Category Archives: Physics

One Giant Measurement for Schoolkids

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School kids measure distance to the Moon

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

Up for Adoption, Again

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Pick me! Pick me! Adopt-a-physicist is running again this spring. Registration/sign-up is open as of today, with the forums open April 13 – May 1.

Help high school students explore what it’s really like to be a physicist – consider participating in Adopt-a-Physicist! In this program, people with physics degrees (at any level) are “adopted” by high school classes interested in finding out about the careers, educational backgrounds, and lives of real physicists.

Run Away!

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Time-Lapse Video: Retreating Glacier

This remarkable image sequence captures a series of massive calving events at Columbia Glacier near Valdez, Alaska. Composed of 436 frames taken between May and September of 2007, it shows the glacier rapidly retreating by about half a mile (1.6 kilometers), a volume loss of some 0.4 cubic miles (1.67 cubic kilometers) of ice or 400 billion gallons (1.5 trillion liters) of water.

Yelling for a Good Reason

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The issue that helped instigate the issue in the previous post was How long would you have to yell to heat a cup of coffee?, which Zz had linked to.

The analysis is fine up until this point

The average human yells at about 80 decibels, which carries along with it about .001 watts of energy

The 0 dB reference for sound energy is 10-12 Watts, so 8 orders of magnitude higher is not a milliWatt. They get this right a little later on.

The average person whispers at about 40 decibels, which translates out to about 10-8 (sic) watts.

Go up by 40 dB, and you get 4 orders of magnitude in power, or 10-4 Watts. But I also read that “loud speech” is equated to 90 dB, not 80 dB, so I’m not sure where the error truly lies. If the power in Watts is what they wanted to use then their answer is fine, but if they really meant 80 dB then the answer is too small by a factor of 10.

When I first saw this problem, it was in the form of a claim that it would take 8 years of yelling, and that fits if you yell at somewhere around 83 dB. The point is that sound doesn’t carry much energy, which is one reason behind Nick’s calculation about how the Electrons per Song on an iPod has been decreasing, and how it’s possible that you can run one for 10 hours on a 3.7 V battery with only 73 mAh of capacity. That’s just 2.7 Joules, but even at 110 dB, this would only represent 1 Joule of sound energy during that span.

On the Job

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How Physics Can Solve Crime And Help To Cure Cancer

Zapperz has a nice little rant, after linking to a story that presents an order-of-magnitude solution to some trivial situation.

I also received a rather nasty and profanity-laced “comment” to the blog, which basically asked why us “MF’s” are wasting out time and not using our brains to cure cancer

Short answer: Chill, bro. “We” (meaning some physicists) are, but OOM calculations are only the beginning of such issues, whereas they were the end of the “how much energy does it take to do X” problem.

Cans in a Blanket

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Rhett asks a straightforward question over at Dot Physics, in A blanket and cold stuff

Suppose you put take two identical cans of soda out of the fridge and place them on the floor in the middle of a room. One can you leave alone and one can you cover with a wool blanket. After an hour, you come back and check on the two cans of soda. Which will be warmer?

The reason I think it’s a great question is that it plays on a common misconception about thermodynamics, and it reminds me of a joke, and a related story.

The joke is about a person declaring that a thermos (which we often call a Dewar) is the smartest thing in the world. When it’s pointed out that all it does is keep hot things hot and cold things cold, the response is, “How does it know?”

Well, some years ago we were giving a tour of our lab to an Admiral (or Sneetch. I have taken to calling military lab visitors Sneetches, a Dr. Seuss creature. The high-ranking ones have stars upon thars) We mentioned the vacuum system, and the Admiral mentioned the lines from the joke — it keeps hot things hot and cold things cold, how does it know? My colleague was so focused in on explaining things, he didn’t recognize the joke, and started explaining the physics of heat transfer: a vacuum is a really good barrier to nonradiative heat transfer. I mentioned that it was a joke before he went in for a second round of explanation.

But the misconception — that blankets heat things up, rather than act as a barrier to heat transfer, is what is pointed out in this example.