cocos2d-iphone – cocos2d box2d：不同的质量,但身体同时下降

for (int k=1;k<=2; k++) {
    int idx = (CCRANDOM_0_1() > .5 ? 0:1);
    int idy = (CCRANDOM_0_1() > .5 ? 0:1);
    CCSprite *sprite = [CCSprite spriteWithBatchNode:batch rect:CGRectMake(32 * idx,32 * idy,32,32)];
    [batch addChild:sprite];

    sprite.position = ccp( p.x+(32*k),p.y);


    b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;

    bodyDef.position.Set(( p.x+(32*k))/PTM_RATIO,p.y/PTM_RATIO);
    bodyDef.userData = sprite;

    b2Body *body = world->CreateBody(&bodyDef);


    b2PolygonShape dynamicBox;
    dynamicBox.SetAsBox(.5f,.5f);//These are mid points for our 1m box


    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicBox; 
    fixtureDef.density = 1.0f/k;
    fixtureDef.friction = 0.3f;
    body->CreateFixture(&fixtureDef);
    NSLog(@"%f",body->GetMass());
}