The gravitational potential energy of an object, measured with respect to the ground, is given by
[tex]U=mgh[/tex]
where m is the object's mass, g is the gravitational acceleration and h is the height of the object measured from the ground.
In our problem, the mass of the plane is m= 5 tons = 5000 kg, while the potential energy is
[tex]U=1000 kJ=1 \cdot 10^6 J[/tex]
so we can re-arrange the previous formula and use these data to calculate h, the height of the airplane:
[tex]h= \frac{U}{mg}= \frac{1 \cdot 10^6 J}{(5000 kg)(9.81 m/s^2)} =20.39 m [/tex]
