CaptainPanic

What purpose would this have? Normally, cargo is at the surface of the earth, and has to go somewhere else on the surface of the earth.

You have to consider whether the weight of your apparatus scales linearly with volume, or with surface area of the displacement volume that you want. If it scales linearly with the surface area of the volume of the "vacuum balloon", then you only have to make it bigger to reach a point where your balloon can lift the apparatus.

Yo! LOL! Absolutely. Couldn't agree more. You're stating the obvious... so it's really hard to disagree. LOL! Anyway, it's really cool that all you kids like the air trecks. I hope some of you are learning some real engineering in the meantime. I really hope that some of you are doing calculations on the power requirements, battery life or torque - to name some random keywords... and not just being cool on the forums.

Jon13 hasn't logged in since May 27, 2008... I don't think he will answer your question soon.

Sounds interesting... I would suggest that you make a comment about your book on our special books subforum, called "book talk"... and maybe drop some seasonal greetings and a 'hello' on the introduce-yourself thread. ! Moderator Note The second part has been taken care of; "Hello" posts are generally moved to this thread

Do IQ tests exist that are completely unbiased towards language? And have they been used to generate enough data to make such a claim? If someone just looked at a number of IQ tests from people in different countries, then who can guarantee that the baselines of value 100 are drawn at the same level?

A woman only needs 1 man to become pregnant... but 1 man can make many women pregnant (unless they're very serious about being lesbian, and don't allow men near them... but the porn lesbians don't tend to be very strict). Just my 2 cents.

... so it's easier to fall on your back when you accelerate?

Fact: Only 4 out of the first 100 posts in this thread contained a linked reference to some background article... A link like this. (I really hope I didn't make a mistake counting the references )

A warm welcome to all new people! I hope you'll enjoy the forum and I hope you can make some nice contributions, or use the forum to learn and ask questions.

The picture in that link is an ordinary skate. Not a motorized one. I guess that the tips on that link measure up to the assistance that was given in this and the other thread... with the difference that we're critical about difficult/impossible ideas while this guy makes it all sound easy. (Recharging the battery by coasting? I'm not so sure that should be point #1 priority). What is interesting is that it's true that some motorized skates exist... and they have exactly the looks that I predicted: They're Big. They don't like the cartoon skates at all. They apparently have combustion engines... which is probably a good idea if you want to actually go anywhere. I guess electric engines will work as well... And I'd like to place a few critical notes at the battery... especially on the link that was posted earlier: Here's a picture of a 12 V, 40 Ah battery. Note that it's quite huge (14.5 kg!). To go at a nice speed, and do some acceleration and braking, you really would like to have 750 W of power (1 horsepower). A 12 V, 40 Ah battery can supply 750 W during 38 minutes... which is really quite reasonable. But you must realize that you're skating around with huge skates and 15 kg of battery strapped to your back. I never said it's not possible. I just don't think it will look and behave like in the cartoon... and I want you guys to be realistic before you build anything that doesn't work.

Are you suggesting to make a perpetual motion device out of your rollerskates? You want to generate all the energy for movement in the skate? Please note that it's impossible to do that... impossible. This whole thread isn't making much sense. I haven't read a lot of good ideas... and frankly, I think all people posting here are just kids (<15 yrs) with little engineering background. All people who post here suffer from a number of issues: -A huge underestimation of the power required. -A huge underestimation of the size of both the energy storage (fuel/battery) and engine -A huge underestimation of the complexity of the control system Therefore my message to everybody here is: please stop dreaming of things that you see in cartoons. Find some device that actually works and delivers enough power, and go from there.

A warm welcome to all new people. I hope you'll be able to find what you're looking for, and I hope that we can also learn something from you guys as well!

Dijon mustard?? LOL!!! Just imagine that any other world leader gets into such trouble for eating at McDonald's If this is true, then it's the best joke ever. I say impeach the bastard! He's eating French food!

Your speed controller should be something of the type of a dead man's switch. If you fall, and let go of the controller, the engine(s) must stop immediately. I do not believe that this essential part is found on a radio speed controller... Please consider safety first, then practicality. As in the other thread - has anyone built anything so far? Anything at all???

Although I think it's a bad idea for a lot of reasons (it's complicated, and the sulphur content of food is very low, and the price of sulphur is low - it's a waste product from refineries) But - You could extract it I guess... if you really don't care about the other components, then just combust it, and check literature for a SO2-removal technology. Then convert this SO2 to sulphur. It's a lot of work, and you'll be working with dangerous gases and it requires many steps. It's not worth the effort. What do you need sulphur for anyway?

1. Electric engines have their specifications in watts or kilowatts. Only combustion engines are generally specified in horsepowers... do you want combustion engines strapped to your feet, or do you prefer electricity? 2. Just look up how much a horsepower exactly is (hint: it's close to 750 W), and calculate it yourself.

It can happen that the reference is restricted. One should attempt to find similar info in an open source reference, but if that's not possible, then the restricted source is good enough. Better a restricted source than no source (although I also find it annoying if it's restricted - it may all be better in the future!). Personally, I have quoted in numerous cases from sources that the majority here cannot check easily. I happen to have some books which contain data that cannot be found easily online (thermodynamic data is just hard to find sometimes). In addition, I have used Dutch sources. You guys just have to trust me that I translated it (or dump it in Google translate). Personally, I think that this should be enough. Requesting that I only refer to open source information would restrict me in answering questions... which is of course not desirable.

I cannot find your claim in the article you refer to. (The article itself has again lots of references - could you link to the correct one?)

I'm looking forward to read your next idea. Lemming powered skates? For everybody who is seriously considering rollerblades with engines: Please check the following things: 1. Jack ass. They tried rocket skates. It's on Youtube. It hurts. 2. The smallest engine that you can find for a bike/motorbike or any other transportation for people. Notice that it's quite big! The engine from your radio controlled car isn't gonna do it. 3. Any mechanism to connect the two rollerblades. If the power output is not exactly the same on the left and right, you're going to hurt yourself a lot. At 10 km/h (not fast), a 1% difference in speed means that your feet are moving 3 cm/s away from each other. That's more than half a meter in 20 seconds. I could come up with lots of other reasons why this is a bad idea... but somehow I get the feeling that a bunch of people fell in love with technology from a cartoon. People: Build a motorized skateboard! It has many advantages: Point 3 is no issue. And you can jump off it when it all goes bad.

lol... poor squirrel. If a horsepower is about 750 W, then how much is a squirrelpower? 0.75 W?

A surgeon, an engineer and an economist are sitting in the bar. They're all boasting about their achievements and their importance. Then the Surgeon says: "I have the most important job of us all". "Oh yeah? Prove it!" replies the engineer. "Well, when God created Eve from Adam, he used Adam's rib. Surely only a surgeon could do that!" The engineer smiles and replies: "Well... where do you think Adam came from? Adam was created from Chaos. And surely that is something that only an engineer can do!". The economist looks smugly and replies: "What do you think it takes to create that Chaos?"

Biologists, physics experts that brew beer, retired computer engineers and shy people who claim not to know too much and are here to learn. Great stuff. Welcome all

Contrary to popular newbie-belief, the introduction thread is read by people who visit the forum regularly. Welcome to all new people!

The thrust of a rocket is expressed in N. It's a force, which (if it's larger than the force of gravity) will lead to an acceleration. The trick with a rocket is therefore twofold: 1. Make enough thrust to accelerate (to overcome gravity) 2. Continue to accelerate long enough to get into orbit, meaning you need to reach something like 6-7 km/s (kilometers per second!). [math]\mathbf{T}=\frac{dm}{dt}\mathbf{v} [/math] (source: wikipedia) In that formula, [math]\frac{dm}{dt}[/math] is the amount of mass you push out of the rocket engine. This can be expressed in kg/s. It can be calculated in many ways, but probably the most common is to simply look at how fast you burn the fuel. The harder part is to estimate the exit velocity of the gas. This is described pretty well on wikipedia, on the "De Laval Nozzle" site. Wiki's rocket engine nozzle site gives additional info. The formula you need is: [math]V_e = \sqrt{\;\frac{T\;R}{M}\cdot\frac{2\;k}{k-1}\cdot\bigg[ 1-(P_e/P)^{(k-1)/k}\bigg]} [/math] The only thing not explained on the wikipedia site is how to calculate the pressure that is developed from the propellant. But a little googling got me also a site which seems to explain that one: http://www.nakka-rocketry.net/th_pres.html - search for the "steady-state chamber pressure". That all is needed to calculate the nozzle velocity. From there on it's a matter of [math]F=m\cdot a[/math], and you shouldn't forget to correct for the air-friction that you'll encounter while you are in the atmosphere (it is definitely significant enough to take into account, but since the pressure and velocity is not constant, it is another nasty formula).