[quote author=Fu-Kwun Hwang link=topic=33.msg7534#msg7534 date=1290087955]
You are thinking about a constant acceleration acc acting for a finite time.
However, the acceleration is always perpendicular to the velocity vector.

The magnitude of the velocity will change if the acceleration is in the direction of the velocity.
Because the acceleration is always pendicular to the velocity so it only change the direction of the velocity.
[/quote]

thank you again ..
but is this a general physics rule ??
that
"the acceleration is always pendicular to the velocity so it only change the direction of the velocity"