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Can You Burn Away Potential Energy?



I recently stumbled upon a question that definitely made me think twice. And then twice more.

If you carry firewood to the top of a hill, and then burn it there, what happens to the firewood's gravitational potential energy? Does it disappear? :camp:

Crazy, I know. The equation for gravitational potential energy is mgh, or the mass times the acceleration due to gravity times the height. If you burn the log up into ashes with substantially less mass, what happens to the rest of the gravitational potential energy the logs had when you carried them up the hill?

Honestly, the hill part doesn't even matter. Whether you burn the wood up on the hill or in the valley below, it's still gone either way. The hill just makes the height component of gravitational energy easier to visualize.

But where does this seemingly lost energy go? It can't just disappear right? All we've ever been taught in physics is that energy is conserved! Right?! :phew:

It's true, it can't disappear - it just becomes harder to find. When the wood is burned, a lot of the energy escapes in the gases released from the fire. :crazysmile: The gases are released over a long period of time as the fire burns, so it's hard to visualize them weighing as much as the wood you're burning - but it's true, just about. If you compressed all the gas that is released from the moment you start burning the wood to the moment the last ember crumbles, it would weigh just about as much as the wood did originally. Thus, the same potential energy.

So the energy didn't go anywhere, it just escaped in a different form. There are some other factors in play here I still don't quite understand, but this is the gist. Maybe I'll look into those other factors and expand on them in a future blog post.

This is where I learned about this topic, and you can too: howeverythingworks.org. This guy really knows his stuff, and probably explains it better than I can. Make that definitely explains it better, since I still don't fully understand it. :blink:

Hope I made a little bit of sense anyway. Thanks for reading!

Until next time,

bazinga818 :ass:

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Just my two cents. I think, some will go to the ground too, since most of the weight will "evaporate", i.e. burn, support structure will bounce back to where it was before the weight was placed on it. But yes most of it will fly away with the molecules, you can think of a dry ice put on a trampoline, trampoline will depress, those heavy pellets disintegrate turn into small peaces - CO2 gas molecules - and fly away and take bits of the potential energy with them and trampoline will straighten out. 

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