbitmask = 0;     redball.physicsbody.affectedbygravity = no;     redball.physicsbody.usesprecisecollisiondetection = yes;      //determine speed of red ball     int minduration = 3.0;     int maxduration = 5.0;     int rangeduration = maxduration - minduration;     int actualduration = (arc4random() % rangeduration) + minduration;      // create actions     skaction *actionmove = [skaction moveto:cgpointmake(-redball.size.width/2, actualy) duration:actualduration];     skaction *actionmovedone = [skaction removefromparent];     skaction *ballcross = [skaction runblock:^{         self.score++;         self.scorestring = [nsstring stringwithformat:@"%i", self.score];         self.scorelabel.text = self.scorestring;         nslog(@"score incremented. score %d", self.score);     }];     [redball runaction:[skaction sequence:@[actionmove, ballcross, actionmovedone]]]; }  - (void)pausescene {     self.paused = yes;     [self addchild:self.pauselabel]; }  - (void)updatewithtimesincelastupdate:(cftimeinterval)timesincelast {     self.lastspawntimeinterval += timesincelast;     if (self.lastspawntimeinterval > 0.35) {         self.lastspawntimeinterval = 0;         [self addball];     } }  //update loop -(void)update:(cftimeinterval)currenttime {     /* called before each frame rendered */     // handle time delta.     cftimeinterval timesincelast = currenttime - self.lastupdatetimeinterval;     self.lastupdatetimeinterval = currenttime;     if (timesincelast > 1) { //more second since last update         timesincelast = 1.0 / 120.0;         self.lastupdatetimeinterval = currenttime;     }      [self updatewithtimesincelastupdate:timesincelast]; }  -(void)touchesbegan:(nsset *)touches withevent:(uievent *)event {     /* called when touch begins */     [super touchesbegan:touches withevent:event];      //starts timer     starttime = [nsdate date]; }  -(void)touchesended:(nsset *)touches withevent:(uievent *)event {     /* called when touch ends */     [super touchesended:touches withevent:event];      nstimeinterval elapsedtime = [starttime timeintervalsincenow];     nsstring *elapsedtimestring = [nsstring stringwithformat:@"elapsed time: %f", elapsedtime];     nslog(@"%@", elapsedtimestring);       (uitouch *touch in touches)     {         //gets location of touch         cgpoint location = [touch locationinnode:self];         nslog(@"touch location x: %f \n touch location y: %f", location.x, location.y);         sknode *node = [self nodeatpoint:location];          //runs when pause button pressed         if ([node.name isequaltostring:@"pausebutton"])         {             nslog(@"pause button pressed");             if (!self.paused)             {                 [self pausescene];             }             else if (self.paused)             {                 self.paused = no;                 [self.pauselabel removefromparent];             }         }         else         {             //prevents destination being in ground             if (location.y < 88)             {                 location.y = 88;             }              //moves , animates player             int velocity = 1000.0/1.0;             nslog(@"velocity: %i", velocity);             float realmoveduration = self.size.width / velocity;             skaction *actionmove = [skaction moveto:location duration:realmoveduration];             [self.playersprite runaction:[skaction sequence:@[actionmove]]];         }     }      nslog(@"touch ended"); }  //collision between ball , player - player dies - (void)redball:(skspritenode *)redball didcollidewithplayer:(skspritenode *)playersprite {      nslog(@"player died");     [redball removefromparent];     [playersprite removefromparent];      //stores final score     [self.storedata setobject:self.scorestring forkey:@"scorestringkey"];      sktransition *reveal = [sktransition fadewithduration:0.5];     skscene *endgamescene = [[endgamescene alloc] initwithsize:self.size];     [self.view presentscene:endgamescene transition: reveal]; }  - (void)didbegincontact:(skphysicscontact *)contact {     skphysicsbody *firstbody, *secondbody;      if (contact.bodya.categorybitmask < contact.bodyb.categorybitmask)     {         firstbody = contact.bodya;         secondbody = contact.bodyb;     }     else     {         firstbody = contact.bodyb;         secondbody = contact.bodya;     }      //red ball collides player     if ((firstbody.categorybitmask & redballcategory) != 0 && (secondbody.categorybitmask & playercategory) != 0)     {         [self redball:(skspritenode *) firstbody.node didcollidewithplayer:(skspritenode *) secondbody.node];         nslog(@"player , ball collided");     } }  @end 

i problem due line

self.playersprite.physicsbody.collisionbitmask = 0; 

instead of set category collisions occur, example

self.playersprite.physicsbody.collisionbitmask = redballcategory;