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### 课程: 宇宙学和天文学 > 单元 4

课程 2: 人类的进化史# 耕种一公顷土地所需的能源投入

耕种土地的能源消耗的思考. 由 Sal Khan 创建

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in the last video we talked about how humans over time have gotten better and better at getting more calories out of a given unit of land and one thing I do want to emphasize in the last video these numbers that I came up with I picked these numbers to make the math fairly simple and to give you the general idea I don't want you to think that the most that a square kilometer can support today is exactly 1,000 people it depends usually on what the land is like how much of the land you're actually using for agriculture what crops you're planting etc etcetera how the people are living how many calories they need the whole point of the last video was just to give you a framework that Wow there is this upper bound based on how much productivity you actually get from the land now what we want to think about in this video that was kind of the last video was this axis right here getting more and more out of the land what I want to think about in this video is how did humans through different technologies how did we get by doing less and less of the labor for getting those calories out of the land obviously you just don't have land and and things spontaneously grow all that well I guess that would happen in the wild but if you're doing agriculture you need to put some energy into the land you got it you got to work the land and so what we have over here in this chart and this chart is derived from information from this book right over here this is energy and society and what we want to think about here is the different ways that humans have gone about to till soil so we're not even going to think about the total process or the total energy required to grow a certain crop what we're just going to focus is one step of the agricultural process and that is tilling the soil and in case you are like me and you have never worked on a farm which I'd want that's one thing I would like to change one day is actually go through that process but tilling the soil is kind of churning it up so you get the nutrients from the bottom layers to the surface you dig you you bury all of the remains from the last harvest you bury all of the all of the all of the weeds so that they die and you also kind of get air in the soil what it does is it essentially it prepares the soil for the next for the next agricultural cycle so it's a process that that humans have been doing antiquity and what I want to do in this video is think about the different ways to do it and how much energy is required to do it and we're gonna think about the energy in two ways how much of that energy comes from humans and how much how much of that energy comes from things other than humans so just as an example when we talk about human power we're talking about someone literally hand plowing this field so this this woman right over here is literally she has this little cart that's digging up the soil behind her when we talk about oxen power we're talking about the oxen doing most of the work they're the ones dragging this plow which is digging up all of the soil and this gentleman has to kind of be there to supervise but this still is a good still is fairly intense labor that this this this gentleman is doing right over here and then when we talk about tractors we're talking about a scenario like this where the tractor is doing most of the work of actually digging up dragging this plow behind it and digging up all of the soil and from this book right over here that's where we got these numbers I'll tell you which numbers I got from them and which numbers I kind of kind of reasoned through because I wasn't fully comfortable with the numbers that they had but these are their numbers that if you're a human power to till one hectare of soil it'll take 400 hours oxen power and this should be a pair not a Perry that should be a pair of oxen 65 hours a six horsepower tractor 25 hours a 50 horsepower tractor four hours and in case y'all are wondering what is a hectare of soil it is literally a plot of land a hectare of soil let me write it a hectare of a hectare of land I should say is a plot of land that is 100 meters by 100 meters and it's roughly it's roughly equal to two and a half acres not exactly two and a half acres it's like 2.4 I think two point four seven something but roughly two and a half acre so we're just think about how much how many hours to essentially dig up all of the soil for a plot of land hundred by hundred meters so what we have over here so clearly human takes a lot longer oxen they can do a little bit faster six horsepower tractor even faster a 50 horsepower tractor very powerful tractor even faster than that now this column right over here is the machine the amount of energy required to actually produce and maintain the machinery used so this is a very unintuitive thing whenever you think about whenever you like for example whenever you think about the amount of energy to to plow this land over here you you tend to think okay well this this individual is going to have to expend a lot of her energy you don't think about the amount of energy required to actually maintain the tool to one build the the the tool that she's using in this case a hand plow and then to maintain that as she does it and so this estimate and I got these two from the these fellows right over here actually one of them might be a gal but it's about 6,000 kilocalories and this is K this is kilocalories for lowercase e and one thing I want to emphasize here one kcal is equal to the same thing as 1 calorie with a capital C which is the same thing as 1,000 calories with a lowercase C and we talked about this in the last video but these when people talk about food calories they're really talking about a calorie with the capital C or you could say they're talking about kilocalories so your candy bar 200 calories they're talking about this right over here in chemistry class when you talk about the amount of energy to raise a gram of water one degree Celsius you were talking about these calories here so in all of these numbers in this chart right over here they are in either you can either view them as this unit kcals kilocalories or calories with a capital C they're essentially the same units that we used in the last video and these are the same numbers that you were used to from a kind of a dietary calories view so for example 6000 calories that's about how much a typical male would expend in 3 days so this is to maintain it over the course of these 400 hours in the case of the hand plow and the total amount of calories it what the total amount of calories that were needed to make the plow divided by the total number of hours so whatever fraction of the plows life is being used here you use that fraction right over here to put this 6,000 calories but needless to say for at least plow for the kind of the the either the human or the oxen scenario this isn't a significant amount of the total calories so obviously if you're doing it either with human power or oxen power you're not using any gasoline you're not using any petroleum and all these scenarios we're going to assume that someone has 10 working hours in the day and that right over here where this is kind of a measure of how hard that person's work is and I kind of estimated these numbers here they're slightly different than what what the original numbers were in this book right over here but we're saying look if you are actually walking along using this hand plow that is actually very very vigorous activity so it is going to require about 400 calories per hour to do this type of activity you do it over 10 hours it's gonna require 4,000 calories just to do that over 10 hours and then we're assuming that there's some rest that the rest of the day you're gonna walk around and maybe you're gonna cook dinner eat breakfast you're gonna sleep some of it we're assuming that the other 14 hours of the day are going to be at about a hundred calories per hour and so this is the total if someone were to using this technique work for a total of 10 hours this is how many calories they would consume in the day and you could see this is the most labor-intensive so it looks like that they would consume the most calories per day this is these two are the least labor-intensive you're sitting on a tractor although that still requires more calories than sleeping or watching TV and so that's the number of calories they would consume now this right over here and this is the interesting number or one of the really interesting numbers based on all of these assumptions this is the total human input in calories to do this task to till this one hectare of soil so over here you're using 5,400 calories a day if you're working 10 hours per day and it requires 400 total hours you're gonna be working 40 days right 400 divided by 10 40 days times 50 400 calories per day you're going it's going to take a human it's going to take a human just a human part not even thinking about the 6,000 calories necessary to maintain and make that plow as the human is going to spend two hundred and sixteen thousand calories to till to plow that one hectare land and if you add the other 6000 into for the actual plow and you can debate what this number should be but it's not a significant number compared to this you get about two hundred and twenty-two thousand total calories when you go to the oxen situation you're acquiring fewer hours and each hour it requires a fewer a little less calories this is this is still labor-intensive but not as labor-intensive is what this woman right over here is doing so on a daily basis you're using a few a little bit fewer calories but since you're only doing six and a half days of this 65 hours divided by ten you have you've significantly reduced the number of calories the total number of calories that the human needs to put into this task now there still is other energy being done now all of a sudden the oxen have gotten involved and if you assume that each oxen consumes about 20,000 calories a day and you have two of them so 40,000 calories per day for just to feed the oxen and you're gonna do that for six and a half days 65 divided by 10 the oxen are going to consume 260,000 calories to do this task so the total energy input here now is has gone up so this is an interesting phenomenon that is going on right over here what the human is putting in as we get better and better technology goes down substantially 216,000 at 33,000 and we'll see what the tractor it goes down even more but the total energy if you include the amount of energy that the oxen have to put in or if you include the amount of energy due to the gasoline that has to be used for the tractor the total amount of energy is going up to plow that field but the human energy goes down dramatically now the last thing I want to highlight here these are where my numbers that depart a little bit or fairly significantly from this original study right over here these are this original estimate is the machinery input on the tractors so if you look this up and you can Google search it no they have much larger numbers here but I did a little research and it looks like for most petroleum based combustion based engines or vehicles roughly 20% of the total energy that's used in fuel 20 percent of that energy is used for the actual production and maintenance of that vehicle over its life so what we did over here is we said okay four six horsepower tractor I use their numbers where you're gonna have to use twenty five hours to do it it's going to use this much petroleum assuming that it uses twenty 3.5 liters of gas or petroleum over twenty five hours and then I just twenty percent of that number for saying well how much of how much how much energy had to be used to maintain that vehicle over over that amount of time and if you think about what fraction of this vehicles life that 25 hours represents that fraction times the total amount of energy required to produce that so remember these things are made out of metal they had to be made in furnaces so just producing a vehicle requires a lot of energy and so this is this right over here is 20% and I just use that rule of thumb for most petroleum-based or combustion based vehicles the 20% of the total energy expenditure over the course of the that vehicles life it was is roughly is roughly equal to the amount of energy used to produce that vehicle but either way you go all the way over here the human has to spend less calories sitting on the vehicle so they spend less calories per day and then the total human input right over here for the six horsepower tractor it's going to take them two and a half days 25 hours at ten hours a day is gonna be 8,500 calories but of course you have the petroleum used and then some estimate of the amount of energy used to produce that tractor and you're just taking the fraction over that 25 hours you're not taking the entire life of that 6 horsepower tractor to produce a 6 horsepower tractor this number would be much much larger if you talked about the total number of energy we're just taking the small fraction of its life that we're using it right over here same thing for the 50 horsepower tractor but either any way you look at it the human and this is the really interesting thing humans by going from human power all the way to a 50 horsepower tractor you're getting almost a factor of 200 improvement in terms of how little energy has to be put in by the human to till that land but you actually get a total increase if you factor in things like the petroleum and then definitely yeah the machine the amount of energy to actually produce that machine so anyway hopefully you found that interesting I find this I know kind of it's something that you don't think a lot about how much energy input has to be put in and often times we only think about the human energy input but we're not thinking about the energy input from other things like like oxen