h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: What Happens When You Drop a Slinky - A36656085
Author: Pinniped - U183682


I'm following a bit of a current fashion here, what with superballs etc, but I wanted to find out if I could do the style.

And we've all got to do our PR bit, right?

Link suggestions welcome.

It certainly looks interesting. I'll have a proper read and review over the weekend.

Lovely entry!


Some people are like slinkies... You can't help but smile when they fall down the stairs!


I have some plastic slinkies with the ends fastened together to make circles for juggling - they do very interesting expand-y things when thrown up in the air.


Neither of those comments helps you in the least, but I'm too tired for a proper review.

So just assume I liked it I can see myself using this one to irritate the statics prof by asking smart-aleck questions

Good work

Just some suggestions:

This Entry is about what happens when you let go of a free-hanging Slinky, and a behaviour that some find counter-intuitive to the point of disbelief. The physics of the Slinky Drop is discussed too.
-->
This Entry looks at what happens when you let go of a free-hanging Slinky (the Slinky Drop) and discusses the physics behind a behaviour that some find counter-intuitive to the point of disbelief.

At the bottom, there is no underslung weight, and so the tension is zero too.
-->
At the bottom there is no underslung weight, and so the tension there is zero.

and the bottom of the broom handle hangs in space when you let go of the top, just as the Slinky did. The only difference is that both the distance travelled by the broom handle-top and the time elapsing before the bottom wakes up are very small.
-->
and the bottom of the broom handle does actually hang in space when you let go of the top, just as the Slinky did. However, the distance travelled by the broom handle top before the bottom wakes up is very small, as is the delay between the top and the bottom starting to move.

Oh and the ’curly MS Word apostrophes’ need changing to 'ordinary decent ones'.

Alex

And perhaps you need to define the "bottom of the slinky" better, because the part experiencing no tension at all is technically just a two-dimensional plane

This is a superb example of what's needed for the EG. Quirky, factual, educational and fun. Written almost like a conversation, great stuff.
Well done Pinniped.

A fine entry.

Seems a shame not to use "in exactly the same way that bricks don't" in a h2g2 entry about hanging in the air, but maybe that's just too contrived.

I'm not entirely comfortable with the force on the bottom of the spring being zero. Clearly every part of the spring has mass and so f = ma suggests that even the tinyest part is under some force due to gravitational acceleration and balancing tension. Seems more strictly correct to say that towards the bottom of the spring the coils are supporting less and less of the spring below and so the force and corresponding tension approach zero at the end. Maybe I'm just being pointlessly pednatic, but I thought I'd mention it. Otherwise, seems like a splendid entry to me.

Wish I knew what had happened to my old sliky Ho Hum.



Ugi

Hi Pin!

Great idea for a piece, and I'm absolutely certain that the visual effect is just how you describe it; still, without actually using this as anything more than a mental exercise, I'm either not understanding the description of the 'why' or it's not what you think.

The slinky, dangled from the top end, has stretched as far as gravity will pull it. The tension of the spring equals the gravity effect. I believe you'll find that when you let go of the top, it is the center of the spring that falls at a normal rate for a falling object, but that the bottom and top are being pulled back towards the falling center at the precise rate of gravity (because it is a gravity level tension being released).

Yes, the bottom stays still during this, but only because it is springing up at the same rate that it would fall down.

This is another version of why astronauts don't feel gravity, but weight just as much (nearly) in orbit as they do on earth. They're just in constant free-fall.

Thanks all

I've used your improvements Tufty, and Ugi's brick observation was irresistible so it's in as a header.

I'm going to include you both as collaborators. (Don't argue. If this turns out a solo Entry, some annoying failed metallurgist guy is threatening to press a dodgy badge on me).

You others suggesting tweeks: Dunno, quite - though thanks anyway.

Malabrista and the bottom-just-a plane-school. Well, sure. You can reduce this problem to some nice maths with convergent series and wave theory, but I think it's more approachable this way, no?

FT - Everything you say is in there already, I reckon. See for example the sideways model with gravity then superposed. Remember too, that seeing it in certain terms is just about the model you choose. There are lots of valid ways to describe any system, never just one, and we mustn't confuse the abstractions in our heads with reality.

You get what I mean? >>The tension of the spring equals the gravity effect<<. Contentious. What do you mean by 'equals'? If you mean absolutely the same, then that's strut/tie tension-think and you're perilously close to being a Soft Rodder. This is continuum mechanics, rather.

Anyhow, I'm not going to be adjusting this for scientific perfection, regardless of where PR pushes. I'm confident it's near enough for the layman. For me, the fun of the piece is ultimately more important than any academic rigour.

Hey, very good


>>the fun of the piece is ultimately more important than any academic rigour<<

This matches the height of the bar that SWL set recently with his assertion that 'being completely correct isn't really necessary - as long as I'm not wrong'

Apallingly, I think I'm in agreement with you both.

Hi Ugi,

I echo the points made above - it's a super Entry

I often use a slinky to demonstrate logitudinal wave motion (i.e.sound). However, not being a physics specialist, the first time I did this, I tried to do it 'up in the air', with a student holding the slinky at each end, and one of them giving his end a slight push to send the wave.

This resulted in the slinky becoming hopelessly tangled and the technician spending a good length of time trying to sort it. In fact, I'm not sure that she ever did.

Thus, when I left the school a year later, I was presented with a 'mini metal springy', which I am now looking at on my study shelf.

I now know that when demonstrating wave motion, one should lay the slinky along a bench or along the floor

I meant Pinni - not Ugi

Looking good, there. And I think the bit about "no underslung weight" says enough about the plane I meant.

Only if it's an article for the layman - I *think* everyone will be able to understand it - you might do something about this sentence or footnote it at least:

All real solid objects exhibit some measure of elastic behaviour, and anything with a finite size exhibits some degree of time-lag behaviour throughout its volume when a local force is applied.

That was the only one I tripped on, it sounded a bit too textbook-formal for the chatty tone of the rest of the entry.

Possibly the bottom end of a broom handle is more intelligent and thus realises it's supposed to fall ?

Physicists - engineers, always looking for complicated reasons for things.

Thanks all.

I'll think about that wordy sentence. At an earlier point in the writing of the Entry there were two voices: a geeky, earnest one and a naive, cheerful one. I overwrote all that, but the offending sentence is a geek residue.

Physicists and engineers aren't all that different in my experience. Physicists tend to be somewhat better at maths. Engineers tend to be somewhat better at drinking.

An engineer thinks his calculations are an approximation of reality.

A physicist thinks reality is an approximation of his calculations.

A mathematician doesn't care.


If you do leave that sentence in as a relic - and there's nothing wrong with it as such, it certainly is a very precise sentence that says exactly what it's meant to *if* you know what all the words mean, append it with a footnote to the effect that the reader should be glad the whole entry isn't like that

What with slinkys and bricks appearing in the same entry - an unwanted image just came to mind that raises this question.

Is 'dropping a slinky' like 'sh#tting a brick'?

(figuratively, of course)

(I think I'll start using that expression IRL. (i.e. "Just off to drop a slinky")

Story of my life, guys.
Everyone else's review thread collects sexual innuendo.
My writing puts you all in mind of defaecatory processes