如何在corona sdk中使子弹碰撞后沿着运动方向改变方向（角度）

我想在碰撞后改变子弹的角度，碰撞后它应该改变运动方向，并且碰撞后必须停止旋转... 请给予任何建议... 谢谢

示例代码如下：

local physics = require("physics")
physics.start()

local obj

physics.setScale(60)
physics.setGravity(0, 0)

display.setStatusBar(display.HiddenStatusBar)
local obstacle = display.newCircle(250, 250, 60)
obstacle.x = display.contentWidth/2; obstacle.y = 200
physics.addBody(obstacle, { density = 150, friction = 0.2, bounce = 0 })
obstacle.isBullet = true

obj = display.newRect(0, 0, 20, 40)
obj.x = display.contentWidth/2; obj.y = 780
obj.isBullet = true
obj.color = "white"
physics.addBody(obj, { density = 1, friction = 0.4, bounce = 0.1 })

local target = display.newCircle(250, 250, 60)
target.x = obj.x; target.y = obj.y; target.alpha = 0

local function Shot(event)
    local t = event.target

    local phase = event.phase
    if "began" == phase then
        display.getCurrentStage():setFocus(t)
        t.isFocus = true

        t:setLinearVelocity(0, 0)
        t.angularVelocity = 0

        target.x = t.x
        target.y = t.y

        startRotation = function()
            target.rotation = target.rotation + 4
        end

        Runtime:addEventListener("enterFrame", startRotation)

        local showTarget = transition.to(target, { alpha = 0.4, xScale = 0.4, yScale = 0.4, time = 200 })
        myLine = nil

    elseif t.isFocus then
        if "moved" == phase then

            if (myLine) then
                myLine.parent:remove(myLine) -- erase previous line, if any
            end
            myLine = display.newLine(t.x, t.y, event.x, event.y)
            myLine:setColor(255, 255, 255, 50)
            myLine.width = 8

        elseif "ended" == phase or "cancelled" == phase then

            display.getCurrentStage():setFocus(nil)
            t.isFocus = false

            local stopRotation = function()
                Runtime:removeEventListener("enterFrame", startRotation)
            end

            local hideTarget = transition.to(target, { alpha = 0, xScale = 1.0, yScale = 1.0, time = 200, onComplete = stopRotation })

            if (myLine) then
                myLine.parent:remove(myLine)
            end

            t:applyForce((t.x - event.x), (t.y - event.y), t.x, t.y)
            t:setLinearVelocity(math.cos(t.rotation*math.pi/180)*150, math.sin(t.rotation*math.pi/180)*150)

        end
    end

    return true
end

obj:addEventListener("touch", Shot)