« on: 04/12/2018 12:01:31 »
Quote from: Jay
I recently got into a debate at work regarding how mass can change in general relativity. In my feeble understanding of general relativity, an object with more energy bends spacetime to a greater magnitude, and so in terms of gravitation it behaves as if it has more mass.It does have more mass. Mass and energy are the same thing, so adding energy is the same as adding mass, and mass bends spacetime, so energy does as well.
But if this is true, does it also work for potential energy - If you lift an object up you put work into it.Yes! This is unintuitive, but if you lift an object, you have expended energy from elsewhere to the object, and it now masses more.
The energy needs to come from somewhere other than the object. If it goes uphill due to its own kinetic energy (like a roller coaster), it is losing its own kinetic energy as it gains potential energy, and the mass is unchanged. The lifting needs to be due to energy imparted from outside the object.
But does it now weigh more (or perhaps more than one would expect compared with simple Newtonian gravity)?The weight doesn't go up since you've moved it away from whatever gravity well defines 'down'. So the mass goes up a tiny bit, but the weight (gravitational force) still drops. If you have impossibly accurate weight and mass scales (spring and balance respectively), you'd measure more more mass but less weight at the top of a building.
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