Rolling, Rolling, Rolling

A few days ago I was relating the cans-in-a-blanket problem, and retelling the vacuum joke and story to someone who had not yet heard them. One of my colleagues commented on a problem he had been given during an interview, also involving cans of soda:

You have two cans, one filled with ice and the other with liquid, but otherwise identical. The cans are rolled down an incline. Which one reaches the bottom first?

Much like the previous problem, I think there is a common misconception at play here for some people who get the answer wrong, and I’ll get to the explanation below. One of the people in the conversation said his first impulse was the wrong answer, but when we discussed the physics, we all agreed on the solution.

I set up to do a demonstration, though my first attempt was thwarted — I filled up a can with water and popped it in the freezer, hoping the can would be strong enough to hold together and have the ice expand vertically. It wasn’t.


I think the problem being that since ice will freeze from the top down and outside-in, the ice adhered to the can too well to let it expand upward as much as I hoped. (BTW — Black Cherry Citrus? Blecch. I bought it by accident when they redesigned their color scheme and introduced the flavor)

So I did it again, adding a little bit of water and letting that freeze, repeating the process several times until it was full, and it worked. Here is the experiment to investigate the problem given above:

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For those who think that the liquid-filled can will roll more slowly, I think I know what the misconception is: most of us have seen or done the experiment with spinning an egg, and a hard-boiled egg spins readily while the unboiled egg doesn’t. So the intuition is that since liquids don’t spin readily, the liquid-filled can won’t want to roll very fast. And, as we can see, that’s wrong.

The reason the intuition is wrong is from a misinterpretation of the reason the unboiled egg doesn’t spin — it’s because it’s difficult to transfer energy and angular momentum to the liquid by spinning the container; the coupling between them is weak. And angular momentum tells you the tendency for something to spin — it only changes when you apply a torque. With the soda cans it means that the work being done, adding energy (gravity acts on it, and there is a torque from the friction of the treadmill causing rotation)but this energy isn’t being added to the liquid, so it must be going into the can itself, which isn’t very massive — almost all of the energy goes into translational kinetic energy. The frozen water, though, does rotate with the can, so the gravitational potential energy has to be shared between translation and rotation of the can + ice system, so the translational kinetic energy (and therefore speed) is smaller.

Beulah, Peel Me a Grape

And nuke it.

Things to do in a microwave #2: Create a plasma

It just so happens that grapes are about the size of the wavelength of microwaves, which is important. And grapes also have sugars, which make them into dielectrics. (There are other fun things you can do with grapes because of this). Both of those together make the coupled grape halves into a dielectric dipole antenna, which is just a fancy way of saying that the microwaves that hit one side of the cut grape will pass to the other side, in a very concentrated way. The result is that there is a huge voltage generated between the two sides of the cut grape. That voltage causes electricity to jump from one grape half to the other (”arcing”). This is what happens when you rub your socks on the carpet and touch the doorknob — that spark is electricity jumping from your hand to the doorknob. The difference in this case is that there is a HUGE voltage generated (3000 volts by one website), and that is enough to ignite the steam from the grapes into a plasma state (a glowing ionized gas, where the electrons have been ripped from the gas molecules by the high temperatures). You can capture this plasma in a glass, as in the video above (wow!)

And, of course, this is preceded by Things to do in a microwave #1: Find your microwave hot spots

In addition to the two methods Stephanie lists, you can use marshmallows or chocolate chips, and look for where the melting starts. And then you can eat the experiment. (Stephanie mentions marshmallows; I missed it)

Update: Not done yet! Things to do in a microwave #3: Ivory Soap Monster

Things to do in a microwave #3: Microwave a CD
#3? Should be #4. (I’ve brought the whole “Five is right out!” counting thing to Stephanie’s attention) There’s an image that shows some mini Lichtenberg figures, i.e. the little tree-like patterns the electrons make.

Things to do in a microwave #5: Microwave a lightbulb

The Billy Preston Effect

Will it Go ‘Round in Circles?

Building The Amazing Steam Candle

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This is a variant of the pop-pop engine — if you point the tubes parallel rater than in opposite directions, you’ll get linear propulsion.

At first glance you might think this couldn’t work. Once you hit steady-state, the rate at which water enters and exits the tube has to be equal. Inside the tubes, that means that the momenta must be equal in magnitude, but opposite in direction, meaning no net momentum for the water, and no propulsion for the boat. The effect is a little more subtle — one has to consider what happens at the entrance to the tube. The water exiting will have its velocity vector along the direction of the tube. But the incoming water is drawn from different directions; it only has to have a component of its velocity in the direction of the tube, meaning the ejected water exerts a greater force.

Smile! Look at the Pinhole!

Pinhole Camera Solargraphy at Astroengine.

[The] solargraphs are taken by a compact camera film cartridge (plus tiny pinhole) strapped to an inanimate object for long periods of time. However, due to the low speed of the camera film and light restriction (plus, as this is Bristol, plenty of overcast days), the six-month exposure brings a surprising amount of detail to the shot. Every day when the Sun was shining (and days when it was struggling to get through the clouds), the path it made through the sky every day was captured.

The original site is Pinhole Photography and has some very interesting pictures and information. Also check out the Solargraphy site.

Blame it on Eddy

“Eddies,” said Ford, “in the space-time continuum.”
“Ah,” nodded Arthur, “is he. Is he.”

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Everyday Electromagnetism

This time, though, Eddies in the penny. And he enforces Lenz’s law.

You can see a similar effect if you drop a magnet down a copper pipe, because the eddy currents will flow, and the induced field is such that it opposes the acceleration, so you get braking.

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If you want to be more practical, instead of moving the magnet you could move the copper around, cyclically, and tap into the current that would flow. Just a thought.

Give My Creature LIFE

Or at least make it spin a little. A simple motor: battery, magnet and wire.

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Magnetic field sees a changing current and that results in a force, which gives you a motor. Quite similar to the Faraday motor, but then, he was allowed to play with mercury (and he never wore a bike helmet, either)

Update: Give MY creature life. Built one myself this afternoon, and even figured out how to upload it to YouTube. She may not look as pretty as these other motors, but she loves to spin. (First attempt didn’t go so well, but I found lighter wire that was still stiff)

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