Monday, 15 October 2007

Exploring Auckland

I've developed a habit of taking our family off on a whim to explore some park or reserve in Auckland because it looks interesting on the map. Yesterday we explored parts of Greenhithe. The primary goal was to check out Taihinu Historical Reserve, which should have a view across Hellyer's Creek to the shores of Beachhaven where I grew up. I found the head of the track down to the reserve (which someone has obscured with a makeshift wire fence, grrr), but the track was a bit steep, overgrown and too wet to tackle yesterday. We'll come back at a more suitable time.

Then we visited Marae Reserve at the end of Marae Road. My map suggests it has tracks heading north and south along the beach but as far as I can tell they don't exist, it's just a clifftop lookout with a nice view over Herald Island. Next stop, a shoreline reserve at the end of Kingfisher Grove. This has a nice view up Lucas Creek, but again, tracks shown on my map don't exist.

So we went back along Roland Road and found a walkway that cuts across a small creek to Churchouse Road. It's in a reserve used to graze pet sheep, and a few were gamboling around. Also backing on to it was someone's paddock with a large pig and some chickens. The kids enjoyed watching the animals and there were a lot of blooming lilies to enjoy too. At Churchouse Road the walkway ends at a playground in Wainoni Park. This playground is quite interesting --- including a maypole, a tractor, a very wide slide, a huge roundabout, and an installation of channels and gates for playing with flowing water. Most intriguing is a set of large plastic cups on stems set in the ground at an angle of about thirty degrees from the vertical. You can sit in them and spin around. The intriguing part is that if you just set yourself spinning gently then through no apparent effort, the spinning seems to speed up over time! It's a very convincing illusion of violating conservation of energy. I think that the secret is that because of the incline, as you spin you unconsciously try to straighten yourself vertically, and this motion drives the spinning somehow. Anyway, definitely worth checking out! Be careful with your kids though, because the spinning can be a runaway process...


  1. If you were trying to find a path from Marae res along the beach it's because it looks that way on a map on account of being a cliff! Check out some of the activities/walks on the Greenhithe website, and feel to submit some of your own.

  2. Robert O'Callahan16 October 2007 02:58

    Hmm, maybe my map uses the same notation for "cliff" and "track". That would be stupid, but it would explain some things...

  3. The spinning is caused by a conservation of momentum. As you move mass closer to the axis of rotation the velocity must remain the same, so the rotation gets faster. I remember a very nice hands on experiment at this expo when I was a child where they had these little mini merry-go-rounds, with just a pole in the middle, and you could stand on them and spin and stick your bum in or out and change your speed of rotation. The effect is also seen in spinning ice skaters.

  4. Robert O'Callahan17 October 2007 00:13

    I'm familiar with that effect, thanks to riding lots of roundabouts at lots of playgrounds with my kids. I don't think that's it; in particular, that effect doesn't produce the illusion of perpetual motion that these cups do. No matter how much you wiggle your bottom, you will eventually slow down and stop.

  5. You mean the spinning cups don't stop?! In that case, you must be right about the unconscious efforts to straighten yourself: that has to be the source of the energy. The effect you discussed above (where pulling yourself closer to the axis would make you spin faster) is surely part of the trick, but I don't quite see how the energy used to pull yourself upright is getting fed into the rotation. it something to do with gravity and centrifugal force offsetting your actual body movement, so that you don't actually get closer to the axis, but the vector of your muscle movement still has that conservation of momentum effect? If you plotted your body movement while not spinning, it would point in a straight line at the axis...and you'd pull yourself up and hit the axis. But while spinning, if you plotted the vector of your body movement relative to the ground, isn't it now rotating, so that your pulling-upright is also a pulling-round? I don't know, but it must be something like that. Next time we're in Auckland we might have to check out that playground!

  6. Robert O'Callahan17 October 2007 23:09

    Yes, the spinning cups won't stop.
    I suspect something like your description is right. You pull yourself upwards to the vertical but the rotation sends you off to one side so you fall as well, driving the rotation.

  7. Denis O'Callahan18 October 2007 05:18

    Hi Rob. I have not seen the spinning cup device but it seems to me that it works like an ordinary swing. The rider moves to change the length of the pendulum and if the movement is at twice the natural frequency of the swing and in the correct phase energy will be transfered from the rider and the amplitude of the swing will increase. Notice there has to be some motion of the swing to start off with.
    This is analogous to what is known in electronic engineering as "degenerative parametric amplification". A pump frequency at twice the input signal frequency is applied to a non-linear capacitor and energy is transfered from the pump to the output signal. Examples of parametric amplification are masers and lasers.

  8. You can also think of it as an energy question.
    When you are at the top you will naturally lean back. You get a vertical reaction force against the ground support, so movement of your body mass upward is work input. This increases your height and therefor your potential energy. You swing down, converting potential energy to kinetic. Even if you don;t react at the bottom you should still have put enough energy in at each top point to go right around.
    But at the bottom you will be hunching forward (trying to keep body vertical). Again you are working against the ground supporting force, doing work to lift your body. This reduces the height through which your weight must be raised on the upward rotation. So less of the kinetic energy needs to be converted to potential energy on the upward part of the cycle.
    You may also be able to analyse it as a gyroscopic question (conservation of momentum)- for an object rotating about X axis, movement about Y axis leads to force about Z axis. However this would be more tricky to get your head around.