Originally Posted By: Bill S.
Originally Posted By: redewenur
Einstein, E=mc^2 (mass–energy equivalence, i.e.,the mass of a body is a measure of its energy content)


Originally Posted By: Kallog
there's no reason that energy should be taken from the mass of the Earth, it could be kinetic which is not included in E=mc^2.


Can you both be right?


Yes, depending on how you define mass and if no other type of energy besides mass and kinetic is present.

m could be rest mass (m0 in the formula below) or relativistic mass, m in the formula below.


I prefer to define mass as a property of an object, rather than a combination of a property and a relationship between the object's speed and the observer's.