# QuickPost: Faking gravity

Very recently, I needed an object to shoot into the air and fall realistically, but I didn't want to import a tweening library or a physics engine. What I needed was some sort of parametric equation that would act like gravity... What would this mysterious equation be... Oh, that's right, Newton's equation of motion for constant linear acceleration is already parametric!

So in my update function, I simply calculate the object's position:

```var elapsed:Number = 0;   // force of gravity var gravity:int = 180;   // initial velocity var v0:int = 100; _update = function(dt:Number):void { _primitive.y = v0 * (elapsed += dt) - gravity * elapsed * elapsed; };```

I just fiddled with gravity until I got something that looked good. One more thing: I wanted the object to do a flip as well. This I could do with a simple linear parametric function, but I needed the total time of the tween so it could land correctly. No problem!

```var elapsed:Number = 0; var gravity:int = 180; var v0:int = 100;   // calculate the total time! var tTot:Number = v0 / gravity; _update = function(dt:Number):void { _primitive.y = Main.SCALE * (v0 * (elapsed += dt) - gravity * elapsed * elapsed); _primitive.rotationX = (elapsed / tTot) * 360; };```