无法对RigidBody.linearVelocity.x 直接赋数值

Creator 2.x
#1

版本cocos creator 1.9.1

问题：
cc.director.getPhysicsManager().enabled = true
this.rb = this.rb = getComponent(cc.RigidBody)
使用this.rb.linearVelocity.x = 800带.x运行不生效，使用this.rb.linearVelocity = cc.p(800,0) 不带才生效，难道这个值只能通过对.linearVelocity来赋vec2值，而不是直接用.x来赋值吗？

const moveLeft = 1
const moveRight = 2

cc.Class({
extends: cc.Component,

onLoad() {
    this.moveFlag = 0
    cc.director.getPhysicsManager().enabled = true
    this.rb = this.getComponent(cc.RigidBody)
    cc.systemEvent.on(cc.SystemEvent.EventType.KEY_DOWN, this.onKeyDown, this)
    cc.systemEvent.on(cc.SystemEvent.EventType.KEY_UP, this.onKeyUp, this)
},

onKeyDown(event) {
    switch (event.keyCode) {
        case cc.KEY.a:
        case cc.KEY.left:
            this.moveFlag = moveLeft
            this.rb.linearVelocity = cc.p(-800,0);
            break

（…略）