5 ms·
> if mass can be converted into energy, then you would have more energy and less mass, so E=mc^2 would no longer hold I'm confused why this wouldn't hold? IANA
by mnsc 9y ago
> if mass can be converted into energy, then you would have more energy and less mass, so E=mc^2 would no longer hold
I'm confused why this wouldn't hold? IANAP so I don't know if E=mc2 comes into play in a "non-nuclear" mass -> force conversion. Say a rocket with x kg of fuel mass lifting off with y J of energy spent. Naively I would say that before liftoff there was x kg of fuel which is equivalent to xc^2 energy and no(?) energy y = 0. At a later time (b) When the rocket has spent half of the fuel, the energy yb spent this far could be calculated as (x/2)c^2 and energy left in mass (xb) would (x/2)*c^2. So that the total amount of energy/mass is the same at start and at time b.
Hmm, this got really confused, so... I just hit "reply".
- plopilop 9y agoThe conversion you are talking about is true only if you annihilate your fuel (which creates photons), and then you convert photons into speed in some way with 100% efficiency. But really this never happens, rockets work by ejecting mass at a tremendous speed. Thus not all the mass of the fuel is not converted into energy. But otherwise, yeah, converting mass to energy completely works. This is what happens in stars and nuclear bombs, actually.
- davrosthedalek 9y agoIt also happens in all chemical reactions. Or: If you pump water up a mountain, the earth-water system has more rest mass.
- plopilop 9y agoSure, but it's less noticeable. The Sun loses 4.4 millions of tons every second (out of a mass of 10^30kg, but still)
- mnsc 9y ago> The conversion you are talking about is true only if you annihilate your fuel (which creates photons), and then you convert photons into speed in some way with 100% efficiency. Maybe it's hard to explain from a real world example like a rocket but if mass isn't converted to energy in that case, in the E=mc2 sense, where does the energy come from?
- plopilop 9y agoWell, in case of a rocket, there are two phenomenon resulting into thrust. The first one is the action-reaction principle: fuel goes in one direction, rocket goes in the opposite one. The second one is indeed the E=mc^2 equation: as the fuel burns, some of its mass (a negligible part, but still results in some decent amount of energy) is converted into energy and this energy can be used to propel the rocket. The rest of the mass is ejected, see first point. So if you compare the mass of the rocket + fuel before the take-off and the mass of the rocket + burned fuel after the take-off, you will indeed see a delta, which comes from a partial conversion of mass into energy of the fuel during combustion. Actually, as pointed out in an other comment, every reaction (or most of) leads to a decrease of the energy of the objects: an empty battery has less energy than a charged battery (yes I know I'm a genius for pointing that out). The difference of energy results, according to m = E/c², to a loss of mass. In the case of a battery, the delta is waaaay to small to be detected, but it exists.
- bmuon 9y agoWhile it might be true that the temperatures of rocket fuel burning are enough to create plasma and some reactions that lead to mass turning into energy, I think Jabavu is right in that the energy comes from the chemical reaction, not from E=mc2.
- plopilop 9y agoWell, yes, but it's the same thing, really. It's what my last paragraph above is about. E=mc² is not about nuclear reactions, it's about every reaction. A chemical reaction of combustion transforms some fuel molecules into new molecules. If you compute the mass of the products of the combustion and compare it to the mass of the fuel before burning, you will notice a sliiiiight difference (probably negligible, barely detectable). The disappearing mass has been converted to kinetic energy (and heat, but who cares).
- deleted 9y ago[deleted]
- 9y ago