box2d 计算自由落体下一帧的位置

var dt:Number=1/30;
var y0:Number=_body.GetPosition().y;
var y:Number=y0+_body.GetLinearVelocity().y*dt+_world.GetGravity().y*dt*dt;
trace(y0, "y:"+y);

b2Island中的实现：

var l_gravity:b2Vec2=b.m_customGravity?b.m_customGravity:gravity;
//==============================added================================
// Integrate velocities.
//b.m_linearVelocity += step.dt * (gravity + b.m_invMass * b.m_force);
b.m_linearVelocity.x += step.dt * (l_gravity.x + b.m_invMass * b.m_force.x);
b.m_linearVelocity.y += step.dt * (l_gravity.y + b.m_invMass * b.m_force.y);
b.m_angularVelocity += step.dt * b.m_invI * b.m_torque;

// Apply damping.
// ODE: dv/dt + c * v = 0
// Solution: v(t) = v0 * exp(-c * t)
// Time step: v(t + dt) = v0 * exp(-c * (t + dt)) = v0 * exp(-c * t) * exp(-c * dt) = v * exp(-c * dt)
// v2 = exp(-c * dt) * v1
// Taylor expansion:
// v2 = (1.0f - c * dt) * v1
b.m_linearVelocity.Multiply( b2Math.Clamp(1.0 - step.dt * b.m_linearDamping, 0.0, 1.0) );
b.m_angularVelocity *= b2Math.Clamp(1.0 - step.dt * b.m_angularDamping, 0.0, 1.0);

角度：

var dt:Number=1/30;
var ang:Number=_body.GetAngle();

var angV:Number=_body.GetAngularVelocity();
angV += dt * _body.m_invI * _body.m_torque;
angV*=dt
angV *= b2Math.Clamp(1.0 - dt * _body.m_angularDamping, 0.0, 1.0);

var nextAng:Number=ang+angV;
/*trace("ang:",ang);
trace("angV:",angV);
trace("nextAng:",nextAng+"\n");*/

LinearVelocity与像素的关系：

不考虑摩擦，刚体质量为1，无重力的情况下
linearVelocity*dt=distancePixel(像素距离)/ptm_ratio(像素到b2World单位)