Potential energy as a form of mass?

[Vitani11]

Homework Statement

Let's say we have a mass of 5kg at a height of 3 m so it's potential energy is mgh = 147J/1.6e-19 = 9.19 e20 eV. Now we know that E = mc^2... so when finding the mass of this potential energy we get 10.2e3 kg. What the hell is that supposed to mean?

Homework Equations

None

The Attempt at a Solution

 

[mfb]

mgh = 147J/1.6e-19 = 9.19 e20 eV

Something went wrong with the units here.

Now we know that E = mc^2... so when finding the mass of this potential energy we get 10.2e3 kg

And again here.

If you fix the calculation, you'll get a result of something like 3.000000000000001 kg (didn't count the zeros). It means total contribution to energy of the overall system (Earth+mass) is not the same as the rest energy of the small test mass.