需要给刚体添加一个自定义的属性：m_customGravity，这样就可以动态的修改每一个刚体自定义的重力，查找box2d源码大约在5486行，加上红色的一句代码
b2Island.prototype.Solve = function (step, gravity, allowSleep) {
var i = 0;
var j = 0;
var b;
var joint;
for (i = 0;
i < this.m_bodyCount; ++i) {
b = this.m_bodies[i];
if (b.GetType() != b2Body.b2_dynamicBody) continue;
gravity = b.m_customGravity || gravity;
b.m_linearVelocity.x += step.dt * (gravity.x + b.m_invMass * b.m_force.x);
b.m_linearVelocity.y += step.dt * (gravity.y + b.m_invMass * b.m_force.y);
b.m_angularVelocity += step.dt * b.m_invI * b.m_torque;
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);
}
..............代码略........................
..................................
.................................
};
使用的方法：
//创建一个圆形刚体
var circle = new Circle({x:mousePoint.x,y:mousePoint.y,r:20,type:sc.dynamicBody});
//给该刚体加上m_customGravity属性，并赋值垂直重力为1
circle.b2Obj.body.m_customGravity = new xengine.EasyB2D.b2Vec2(0,1);