I see, thanks for the response.
Another random physics question though:
Say, I've got a 5 kilogram stone crashing into the ground at 40 m/s. Thus, the stone's energy is 50 kg * 40 m/s = 2000 joule. Now, if I look at the back of a glass 'o Peanut Butter (which I ate in one day btw), the peanut butter has an energy of 3.5 (350 gram) times 2160 kilojoule, which results in a whopping 7'560 kilojoule. This is an insane amount of energy! How can that be? Do nutritions use some kind of other joule?
Well... Yes. The energy in foodstuffs is the heat value of the stuff, ie. when the stuff is dried and burned, the exothermic reaction frees up that much energy.
You need to remember that cells and organisms are rather ineffectual in terms of energy usage. There's also a static, constant energy consumption of keeping body temperature up and living functions normal, it's not simply possible to convert the energy from you eat into mechanical work - it doesn't work that way. Brain work consumes energy, digestion consumes energy, heart pumping and lungs breathing all take energy, walking, talking and sitting upright take energy, and what's left of the energy received from food after all that is taken care of is stored into fat or used to generate mechanical work with muscles.
Also, a good way to relate joules to reality is that ten joules is roughly sufficient to lift one kilogram mass to height of one metre in normal gravity potential (g=9.80665 ms^-2). One kilojoule would be sufficient to lift hundred kilograms to height of one metre height. But that is all mechanical work. If you assume that the efficiency of human is 10% (which is rather optimistic value), one kilojoule would be sufficient for lifting ten kilograms to one metre height.
Aside from that, you got the kinetic energy equation wrong. If the stone is 5 kg, then the equation goes
E(k) = ½ m v ^2 = ½ * 5 kg * (40 m/s)^2 = ½ * 5 kg * 1600 m^2/s^2 = 5*800 kgm^2/s^2 = 4000 J = 4 kJ
and if the stone weighs 50 kg, then the energy is tenfold.
If you calculate with mass times velocity, you get momentum instead, another very important physical quantity (one that you will meet again in the definition of force, for example)... The stone's momentum in this case would be 5 kg * 40 m/s = 200 kgm/s, and in the impact the momentum transfers to the combined mass of Earth and the stone, which obviously doesn't really move Earth noticeably.
Regarding calories, kcal, joules and kilojoules in foodstuffs, I find it easiest to look at the kilojoules rather than other units...
