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For example, we have a ball with mass m lying on the Earth. Then we lift it to a height h. So, now system Earth-ball have potential energy $E_p = mgh$. From Mass–energy equivalence system got $mass = \frac{E_p}{c^2}$. Does it mean that potential energy increased inertial mass of system and it's harder to accelerate that system, or potential energy just increased rest mass of system Earth-ball?

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If the energy to lift the ball came from outside of the Earth-ball system, then energy has been added to the system. And that means that the rest mass (energy) of the system has in fact increased.

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    $\begingroup$ Let me complete your argument. If instead it's you, staying on Earth, who lift a heavy rock, it's true that potential energy of rock-Earth system has increased, but at the expense of your internal energy. So total mass you-rock-Earth is unchanged. $\endgroup$ – Elio Fabri Oct 29 '18 at 10:48

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