Code?

Your formula's perfect. Gravity is nothing more than linear acceleration, and that's what your code is doing.

The reason it goes super fast is because consider that by then end of 100 frames, or 1 second, if there was no floor your ball's y-speed would be 400 units every 10 ms. You need to either switch to floating point values and use much smaller speed increments, reduce the game logic to process at a slower rate, (such as 20 ms instead of 10 ms), or both.

--- Kris Asick (Gemini)
--- http://www.pixelships.com

If you're trying to model any kind of real-world physics, use real-world units in all the calculations and only scale when rendering. If not, forget about the real world completely and think only in terms of your game's world space.

Related to the previous point: if you're going to use real units anywhere, at least use logical ones.

And use floating point. It's not 1995 anymore.

Gravity is acceleration, so its unit will be pixel/second^2.

EDIT: Also I strongly suggest using floats or fixed point fractional numbers. If you do calculations every 10 miliseconds you'll surely need subpixel precision.

How does pixels per seconds^2 affect my code?

EDIT: I don't want to play with, I just want to set the variable to what it would look like in real ife. (Depending on gamespace and screenspace)

So it has realistic effects on the game. (This is if I'm making a game that is "real", like basketball or something).

Then re-read the thread. Or read it, as it may be.

if(bally>=//The screen hieght. I.E. 600
      ballyspeed*=-1;
if(ballx>=//The screen width. I.E. 800
      ballxspeed*=-1;

It's off-topic, but... wouldn't this

if(bally>=SCREEN_H)
      ballyspeed*=-1;
if(ballx>=SCREEN_W)
      ballxspeed*=-1;

be better?

In case you have already implemented that in your code and just put a comment there for some unfathomable reason, ignore this post.

Helping your problem was not the point of my post. Pointing out stuff that might be of some use in the future, was. I just thought I'd point it out in case you didn't know about it.

You're trying to model real world physics with too few of the components.

If you want to implement real gravity you must also create systems for units of measurement, object mass, volume, friction, drag, kinetic transfer of energy, time...

The only solution to your problem is to design all that. Then and only then will your gravity be realistic. Anything beforehand will be arbitrary, and 99% of game developers settle for arbitrary because as you can see, modeling real world physics is a major chore and not to be attempted by anyone but the most dedicated and experienced in both programming and physics.

--- Kris Asick (Gemini)
--- http://www.pixelships.com

Short: Yes.

That's what everybody else does...
To be honest, a gamer doesn't care how realistic it is, they care how realistic it looks.