Sunday 18 January 2015

Wet dogs, kids, Nazis and roundabouts.

Note: This post was published in 2011, but the original was invaded by scammers to the point I copied the legitimate bits and deleted the original. :-)

We know what happens when a wet dog shakes itself and some always seems to get into your mouth! At the moment of impact have you wondered in what possible abominations your temporarily despised pet might have been rolling? Maybe you have, but then did you wonder at the forces that propel those contaminated droplets in your direction at speeds beyond your ability to avoid them?
Centripetal force, circular motion. That’s what it is. It is that force that makes a stone fly from a sling or a satellite stay up there.
Now, I direct your attention to a previous post that talked about the globe slowing a little (I postulated a change of 0.0137 seconds longer days by the year 2050) and then take your imagination back about four and a half billion years, when the whole glowing hot mess of oxides nitrogen and iron settled itself into orbit, third planet from its birth mother.

As mainly gases, imagine how much larger it would have been and how far from the axis its centre of spinning force must have been!
I am not about to try the math, but if we take ocean water again then imagine it all as vapour (steam to me and you), and if you like, include coal and oil, no longer buried, but in the atmosphere as CO2 , we realise that our planet would have been, by volume at least, predominately atmospheric gas, extending its weight hundreds, perhaps thousands of kilometres further out from where it is now, way below sea and ground level.

Don't go, because here is where the story really starts. It includes a confession of the method I used to teach ‘Conservation of Energy’ to otherwise disinterested eleven year olds.

We all went to the local park where there stood an old fashioned roundabout, all weathered wood and rusty tubing. That wonderful piece of scientific equipment is long gone, removed by Child Safety Nazis, possibly as a result of the use to which it was put by me. Nevertheless, let me confess what we did. It was really me, but I use the royal we to suggest shared responsibility.

We placed six of the smallest children at the centre, instructing them to hold on tightly no matter what. Then at the outer end of each of the six radiating bars, a larger child, with feet on the edge and body hanging out as far as possible, was instructed to listen for a signal from me. Other children spun the roundabout with its human appendages until they could impart no further energy.
Then, on the signal, the hangers-on pulled themselves towards the centre as quickly as they could.

What happened? Well, the whole system accelerated alarmingly and all were screaming with delight or terror until the signal was given for the moving bodies to slip back to the perimeter when it slowed again, allowing the guinea pigs off, to laugh hysterically, run for the washroom to vomit or demand a second go, depending on the strength of their stomachs or predilection to self harm.
But I guarantee their understanding of conservation of energy, levers and centripetal force were never forgotten.
So what has that got to do with the speed of rotation of our planet at its birth billions of years ago?

OK, let’s go find one of those old fashioned roundabouts and we can pretend we are water vapour and CO2 condensing towards the centre of the planet, and see what it does to its rotation.
Here it is expressed as a mathematical equation: v2 / r =a, where ‘a’ is centripetal acceleration, ‘v’ is velocity in meters per second, and ‘r’ is the radius of the circle in meters. Don't let that worry you. Suffice to say, any reduction in ‘r’ really gives ‘a’ some ‘wow’!

That suggests our planetary rotation speed has accelerated considerably over the last few billion years from when there might have been only a couple of hundred days or less in a year.
You understand? No? OK, climb onto the roundabout and hang on!


  1. Whacko Stafford, what a great experiment. I was one of the chuckeruppers. Those things always brought my lunch up. Where would that have ended up in the ancient scheme of things. Hopefully not returning to slap me in the face.

    As primary school teacher eons ago my only science experiment along these lines was to smack a very tiny and incredibly annoying grade onner so hard it nealy put him into orbit.
  2. I hate to admit I am so slow... but you leave me wondering... I enjoyed reading about your experiment, and I'm sure the young people never forgot, if they understood at all. But a fun way to learn.
  3. child safety Nazis ...we have them here too
    No more fun at the playground
    I'll get on if you do the spinning
  4. No idea what your point is Stafford but it still made an entertaining read :)
  5. Stafford,
    It must have been real education for the kids. Such events stayed on in memory longer than trying to understand through just reading the difference of movements between centripetals and centrifugals.... Whew! There's a lot to chew in that one. Great!
  6. Teachers who make lessons interesting are never forgotten by their students :)
    Have you been watching Wonders of the Universe (Tues ABC1)? The enthusiastic young man presenting it makes it all very interesting and understandable. He doesn't look old enough to be a professor though!
  7. hmmmm....does that mean I get a lie in or not...?
  8. My blog, facebook account and email account were all hacked and my passwords changed so that I couldn't get back in. Hate mail was sent in my name. Horrendous posts were published in my name, and I had to replace my computer.

    All in the name of vicious people expanding as if by centrifugal force into the world.

    I went to the sheriff (no help there) but the cowards thought there MIGHT be help there, so they shut down LIFE IS GOOD.

    Somehow, I found you again.
  9. I must say to anyone with the temerity to venture back here, as a non-attached casual science/maths/drama/music/literacy teacher, I was given freedom to do lots of interesting stuff like that. We worked out what Pi was using swings as pendulums, and moved a gigantic log using levers and cooled our drinks in water by adding salt, etc, etc.
    My question was always; "Work out why that happened and tell us next time and remember, there are no wrong answers!"
    I do miss it.