How do you calculate the gravitational potential energy of a 2 kg bottle of soda falling?

If the bottle is falling, then its gravitational potential energy is also falling.  That's because gravitational potential energy is  (mass)·(gravity)·(HEIGHT) .  

So the best I can tell you is that at any instant of time, the bottle's GPE is

(2 kg) · (9.81 m/s²) · (HEIGHT above ground at that instant) .

That's going to be  (19.62 x height above ground in meters), and the unit, of course, will be 'Joules'.

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