很奇怪的情况，如果连接多一点物体碰地板就没事？
mapmaker = Mapmaker::create();
mapmaker->square(world, b2Vec2(10, 0), b2Vec2(20, 1)); //地板 普通的长方体
b2BodyDef bodyDef;
bodyDef.type = b2_dynamicBody;
bodyDef.position = b2Vec2(5,2);
b2PolygonShape bodyShape;
bodyShape.SetAsBox(0.7f, 0.7f); //方形
b2FixtureDef bodyFixture; //定制器
bodyFixture.shape = &bodyShape; //形状
b2Body* bodyA = world->CreateBody(&bodyDef);
bodyA->CreateFixture(&bodyFixture);
bodyDef.position = b2Vec2(5, 4);
b2Body* bodyB = world->CreateBody(&bodyDef);
bodyB->CreateFixture(&bodyFixture);
b2WeldJointDef jointDef;
jointDef.Initialize(bodyA, bodyB, bodyA->GetPosition());
world->CreateJoint(&jointDef);
