dinamin:
i just need values when there is weight (person standing) and when the weight is zero (person in air after jumping)
If you use an accelerometer to measure vertical acceleration, you could detect when somebody holding the Arduino was jumping. Or you could use any form of inertial switch to do the same thing. (This isn't at all novel, hand held game controllers already do this.)
If you want to measure the weight on a floor, the simplest way I can think of to do that is to have a suspended floor which flexes slightly when weight is applied, and use that movement to operate a potentiometer. There are other ways to do it using pressure sensors etc but they seem likely to be more expensive and harder to get working.