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Subbing your Space Shuttle entry

Post 1

Lbclaire

Hi Pinniped,

I'm subbing your entry on 'The Physics of Space Shuttle Re-Entry: Bringing Home the Brickyard' (A6381038). I'm enjoying it, and there's not much to be done that I can see so far, but I wanted to insert a few explanations for the less-physics-knowledgeable amongst us. However...here I run into a problem. I am not a scientist, by any stretch of the imagination and I dropped physics like a hot brick as soon as I could at school. So I'm finding it difficult to formulate a correct, yet accessible explanation. smiley - headhurts

I want to define kinetic and potential energy. Kinetic energy's not too bad - I have 'The energy which a moving object possesses because of its motion'. It's the potential energy bit that I don't understand myself, let alone can explain to anyone else! At the moment, I have 'The energy stored in an object due to external factors such as its position relative to other objects.'

Can you think up a better way of putting it?

Also, you've used the word 'perigee' a couple of times - could you give me a definition of this for another footnote, as I've never heard of it?

Finally, I've inserted a definition for Mach: 'The term used to describe the ratio of the speed of an object to the speed of sound. The numerals indicate the number of times faster than the speed of sound the object is moving, for example, Mach 2 is twice the speed of sound.' Does this sound OK to you?

I could probably ask some scientific bods onsite for help, but I'd rather you had the chance to use your own words, as it's your entry.

Let me know what you think.

Thanks,

smiley - smiley Lbclaire


Subbing your Space Shuttle entry

Post 2

Pinniped


Hi Lbclaire

smiley - cool that you're doing this one.

Your points :
1. Kinetic/Potential Energy.
There are lots of systems that exchange energy like this. A familiar one is a pendulum. At the ends of its swing, the bob stops moving. All of its energy is now potential. At mid-swing, its kinetic energy is maximum (the bob is moving at its highest speed). It took that energy to move from the potential energy store, and as it swings through, slows down and stops at the opposite top of its swing, it's putting kinetic energy back into the potential energy store.
Get the idea? In fact the space elevator is a bit like a pendulum whirling, ie being swung through 360 degrees round and round so it never stops. Think of a long rope with one end tied to the ground that's whirled round by the rotation of the earth. Now if you work it out, you can prove that a rope that's long enough and light enough has got enough kinetic energy to keep the free end up in space.
OK so far?
Right - we don't stop there, ie with a rope only just long enough to stay up. If we make our rope longer still, there's an excess of energy in the working system and the excess gets stored in the cable as tension. This stored energy is potential energy, and we can borrow some of it to pull something up the rope (a space shuttle orbiter maybe?) without the free end falling down. The really neat part is that when the something comes back down, its borrowed energy (which has been kinetic while the something was orbiting) now goes back into potential energy in the rope/elevator.
This might sound like picking yourself up in a bucket, but in fact it's seriously sustainable physics. There are a few engineering details to sort out, though...
I hope that'll help you understand. I wouldn't recommend trying to footnote anything on potential/kinetic energy, though, unless its a good external link that you find. It's enough of a subject for its own Entry.
2. Perigee. This means the near-point of an elliptical orbit (apogee is the far point). I put this one in a footnote itself, because the proper maths is really quite difficult. I thought it was best for the layman to think of the re-entry burn as slowing the orbiter down, so that it starts to fall back to earth under gravity. In reality (as the footnote says), the re-entry burn alters the shape of the orbit from near-circular to a flatter ellipse, with the perigees low enough to be inside the upper atmosphere. The orbiter is brought down by the plasma braking near to perigee, but perversely enough, it's actually getting faster as it approaches perigee because that's a consequence of energy conversation in elliptical orbits.
I don't think I'd try elaborate on that either, honestly! Another Entry in its own right, perhaps (and if you want to know more, read up on Kepler)

The Mach definition sounds fine.

OK? There's only so much you can do to simplify this particular Entry. Believe me - in some ways you could argue that it's already been oversimplified in the form I wrote it.

Kind of ironic that someone you know collaborated on the EG Entry on why scientists are incomprehensible, yeah?smiley - winkeye

Pinsmiley - smiley


Subbing your Space Shuttle entry

Post 3

Lbclaire

Hi Pin,

smiley - ta for the explanations - there is now a glimmer of understanding on my part smiley - winkeye. It must be very tricky to explain many concepts of physics, and I think you did a good job there!

Thanks also for the vote of confidence - subbing's great for learning about subjects you never knew about, but I sometimes have to beat my brain into submission to get some of it so that I can make a judgement on whether it reads OK.

Not that your entry doesn't, smiley - yikes I just feel a few of the terms would benefit from a quick explanation. I was thinking 'soundbites' I suppose, something like you get in a dictionary only a bit more accessible. I appreciate that kinetic and potential energy and perigees could easily have their own entries, but I still think it's worth popping in a very basic definition like the one you've given me here for the perigee, to help people understand the rest of the paragraph.

But...I can't think of a 'soundbite' defintion for potential energy, although your pendulum explanation is very helpful. But as you already mention storing the potential energy, I think that's OK. So...I will leave the footnote on kinetic energy, if that's alright, and add your explantion of perigees in brackets after the first use of the term in the footnote.

smiley - smiley Lbclaire

PS The good thing about actually living with a scientist is that he can see by the vacant expression in my eyes when he's crossed the line of comprehensibility. He's getting very good at 'layman speak'. smiley - winkeye


Subbing your Space Shuttle entry

Post 4

Lbclaire

Hi Pin,

I've sent this back now - hope that's OK.

smiley - smiley Lbclaire


Subbing your Space Shuttle entry

Post 5

Pinniped

smiley - book


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