The Geoff

Weirdly, I think we should encourage people to ask questions in forums/fora like these and many others. "Just Googleing it" is a cheap option. It's cheating in a pub quiz. All of the fun and education is in the conversation, that's why you get five or six blokes at my local pub figuring out crosswords between them of an evening. The standard line-up is a bookseller, a banker, a joiner, a chef, a barman or two and several disreputables who have never admitted to a real job but have money for beer and know what a homonym is when they see one. That's the beauty, you get some chat and learn how reliable your sources are. We used to call it "making friends" or "reputation" if you're an academic. It's lovely. Don't get me wrong, the folks who write the Internet are lovely, but they've never bought me a pint.

One of the foundations of GR as we were taught at school is that there is no way to differentiate between a force felt due to acceleration and a force of the same magnitude caused by a gravitational field. However, the acceleration caused by a rocket (for example) is linear, everything is being pushed in the same direction. A gravitational field, however, is invariably curved in some way....if I hold my arms out and drop a coin from each hand they won't fall in the same direction, they'll converge slightly because they're both heading towards the same place, the centre of the Earth. So given that an infinite plane (to give a perfectly linear gravitational field) is impossible, you can always differentiate between acceleration and gravity if your experiment is sensitive enough. Does this cause GR any problems? Or is it one of those "good lies" to try and explain the maths?

Step out of a third floor window and say that... It is a very interesting question - there's a good (but pricey) book called "In The Grip Of The Distant Universe" that covers a lot of the history and weirdness of inertia.

Right......so in effect you can change the direction of the vector all you want with two bodies, but it takes a third to provide an actual transfer of kinetic energy?

Why thank you Really? I was under the impression it was a simple result of the relative motion of two massive bodies? Simplified version: Picture a spaceship approaching the earth with velocity v (for clarity, the ship is moving left to right in our external field of view). The ship skims past the earth at the correct speed to be caught in half an "orbit" and exits moving right to left, new velocity is -v. Total change 2v, no sun required? To phrase the idea differently, a ship comes in with such a velocity that a "normal" gravity assist would only divert its course by a few degrees, but instead it catches the end of a tether and holds on until it completes a 180 degree change of course, in effect amplifying the delta-v many times. The earth would of course be diverted from its normal course, sun-orbiting or not, to balance the books. Yup, I'm perfectly aware that the practicalities are firmly in the SF realm

I'm hoping this isn't speculation as it's part of a SF plotline I'm playing with and I want it to be as good as Clarke's stuff We know the slingshot effect works from various spaceprobes that have used it. We know (theoretically) that the space elevator works. So....if you had a ship in geosynchronous orbit, on the end of a space elevator, and then rapidly shortened the tether from the ground before releasing it, it should impart a slingshot effect on the ship? Basically the energy put in by the power station on the ground would accelerate the ship without having to carry all of the fuel into orbit? I'm pretty sure it works in theory (and the practicalities are what makes it SF rather than a proposal for a paper!)

This reminds me of something on the IRC the other night.... Person X: What's a really quick and easy thing to do check that Sulphuric acid is exactly 93%?

As said, there's no rushing need to specialise. Your son sounds very like me at that age, I hope my experience is useful: I was very into science from a young age, my dad is/was an engineer/metallurgist by trade, and I was one of those "but why...?" kids. I got into all forms of science as a kid but fairly swiftly gravitated (no pun intended) towards physics. This was in combination with my hobbies which were mostly outdoor sports (climbing, rugby, mountain biking etc) and a keen interest in computers and technology. All three; science, sports and technology were a result of the same mindset I think. This was further encouraged by an excellent physics teacher at my state school (by the name of Mr Cowan) who allowed me to do my final year project on mountain bike braking systems. I spent several pleasant hours throwing myself, my bike, and an expensive and well padded laptop down some proper Scottish mountains to collect data from thermocouples and pressure sensors I'd built from scratch, and then analysed it with software I wrote myself. That's pretty cool when you're 16 and I'm still darned proud of it. Even today it's the same in many ways - I work in a big bookshop running the science section that supplies the local universities and colleges, I regularly kitesurf and find myself doing resolution-of-forces calculations whilst pulling on a wetsuit, and I recently wrote my first full-scale web application which I have a feeling could become a commercial product. Oh, I went to uni to study physics and just wasn't any good at academic discipline. I dropped out after two years. The point remains, a properly encouraged science education when you're young will stay with you for life, and that's a wonderful thing. Your son may end up being a hairdresser for all any of us know, but keep him enthralled in science and he'll produce some fascinating advances in hairdressing technology. (If you think I'm being flippant look at the money that goes into the men's razor market). Nothing ruins a good dream like too many expectations Serious, practical advice? Teach him some basic engineering skills like working on the car or fixing bikes, and get him a cheap computer to install a linux based OS on. He'll learn some real-life skills that are all too rare amongst some of the best graduates.

Hi, I'm the flatmate and yes, my ears are really quite small. I'm not on here simply to copy my flatmate in a weird SWF way (the film, not the file extension), that got boring several years ago. However he did point this forum out to me and I thought it was worth a look....turns out I was right. My background starts with a childhood combination of Clarke, Asimov and taking toys apart. Remember those toys with the wire you had to guide a hoop around without touching it? I dismantled one of those and used it to make a Santa-alarm when I was six, and I'm still quite proud of it. Would have worked too if my dad hadn't come blundering in at 2am. I tinker...there's nothing that I can't look and and think "Hmmm....I can improve that". The I'll take it apart, find I've broken it and put it back together in a distinctly Heath-Robinson style until I've got something that works nearly as well as before. It's called customisation. That's why I'm typing this on a linux system that uses a network of three different machines to get all of the hardware to play together properly and requires regular doses of coal to keep the steam pressure up..