What Is Horsepower & Why It's A Dumb Unit - America vs Metric

hello everyone and welcome in this video we are talking about horsepower arguably one of the most important units when talking about cars so we're going to be focusing on three questions in this video first of all what is horsepower second of all why does a car that's sold in america and sold in europe have a different horsepower rating in each country when it's the exact same car and then our third question is why is all of this nonsense here on the whiteboard pointless and what unit should we be using so we're going to get through that starting with what is horsepower and horsepower is a unit of power well what is power well power is work over time now that sounds useless and confusing so what is work over time well work is force times distance so if you have a car it takes a certain force to push this car a certain distance that's work now how quickly you move it from here to here that is power so power is simply force times distance divided by time now we want to know what is the unit horsepower and so this is a unit developed by james watt now james watt is known for his steam engine and so his steam engine was often replacing horses which were doing the work and so he wanted to be able to communicate okay what can my engine do how many horses can my engine replace and so he needed to come up with a unit in order to make that comparison that unit being horsepower and so there are many different stories out there and derivations of how this came to be how this unit came to be um and many of them are not cited i found a journal american journal of physics entry in 1936 that detailed how he came up with this um and looked at his actual notes and so that's what we're going to be using as the basis for how he derived this unit and in this article they talk about a horse rotating a mill and it has a 12 foot radius from the center so that horse presses a force to rotate this arm right here and then you have a grinding mill in the center and so it goes around and around and around in circles and so it has a certain force that it's applying and has a certain distance that it travels and then it travels that distance in a certain amount of time so we have all the things we need in order to figure out what is power and so james watts says that the horse pulls with that force of 180 pounds of force and an interesting thing from this article what it said about how he derived this unit nothing is said about how he arrived at the 180 pound pull exerted by a horse and so that's kind of the unfortunate thing is like we're using this unit i mean he probably didn't know when he made this unit that we would be using it uh today and it would be so meaningful and he just kind of you know this is back of the napkin math obviously the guy was very smart but i don't think he had the foresight to see how much how often this unit would be used and it's kind of very nonchalant how we arrived at it so we don't know where 180 pounds of force comes from but that's what the horse is pulling and it's at a radius of 12 feet and so we can calculate the distance that it travels to pi r uh and it is able to travel around 2.5 times 2.5 circles per minute so this is what his uh james watts notes showed and so we can do the math here to figure out how much horsepower is that so power is equal to the force that's our 180 multiplied by our distance so that is going to be 2 pi times our radius which is 12 times and then 2.5 times per minute so multiply by 2.5 times per minute and so we can multiply all of that out and that gives us 33 929 foot-pound-force per minute now he did a bit simpler than this again back then napkin math that he's kind of going off of and he just said well 2.5 times per minute at this radius that's about 60 yards per minute so that equals 180 pound force multiplied by 180 feet per minute and he arrived at 32 400 foot pound force per minute now later in his writings he starts to write this as 33 000 foot pounds force per minute maybe he's just rounding up we don't exactly know but that's what he starts going with again this is like it's crazy this is the unit we're using today and this is how we got to it but this is the unit today so one horsepower today is equal to 33 000 foot-pounds of force per minute or 550 we can simply divide that by 60 550 foot-pound force per second okay so just looking at these numbers and these units it probably doesn't mean anything to you directly so what do these numbers actually mean well if you were to have a horse and it's got this pulley system so it's got a rope going across this pulley and it has 550 pounds hanging from it this horse would be able to lift this 550 pound weight one foot in one second so that's 550 pounds of force up one foot in one second so that's what that one horse is doing so if you had a car engine that was then strapped to the same system well it would be able to either take that 550 pounds of force say it has a 200 horsepower car engine that is doing the work here well it could lift that 550 pounds 200 feet in one second instead of just one foot or it could do a lot more weight multiply the weight by 200 110 000 pounds uh lift that one foot in one second so you can see that you know you multiply the horsepower by 200 you can do significantly more work uh in the same amount of time now how does this actually apply to cars well ultimately it's coming down to you know when a car is driving it has certain resistive forces aerodynamics inertia by accelerating rolling resistance and internal friction within the bearings in the drivetrain of that vehicle and so it's trying to accelerate against that and that's what it needs power for so power ultimately allows the vehicle to put down a force at the wheels and accelerate so it's determining its top speed its acceleration its rate of work in other words how fast can that car move from one spot to another the more horsepower it has keep everything else equal then the faster it can move from that spot to another spot okay so what is metric horsepower and you may see this written as ps cv ch other uh abbreviations so all of these are simply abbreviations for the words horse and power in different languages ps being german cv being italian ch being french and so on there's many different ones out there but they all represent metric horsepower and so if in sae units we are looking at foot pounds force per second then in metric instead of feet we have meters and instead of pounds force we have kilograms force so in metric we are looking at kilogram force times meters per second so one horsepower which we just figured out is equal to 550 foot-pounds force per second whereas one metric horsepower is equal to 75 kilogram force per meter meter per second squared and these are fairly close uh one of the interesting things we need to look at here though is that we have a unit of force and so a pound force force is equal to mass times acceleration so a pound force is equal to a pound of mass multiplied by gravity a kilogram force is equal to a kilogram multiplied by gravity and so for example with our 550 foot-pound force per second if we multiply that by gravity here to write it all out in complete units so we have 550 multiplied by one multiplied by 32.2 feet per second squared that's gravity's pull in sae units then we get a number of 17694 pound-feet per second pound-feet squared per second cubed now i know looking at this it's like what is all this it's meaningless and it truly is it's truly just meaningless so let's just power through and we'll get to some very interesting conclusions so if we were to convert this to metric we would get 745 kilogram meters squared per second cubed now one uh metric horsepower we know is 75 kilogram force meter per second so we're multiplying that by gravity 9.807 and we get 735.5 and as you'll notice these two numbers are not exactly the same thing in fact one metric horsepower is equal to 98.6 if you take this number and divide it by this number 0.986 98.6 percent of one american horsepower so that is why you will see cars have a different horsepower rating in europe versus in the us so for example a ferrari 812 superfast its name actually gives some hints about it so the 8 standing for 800 cv uh and the 12 standing for a v12 but that 800 cv or metric horsepower is equivalent to just 789 horsepower which is what you'll see it rated as in the united states same engine same horsepower just a different unit being used mclaren 600 lt the 600 standing for 600 metric horsepower but in the us that is 592 horsepower so again same car but using different units when discussing horsepower so finally we get to our question why is all of this nonsense and what units should we actually be using so what if or should i say what if oh boy oh boy okay so what if there was a unit that equaled one kilogram meter squared per second cubed instead of this 745.6 which doesn't make any sense 17 694 pound feet squared per second cubed which makes no sense 735.5 kilogram meters squared per second cubed which is the metric horsepower again doesn't make any sense what if there was a unit that just equaled one you know wouldn't that be great and there is the what so that's the fantastic thing named after james watt and you can't fault him this unit came about after his time named after him and so one watt is one kilogram meter squared per second cubed or you could use a kilowatt a thousand watts and that is equal to a thousand kilogram meters squared per second cubed so these units make a ton of sense you can derive them and you have you know these base units that all line up in these beautiful neat one 1000 numbers rather than this nonsense 1700 735.5 none of that makes any sense so why use it when you could use the what and then people will say well i'm not familiar with talking about cars in terms of kilowatts but but yes you are you know when you look at a light bulb this is a 10 watt bulb you've heard that before this is a 45 watt bulb you've heard that before so when we talk about tvs computers vacuum cleaners microwaves we talk about electronics all the time we talk in terms of watt which is a unit of power and it's a unit that makes plenty of sense uh versus this back of the napkin mat which was used to derive one horsepower doesn't even actually represent one horsepower a horse could have more than one horsepower so it's silly to use this and this is just such a logical unit to use and thankfully some electric cars we are starting to actually talk in terms of kilowatts which is fantastic so thank you all so much for watching and if you have any questions or comments of course feel free to leave them below and i'm going to keep talking in horsepower because you know what this is america and that's what we do here we just make it difficult and we use the units that don't make any sense why not

Channel: Engineering Explained

Views: 593,287

Keywords: horsepower, what is horsepower, hp vs ps vs cv, difference between hp and ps, metric horsepower, american horsepower, horsepower vs torque, james watt, engineering explained

Id: gC2-JKO0c2I

Length: 12min 22sec (742 seconds)

Published: Wed Oct 28 2020