HP vs Energy Calcualtion

the object which is moving will have kinetic energy E=MV^2. So if the object doubles it's velocity it will have 4 times the kinetic energy than previously. That's not the power required to accelerate it.

does that help?

A couple things:

First, you may know this, but when you say "move an object" you should really say "lift an object." In other words moving a 550 lb object across the floor requires less hp than lifting it.

Next there is a principal called work-energy equivalence, which simply says that if you want to do 10 joules of work, you must expend 10 joules of energy. Or in the English system, if you want to do 550 ft lb of work, then you must expend 550 ft lb of energy. Even more simply said, the units of work and energy are the same.

Then, there is power, which involves both time and energy. So getting back to hp (horse POWER) one hp is 550 ft lb per second. So power is telling you how much work you are doing in a given amount of time. Closely related to , but different from energy.

Before saying more, I'll give you an assignment. Find the English unit for mass that is closest in value to a few KG. Find the English unit for force that is closest to a Newton. Here's a hint re the English unit for mass: if you get the wrong answer, I won't slug you.

An excellent site for researching this stuff is here. When you're sure you have the force vs mass distinction in hand, then look into F=MA. When you look at A, you'll find a mysterious squared factor, that may have something to do with your original question.

BTW, (further editing your words) you should be saying "... if you double the speed, you quadruple the amount of energy required."

After you've poked around hyperphysics (which is endlessly linked) please come back with any further questions.

First point: work done = force x distance

Second : Power = rate at which work is done = (force x distance) / time

Energy is a sort of work kept in abayance or "promiss to do work when called upon"

Kinetic energy = 1/2 x mass x velocity square

Notice that there is no time term in work and energy. So the power to speed up a mass to a certain velocity becomes greater as the time involved is reduced. This is why they say zero to 100 km/hr in 6 seconds when they try to impress you on the power issue. However the work done only depends on the square of the final velocity.

I hope now you can think it out from here.

If you want to learn about science, the first thing you should do is being careful about the variables you are dealing with. Most of the mistakes I've seen come from mixing variables and units.

You said in the horsepower sentence: weight ... Well, this is a force magnitude.

In the energy sentence you wrote: M ... which stands for mass.

Do you think that force is the same magnitude as mass?

Do you know some equation that links force to mass?

What do you know about the equivalence of the different forms of energy?

Please read the first chapters of your physics book and find your own answer to the question you made.

I read all tips. Mario's is the closest I think is to follow.

My way to look at & explain any confusion with equations (some of them are using different letter to express the same "oranges") was to always write their units,

e.g. V {Veleocity in m/s (meters per second) or ft/min (feet per minute)}

Weight = mass * gravity (the place of many confusion, especially when use lb/ pound as a unit without showing if it is mass or force one)

Force uses (Newton = kg * meter/squared second).

Starting any of mechanical calculations before you will undestand (and memorize) idea of the function use SI system first, then go to US Customary (used still in US engineering calculation).

I agree with southern 123 re SI system. You will find it much easier and logical to work in SI units, because it (usually) avoids the use of units of the same name (e.g., pounds) serving as both a mass and force. (Also, most textbooks in physics will use SI units.) (Of course, in English units, there is a unit for mass -- it's simply that most people on the street have never heard of it.)

As you study further, you will come across the terms ft-lb, and lb-ft. Even engineers use these incorrectly frequently -- and they are used incorrectly so often, that now, the meaning of either has to be taken from context (just as in pounds mass and pounds force). In the 60's in the US, your physics teacher would cut your hands off if you said foot-pounds to mean torque or pound-feet to mean work. In the SI system, torque is measured in Newton meters, and work in Joules: no confusion.

Am I long-winded, or what?

Please continue with the independent and curricular study, particularly with regard to that branch of physics called mechanics, and discuss it with tutors and peers to increase understanding. Of particular need is to bring on board the definitions of units and the concepts of momentum, energy and power.

I'd like to second what PWSlack has said, especially as regards independent study. As home schoolers have found, a kid with a lot of interest in a subject, working at his/her own pace, can learn as much in a couple weeks as he otherwise would in any organized classroom in a year. An advantage that you have, that I did not as a kid, is the Web, and hyperlinks. When I was a kid, (when our TVs ran on whale oil, and when we walked uphill both ways to school) when we found a topic in a book that was not explained in sufficient detail, we'd have to make a trip to the library, and search through perhaps many books before finding an answer. Now, you can find, in Wikipedia, infinitely more stuff than I've learned in a lifetime, much of it all neatly hyperlinked.

I home-schooled my son for just one term (being utterly and totally disappointed with what was then his public school). To say that "I home-schooled him" is technically incorrect: really what I should say is that I simply got out of his way, and let him school himself. Now, as a tenth grader, he is finishing his second college level calculus course, and has finished AP physics and AP chemistry. Fortunately, he is now in a public magnet (science and technology) school where such things are not only possible, but encouraged.

In my experience, at least, what you learn in a classroom might provide a good foundation for further study, but it's what you learn on your own that will ultimately give you the most job and life satisfaction.

Hi Ken,

Congratulations on your son!

In my view, the purpose of lower education is not so much to "educate" a person as to teach "how to learn" by giving one the tools needed in the form of a good set of basic skills and required knowledge to be able to acquire and assimilate further information and knowledge: a springboard so to speak. That our Public School system seems to be confused on this point is a source of much frustration to many, including myself.

College then builds on that by improving your learning tools and enhancing your basic knowledge, especially in your major where you are now actually becoming "educated". This is even more so in advanced degrees and in the case of doctoral programs, where you are actually adding to the body of current knowledge. But then doctoral studies are a form of self-motivated study and research (I know some harried souls pursuing their doctorate will disagree) but overall it's true.

My point is that education is a never ending process as opposed to some fixed achievable goal, and self motivation is the key. Just sitting in classes, doing your assigned homework, studying what you have to and passing tests is not what a real education is about. Many, many historically "great" people who accomplished much were largely or almost entirely self educated. Were they geniuses? In most cases not, but they shared a love of learning, and/or the determination to learn.

Greg

Thanks for all the replies, but I am still a little fuzzy on one thing:If you elevate an object to 32 ft in height in 2 seconds, and then release the object and let it free-fall to the ground, it will strike the ground in 1 second.This means it produced 4 times as much energy as was required to raise it, which would be able to raise 4 times the mass to the same height.

This cannot be, I know, so I figure I am tripping up over something very simple and basic.Sometimes I can swallow a mountain, and yet choke on a gnat. The light will eventually come on, and I will realize how simple the answer was, but right now, I cannot seem to get a handle on it.....

You're right, it is simple -- you'll kick yourself. If the acceleration due to gravity is 32 feet per second per second, then, when the object is released, it starts at zero speed, and begins to pick up speed. By the end of the first second, it is going 32 feet per second. So, in the first second, the average speed is 16 fps, and the object will have fallen 16 feet. As you can guess, it won't take a full second to fall the remaining 16 feet.

The speed at which you raised the object in the first place has nothing to do with its potential energy: an object 32 feet above the reference height has a particular potential energy, regardless of how long it took to get there: seconds, months, years... does not matter. This makes sense, because you know that when you release the thing, its history is going to have no effect on how it falls.

Potential energy can be potentially (??!!) confusing. Imagine you have a 100 pound stone, sitting at ground level. What is its potential energy? Now turn around and look behind you: there is a 100 foot canyon. You start thinking about pushing the stone off the cliff. What is its potential energy now? If you have not already thought about this, poke around the web and physics texts.

Have fun. You can be proud you are undertaking this.

You are confusing power and energy once again. A good part of the problem is due to your American reluctance to adopt the SI system of units which the rest of the world uses. Any object at a height of 32 ft has a potential energy equal to its weight (say in pounds) multiplied by 32. But when the object falls it hits the ground at a velocity = square root of twice acceleration due to gravity (32 ft per second per second) multiplied by the height (again 32 ft). The rub is that the kinetic energy is 1/2 x mass x velocity squared. Unfortunately mass is not the same as the weight of the object ( pounds) but is weight divided by the acceleration due to gravity. I think in your system mass is called "slug"; which is the 1/32 of the weight. Taking this into consideration a weight of one pound at 32 ft height has potential energy 32 ft.lb. Now when it falls it hits the ground at 45.2548 ft per second. And (1/2) x (1/32)x (45.2548 squared) = 32 ft.lb So the PE has been converted into KE and all is in order.

I hope you follow the logic.

To further illustrate Yanthrum's point: Let's call 32 feet 10 meters. Let's use a real unit of Mass, 1 Kg. Let's use a little rounding and call the acceleration due to gravity 10M/S^2. We know this object will accelerate: V=sq rt (2GH) = sq rt (200) = 14.14

AT THE TOP:

PE = MGH So, the potential energy of this object 10 meters up is: 1*10*10 = 100 Joules

KE = 1/2 Mv^2 So the kinetic energy is 1/2 * 10 * 0 = 0 Joules

AT THE BOTTOM:

PE = MGH So, the potential energy of this object 0 meters up is: 1*10*0 = 0 Joules

KE = 1/2 Mv^2 So the kinetic energy is 1/2 * 10 * (14.14)^2 = 100 Joules

As you can see, the total energy is the same at the top versus the bottom, (and anywhere in between, if you do the math). You also see that the calculation is easy, because you don't have to divide by 32 to convert pounds (force) into slugs (mass). You might ask, what happens when the object hits the ground, making its speed zero? Where did the energy go? I'll leave that for you to solve.

Oh -- and here is a link where you can play around with this in an automated way.

If you insist on thinking in terms of pounds, etc. a possible solution is to do the calculations in SI, and then simply convert the final answer.

Oh -- another thought. Check this blog re using units -- which should be in your calculations, and properly cancelled out, etc. Do as I say, not as I do.

Thanks for the link...very helpful!

Thanks! That cleared it right up.!

Yes, this is IT!

It is nice to use all units (e.g. m/s) in SI system, it is almost MUST to use units in calculation in US system. Velocity here could be: Ft/min, Ft/sec, in/s etc.. so it is easy to get wron number as a result of calculation in US C system.

I agree with you and Yanthrum: SI makes so much more sense. The mass issue alone should be enough to get us to convert, but still we cling. 1/1000 of the general population has heard of a "slug" -- it is only used routinely in aerodynamics, and a few other applications. Otherwise, we use this bizarre pound force vs pound mass concept -- but in routine use, we drop the "mass" or "force" part -- the only way you can know which is from context -- and if you are just learning physics, getting the context can be tricky.

That anything engineered in the US actually works, is a miracle!

That issue seems to crop up in a lot of places.The US put a man on the moon using a slide rule, very basic computers and the English System.We crashed a probe on Mars because someone forgot to convert to Metric, or vice-versa.I agree there should only be one standard.Converting between two systems leads to errors and confusion, even with seasoned engineers.

Standard ball bearings are metric, and to convert to an inch diameter for a press fit on the bore, you have to multiply the last two digits by 5, to get MM's, then multiply by .03937 to get inches.Much simpler to use metric measurements.Inch bore bearings are available, but are considered special sized.

Either system works if you are properly trained in it, and humans are more comfortable with the system that they grow up with.I daily convert inches and feet to metric,mentally, without much effort, but it took a while to get used to.

The only way to change the way people are taught is to train a new generation from the start.Then it will be natural.It is being done, but old ways die hard.

US education system uses still books (physics, mechanical & other tech domains) written by old-good educators who, I recon, had no time to read and apply SI system of units. The one that should be realy Internationally accepted and used.

Also someones think metric is SI - wrong. So I see in "modern US" physics examples uses cm (= centimeter) insted of m = meter as a basic unit. In the mechanic small world EU engineers use mm = millimeter = 1/1000 m.

Degrees Celsius are metric but SI uses K (Kelvin without degree symbol) as temperature unit etc.

US system, and francly, English system is no longer a basic system in UK.

Some conservative users are in the marine world where you can find even different miles, tons.

Guess: is German HP = US HP?

Sizes of wires (electric conductors, spring) are simpler in EU = mm^2 (squared millimeter) that can be easy used to calculate e.g. a drop voltage without asking for calcukation - data tables/sheets. Drill bit numbers cannot tell you directly what diameter of the hole would be after drilling is done.

These ways keep away some amatouers from master electricians in USA, right?

Like japanise tools from repairing Hondas.

But they took so many hour of young people learning time so they could not be ready for high level competisions before their 30s (sarcastic!). Thanks God, the computer logic uses binary system, and waffer production measures in SI.

Sorry I did not used Speller Checker so my old fingers could not hit proper keys on the keyboard.

You bring up lots of good points -- fun to read. Having worked as a mechanic on European, British and Japanese cars and motorcycles, I have the most peculiar assortment of wrenches. Even the metric world is not free of inconsistencies (DIN vs JIS, etc) but if you added up all those inconsistencies, they would have to be a small fraction of the inconsistencies in the US system alone. Although I was taught the metric system in the late 60's, (early "Metrification') my kids seem to be the first generation to really "think" in metric here: they are more likely to state a size in cm than inches. SI: not so consistent here. The kids, at least, tend to use whatever unit seems to "fit" best.

Me -- I am utterly and totally inconsistent. One HP= 167Meter-pounds/sec

In the interest of clarity it is high time all enginers use the SI units. In future we have to avoid horsepower and give engine outputs in killowatts. Then there will be no confusion over German HP and American HP or British gallon and American gallon if we all stick to litres. The object of a language is to convey ideas and using the same term for different things is absurd. A killo of some stuff gives a clear idea where as a pound of it will be different quantities at different places even on earth.

I agree completely.

It's amazing how many differences there are between British usage and American, given that we were a British colony. Our US gallon was defined by British Parliament in 1707, and has remained exactly 231 cubic inches ever since. So it is especially ironic that the units are different now. In 1824, British Parliament abolished the older units, and came up with the Imperial Gallon. By that time, we were no longer a colony, and the idea of changing our measurements to match the British measurements would have been completely absurd, given that we fought a long and bloody war with them not long before.

So the difference stems from hatred, really. That hatred has waned over the years and has been transformed into mutual affection -- but the difference in units remains.

Now, I suppose the main reason that the US clings to outmoded units has to do with a basic principle of physics: Things in motion tend to stay in motion. There is also a comforting human quality about the US system: my foot, in a shoe, is within a couple mm of being a "foot" long, so I can pace off feet very precisely (and can pace off yards with a comfortable stride). 5' men are short, 6' men are tall, and 7' men are basketball players.

But while the US system might "feel" good, there is little logic to it, and for scientific purposes, its clearly more difficult. In the pure sciences, I think the use of US units is very rare in the US. But in engineering, it remains common -- so joint scientific/engineering pursuits (Like the Mars Climate Orbiter) can go haywire, when people are careless.

No my friend

One Horsepower is the force required to move 75kg. in one second the distance of one meter so this would mean a 200hp car would be twice as fast as a 100hp car if both cars weigh the same and this is far to be true.

The answer is around the fact that "Response is not linear". The more power you apply the faster you move, it´s true but not in a linear response because there are limits in nature besides, external factors play an importan role too. But even if we get rid of external factors such as wind resistence and your conditions are ideal you will never get a linear response.

Regards JR mexic777@hotmail.com

JR brings up a good point. In physics texts, you will often find little notes (in problems to work out) like "ignoring friction" or "ignoring air resistance". Sometimes, the things eliminated from the question can be the predominate effect on how things actually work. The example of car speed vs hp is an excellent one. I'll use the dreaded US units below, because for our student, the idea of a 75 Kw car will not be intuitively easy to grasp.

At a constant speed, the main things keeping a car from endlessly accelerating are rolling resistance and aerodynamic drag. At highway speeds and beyond, rolling resistance is the smaller effect, and the slope of the line (resistance/speed) is quite gentle, so that with a 2000 lb (mass) car, at 50 mpg, the rolling resistance might be 25 lbs (force), and at 100 mph it might be about 35. But aero drag goes up with the square of speed, so if it is 50 lbs at 50 mph, it will be 200 lbs at 100 mph. So, at 50 (73 fps) the drag is 75 lbs, and at 100 (146 fps) the drag is 235 lbs.

Hp required at 50: 75*73/550 = 9.95. (This number is very close to actual test data for a very small car.)

Hp required at 100: 235*146/550 = 62.38 (This number also squares very well with data from an old Daihatsu I have, which will do 90 on 53 hp)

As you can see, 62.38 and 9.95 are in anything but a 2:1 ratio.

So when the physics text says ... "ignoring aerodynamic drag" you may be ignoring more than just a minor thing.

A little side note on the British vs American systems:

A Billion in England, is a Million-Million(1000000000000)

In America, it is 1000 Million(1000000000)

Go figure.