Math behind a sine curve

O'hoy fellow Scir'vees!

I have this top down game in which I want the player to be able to jump. Since there is no jump or height function in the 8 directions behvaiour I needed to hack something together myself and thus my following conundrum..

I created an invisible sine object do to a nice sine curve for me when you press the jump key, however, there's not way to reset the oscillation, the curve always starts where it was inactivated, making it quite a hassle to get it to work right. So I decided to just do a nice sine expression myself... I'm incredibly inexperienced when it comes to this type of math though so...

How do I make an expression that has an initial force that de-accelerate to 1 and then accelerate to 0? (faking gravity after a jump etc)

~B

The simplest way I can think of add jumping to 8direction movement is to use a hidden object with the platform behavior to get your jump height from and set it up like this:

http://dl.dropbox.com/u/5426011/examples12/8dir_jump.capx

That is very simple indeed :) Thanks!