零重力box2d世界的速度降低

Question

我正在尝试创建一个在Box2D中围绕静态体旋转的动态体.我有一个零重力的世界,一个DistanceJoint连接两个身体的世界.我已经消除了身体和关节的所有摩擦力和阻尼,并对动态体施加了初始线速度.结果是身体开始绕行,但是它的速度随着时间的推移而减小 - 我没有想到在没有摩擦的零重力环境中.

难道我做错了什么？是否应该在每个步骤重新创建线性速度,还是可以将此工作委托给Box2D？

这是相关代码:

// positions of both bodies

Vector2 planetPosition = new Vector2(x1 / Physics.RATIO, y1 / Physics.RATIO);
Vector2 satellitePosition = new Vector2(x2 / Physics.RATIO, y2 / Physics.RATIO);


// creating static body

BodyDef planetBodyDef = new BodyDef();
planetBodyDef.type = BodyType.StaticBody;
planetBodyDef.position.set(planetPosition);
planetBodyDef.angularDamping = 0;
planetBodyDef.linearDamping = 0;

planetBody = _world.createBody(planetBodyDef);

CircleShape planetShapeDef = new CircleShape();
planetShapeDef.setRadius(40);

FixtureDef planetFixtureDef = new FixtureDef();
planetFixtureDef.shape = planetShapeDef;
planetFixtureDef.density = 0.7f;
planetFixtureDef.friction = 0;

planetBody.createFixture(planetFixtureDef);

// creating dynamic body

BodyDef satelliteBodyDef = new BodyDef();
satelliteBodyDef.type = BodyType.DynamicBody;
satelliteBodyDef.position.set(satellitePosition);
satelliteBodyDef.linearDamping = 0;
satelliteBodyDef.angularDamping = 0;

satelliteBody = _world.createBody(satelliteBodyDef);

CircleShape satelliteShapeDef = new CircleShape();
satelliteShapeDef.setRadius(10);

FixtureDef satelliteFixtureDef = new FixtureDef();
satelliteFixtureDef.shape = satelliteShapeDef;
satelliteFixtureDef.density = 0.7f;
satelliteFixtureDef.friction = 0;

satelliteBody.createFixture(satelliteFixtureDef);

// create DistanceJoint between bodies

DistanceJointDef jointDef = new DistanceJointDef();        
jointDef.initialize(satelliteBody, planetBody, satellitePosition, planetPosition);
jointDef.collideConnected = false;
jointDef.dampingRatio = 0;

_world.createJoint(jointDef);

// set initial velocity

satelliteBody.setLinearVelocity(new Vector2(0, 30.0f)); // orthogonal to the joint

Answer 1

在身体上,你是对的.能量守恒应确保身体的速度保持不变.

但是,Box2D无法完美地代表物理学.每一帧都会有一个小错误,这些错误加起来.我不知道Box2D如何处理关节,但是如果它将对象的位置投射到圆上,这将导致框架中行进的距离略小于没有关节的情况.

底线:期望速度保持与您开始时的速度完全相同是不合理的,您需要进行补偿.根据您的需要,您可以每帧手动设置速度,也可以使用电机固定在行星上的旋转关节.