I'm not sure if I should be using a func or a sub, but it looks like my
values get passed from one sub to another, and all of the values are OK at
the end of the called sub, but my variables "x" and "v" are being reset to 0
when the procedure continues after I call the 2nd sub. My other values are
OK.

If you want to try this out in Excel, set some values in B1:B4 and B6:B7.

I have put in a couple of msgBox's just to see what values the variables
hold. Those will be commented out when this is working.

Option Explicit
' Sub Worksheet_Change(ByVal Target As Range) ' this will trigger if any
part of the sheet is changed
Sub One_D_Motion()
Dim t As Single
Dim t0 As Single
Dim time1End As Single
Dim time2End As Single

Dim x As Single
Dim x0 As Integer

Dim v0 As Single
Dim v As Single

Dim acc As Single
Dim acc1 As Single
Dim acc2 As Single

Dim row As Integer

' This will find the position and velcoity for the first 20 seconds
' The user must set the initial conditions and the time period for the
accelerations
' Written 6/12/2008 by Jeff Ciaccio

' Clear out the range where the data will go
Range("D2:g202").Value = ""


' Record the values the user has chosen
x0 = Range("B1").Value
v0 = Range("B2").Value
acc1 = Range("b3").Value
time1End = Range("b4").Value
acc2 = Range("b6").Value
time2End = Range("B7").Value

' Set the first row cells
t = 0
row = 2
Range("D2").Value = 0
Range("E2").Value = x0
Range("f2").Value = v0
Range("G2").Value = acc1
Range("d3").Select


Call calc(t, time1End, row, x0, v0, acc1)
' TROUBLE
MsgBox "x = " & x & " v = " & v
'MsgBox prompt:="Time period 1: " & vbLf & "The ending position was " & x &
" meters, and the starting position was " & x0 & ", so the displacement was
" & (x - x0) & " meters", Title:="End of first period"
'MsgBox prompt:="Time period 1: " & vbLf & "The ending velocity was " & v &
" m/s, and the starting velocity was " & v0 & ", so the Delta v was " & (v -
v0) & " m/s", Title:="End of first period"
'MsgBox ("Time period 1: " & vbLf & "Since there was constant acceleration,
the average velocity is:" & vbLf _
& "avg vel = (" & v0 & " + " & v & ") " & Chr(247) & "2 = " & (v0 +
v) / 2) & " m/s", Title:="End of first period"

Call calc(t, time2End, row, x, v, acc2)

'MsgBox prompt:="Time period 2: " & vbLf & "NOTE: To find the average
velocity for the whole period, you CANNOT simply average the starting and
ending velocities!" _
& vbLf & "Just find the displacement and divide by 20.0 seconds." & vbLf
_
& "The average velocity was " & (x - x0) / 20 & "m/s^2", Title:="End of
simulation"

End Sub

Sub calc(t, timeEnd, row, x, v, acc)

Dim t0 As Single
Dim x0 As Integer
Dim v0 As Single
MsgBox "Thanks for passing me t = " & t & " end time = " & timeEnd & "row =
" & row & " x = " & x & " v = " & v & " and acc = " & acc
t0 = t
x0 = x
v0 = v
Do Until t = timeEnd
t = Round(t + 0.1, 1)
row = row + 1
x = x0 + v0 * (t - t0) + 1 / 2 * acc * (t - t0) ^ 2
v = v0 + (t - t0) * acc
Range("d" & row).Value = t
Range("e" & row).Value = x
Range("f" & row).Value = v
Range("g" & row).Value = acc
Loop
MsgBox "x = " & x & " v = " & v & " at the end of calc."
End Sub


--
Jeff Ciaccio
Physics and AP Physics Teacher
Sprayberry High School; Marietta, GA
Blog: http://sprayberry.typepad.com/ciaccio