There’s a fairly well-known science question which asks
How does the amount of energy per gram of TNT compare with the energy per gram of a chocolate chip cookie?
I’ve discussed before why I think the answer should be, “About the same,” if you’re doing a first-order approximation, and depending on what options you give for an answer.
Throwies are simple LED circuits — the LED and a battery, with an optional magnet so they will stick to ferromagnetic materials.
Evil Mad Scientist Laboratories does a pretty exhaustive analysis on the circuits, looking at battery life and potential danger of these simple circuits. Some thoughts on throwies
This data shows a couple of interesting things. First is that the power-law model seems to hold fairly well. Second, the power function that pops out is not very different from that of the data from only the first half hour– integrating both out to 24 hours gives two answers– 150 mAh and 186 mAh –that differ by only 25%. The estimate based on the long data record (150 mAh) is the more accurate one, but this does suggest that we should be able to use the data from the first half hour alone to get a fairly good “factor of two” estimate of the performance over 24 hours.
All you need is a big trash bag and an industrial strength vacuum cleaner, and a willing victim (er, “faithful subject of science.”) The victim (aka “subject) gets inside the bag, and once you suck all the air out of the bag with the vacuum cleaner, they’ll feel an intense pressure. SAFETY FIRST! Read this PDF writeup of the activity (from the Exploratorium’s Eric Muller) for all the ins-and-outs and safety factors in doing this with your kids. (Words to the wise — don’t put your head inside the bag!) It’s stunning — try it if you can.
I’ve done several of these, including a version of the homopolar motor. The eddy current damping is fun, too — you can make nonmagnetic metals react to magnets by inducing current flow in them. Lenz’s law.
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:
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.
The students analysed an mp3 recording of the conversation between Neil Armstrong on the surface and ground control in Houston in which he utters his famous “one small step” speech. The recording is available on the NASA website.
They noticed an echo on this recording in which sentences from Earth are retransmitted via Armstrong’s helmet speaker through his microphone and back to Earth. They used the open source audio editing program Audacity to measure the echo’s delay
