Saturday, November 24, 2007

G'day Bart and Lisa

G’day Bart and Lisa
In the Simpsons episode "Bart vs. Australia," Bart and Lisa explore the physical phenomena called the "Coriolis effect," and compare its influence on plumbing in the Northern and Southern Hemispheres. For my book on the science of the Simpsons,What's Science Ever Done for Us?, I decided to contact an Australian physicist, Joe Wolfe, who has written about the Coriolis effect and runs a great physics website. I asked him to imagine what it would be like if Bart and Lisa visited his university (New South Wales) to find out more about the phenomenon.

Much to my delight, Prof. Wolfe speedily replied with a story he wrote imagining such a visit. I excerpted the story for my book, but here, for you loyal readers, is the full version:

"Bart and Lisa visit Prof. Joe Wolfe," by Australian physicist Joe Wolfe:

G'day Bart and Lisa!

delighted to meet you. No, I don't have a TV, but my nephew talks about you. Apparently you play saxophone. So we have that in common. (BTW, you might like to see: )

So, water in the bathtub. As you've come all this way, let's take some time to look at this carefully. Sydney is suffering a drought at the moment, but in the interests of science, let's put several cm of water in the bath, the sink and the hand basins at my place. We'll also put these pieces of wire in the plugs so that we can pull them out without disturbing the water. Now let's go across to Coogee Beach and catch a few waves while we leave the water settle -- we're
allowing any currents caused by filling the basins up to die away.

* * *

Yes, isn't it amazing how much sand gets in your shorts and how much water gets up your nose? That's what's called a Coogee dumper: the sand at Coogee is often steeply sloped and so the waves break suddenly and give you a lot of torque. No you can't have a shower yet -- it might disturb the experiments.

Well, what do you know? By pulling the plugs out really carefully, and not disturbing the water, we saw it run out smoothly with no rotation in either direction.

What will happen when you go home? Well, in general, people probably wouldn't take the trouble to let any motion in the water die away. In that case, the direction in which it drains might depend on the location of the tap you used to fill it, because that can set up a circulation pattern during filling. If you have hot and cold taps on opposite sides, you might get different results for hot and cold water! Also, some basins might not be symmetric, so in some basins you might tend to get more than 50% clockwise, while others would be less than 50%. Nevertheless, these effects should cancel out. People who have done the experiment in the US report, on average, 50% each way.

Yeah, I know. But people often confuse what they expect to happen with what really does happen. (Well, better to be prejudiced about physics than prejudiced about people!)

Certainly, I can show you a couple of things that work differently in this hemisphere. Let's go to the University of New South Wales, where I work. In the foyer of the School of Physics we have a 14 m long pendulum.!/FOUCAULT_PENDULUM/foucault_pendulum.htm

It's called a Foucault Pendulum, after Jean-Bernaud-Léon Foucault, who put a long pendulum up in the Pantheon in Paris, to give a demonstration of the rotation of the Earth.

We use the wire and the line behind it to line up the cable of the pendulum, then we let it swing in that plane. Good, that's it: it's now swinging exactly North South.

Let's go down the corridor and look at some of the other demonstrations. Have you ever wondered how a computer works? We have a set of displays on that.

Wow, that's fast -- you only took 10 minutes to get through them all. Let's have a look at the pendulum now.

No, not quite the same. Line up the wire and the line and you can see that the cable of the pendulum is now swinging across the North South vertical plane - the plane of the pendulum has precessed by a bit more than a degree. (Yes,
that's why you had to line it up carefully in the beginning.) Correct, from our point of view, the plane of the pendulum has turned (scientists say 'precessed') in the counterclockwise direction.

Okay, here's a globe of the World. We're in Australia, so we're looking upwards at it from the South side. Now I want to spin the globe so that the sun -- let's say the light over there -- seems to rise in the East -- I want it to appear over the Pacific Ocean. That's it. We have to turn the globe to spin clockwise.

Now Lisa, stand up so that you can see the United States on the North side of the globe. What direction is it spinning?

Yes, from your position looking at the US, the Earth is spinning counterclockwise. For Bart and me, down here looking up at Australia's side of the globe, the Earth is spinning clockwise. That's why the Sydney pendulum precesses counterclockwise.

No, it takes longer than a day to make one complete cycle: the period is proportional to the reciprocal of sine of the latitude. The maths is a bit tricky but it's here:

Now, when you get back home, I want you to go to the Smithsonian and have a look at the Foucault pendulum there. What direction do you think it will precess?
Yes, but don't just think that -- check it out! That's what makes it science, the experiment. Remember all those people who never checked out the water in the basin!

Oh yes, the other thing I promised you. Well for that we need a boat, or you have to be very good swimmers. Back to the beach.

See the island out there? If we go a few km further out to sea, we'll find a current going South at about 1 or 2 knots (2 or 4 km/hr). That is part of the major current in the South Pacific Ocean. It goes South to near Antarctica, then East to Tierra de Fuego, then North past Chile, then West across the ocean near the equator, before heading South past the East coast of Australia again.

Yes, so it turns anticlockwise, like the Sydney pendulum. And when you look for the ocean current off California, you'll see that the North Pacific current goes clockwise. So does the North Atlantic current -- Americans call the nearby bit of it the Gulf Stream.


PS Here's a slab of Cooper's Sparkling Ale to take back to your dad. It's much better than Foster's.

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