想做一个小游戏，建了一个role类，想添在类中添加一个重力加速度的效果(sub gravity)，可是不行，下面我已经把问题作了简化，请帮忙看一下毛病出在哪里
在窗体上放一个image，改名为body；放一个timer，interval值设为50,然后添加如下代码：
类模块role:
Option Explicit
Public body As Image
Public Vy As Integer
Public g As Integer

Public Sub gravity(body As Image, Vy As Integer, g As Integer)
Vy = Vy - g
body.Top = body.Top - Vy
End Sub

窗体:
Option Explicit
Dim p As New role

Private Sub Form_Load()
body.BorderStyle = 1
body.Left = 2000
body.Top = 6000
Set p.body = body
p.Vy = 100
p.g = 20
End Sub

Private Sub Timer1_Timer()
p.gravity p.body, p.Vy, p.g
End Sub

运行时body作匀速运动，并无加速度效果，也就是说似乎Vy并没被Vy = Vy - g改变，用F8逐行执行发现,每次虽然在过程sub gravity内部Vy的值确实被Vy=Vy-g改变了，但是sub gravity结束时Vy又变成了100，这是怎么回事儿啊？每么样才能使Vy的值能够被sub gravity过程改变？

[此贴子已经被作者于2006-7-21 15:52:22编辑过]

Option Explicit
Public body As Image
Public Vy As Integer
Public g As Integer

Public Sub gravity(a As Integer, b As Integer)
a = a - b
Vy = a
body.Top = body.Top + Vy
End Sub

Option Explicit
Dim p As New role

Private Sub Form_Load()
body.BorderStyle = 1
body.Left = 2000
body.Top = 6000
Set p.body = body
p.Vy = 44
p.g = 20
End Sub

Private Sub Timer1_Timer()
p.gravity p.Vy, p.g
Text1 = p.Vy
End Sub
=============================
Option Explicit
Public body As Image
Public Vy As Integer
Public g As Integer

Public Sub gravity()
Vy = Vy - g
body.Top = body.Top + Vy
End Sub

Option Explicit
Dim p As New role

Private Sub Form_Load()
body.BorderStyle = 1
body.Left = 2000
body.Top = 6000
Set p.body = body
p.Vy = 44
p.g = 20
End Sub

Private Sub Timer1_Timer()
p.gravity
Text1 = p.Vy
End Sub