Category Archives: Physics
Toil and Trouble
More bubbles in microgravity
I find it interesting that, contrary to terrestrial experience with drops on a surface, the colliding drops tend to scatter rather than coelesce. In microgravity the drops are spherical, which is the minimum-energy configuration, so I expect this presents an activation barrier of sorts. The collision needs to be strong enough to perturb the shape of the drops, so that combining them doesn’t represent a configuration that requires more energy than is present.
No Artificial Motion Added
Powering the World Using Natural Motion
Scientists at the University of Houston recently found that the electrical sweet spot producing the most efficient charge occurs when a piezoelectric filament is about 21 nanometers long. To put that into perspective, if you lined up 4,000 of these filaments next to each other, it would be about the width of a human hair.
The Hubble Advent-ure
Hubble Space Telescope Advent Calendar 2008
As we head into the traditional western Holiday Season, I’d like to present this Hubble Space Telescope imagery Advent Calendar. Every day, for the next 25 days, a new photo will be revealed here from the amazing Hubble Space Telescope. As I take this chance to share these images of our amazing Universe with you, I wish for a Happy Holiday to all those who will celebrate, and for Peace on Earth to everyone. – Alan (25 photos total – eventually)
Cleanup on Aisle Six
Here we have a beautifully illustrated example of Newton’s First Law of motion involving shopping carts. Did some force push those carts out the back end of the trailer? Not at all.
Well Blow Me Down
Over at Good Math, Bad Math, Mark has a takedown of a device purported to move directly downwind, faster than the wind. Wind-Powered Perpetual Motion (and Dave Munger thinks he’s wrong.)
Here’s the video
The objection is simple: when you are traveling at the wind speed, there is no more wind in the cart reference frame, so there’s no force. The treadmill analysis is flawed.
If you’re testing a wind powered vehicle, then in a closed, windless room, putting the vehicle on a treadmill moving at 10mph is not the same thing as putting the vehicle on a stationary surface in a 10mph wind.
By putting it on a treadmill, you haven’t recreated the real-world situation — you always have your wind, and the treadmill doesn’t remove that. You never test the condition of having the wind relative to the cart drop to zero. So while it’s not faked, it’s still a sham.
It shouldn’t be hard to engineer a device such that the wheels rotate faster than the propeller, i.e. whatever the propeller’s rotation rate is for a wind of speed X, the wheel edges move faster than X. Since the wind is always present, the cart will move forward on the treadmill moving at X. Even uphill.
My question is this: if this works, at what speed does the cart stop accelerating?
UPDATE: Or with no wind present, as in the test (On the first viewing I thought they had a fan turned on) what you’re doing is converting treadmill kinetic energy into propulsion by turning the propeller. But you don’t need to have much propulsion to move forward, even uphill. Not a valid test.
Update, Mark II. See the comments — I was viewing this from the mistaken notion that the propeller was acting as a turbine while on the ground and at low speed, which isn’t the case.
This has the implication, I think, that the cart must have enough mass to ensure that the propeller acts as a propeller. My question of what the maximum speed is still stands, because I’m sure it involves fluid mechanics and that’s not something I’ll win should I tangle with it.
Physics or Driver's Ed?
Helpful for when you have to walk in a straight line.
It … is … ALIVE!
We already know that cornstarch and water mix to make a non-newtonian fluid. Here’s what can happen when you stress and relax it rapidly (where rapidly is 80 Hz)
That's Dr. Time to You, Pal!
Meet the world’s director of time
An interview with Dennis McCarthy, who is the Director of the Directorate of Time (or was at one point; I’m not sure how his retirement and subsequent resurrection affected the job title)
Though the BBC filmed in the lab, none of that footage made it into the embedded clip. Perhaps there’s footage in the show that’s airing on BBC 2, as I type this. I’ll have to check it out.
More than anyone, Dr McCarthy appreciates the need for the world’s population to be synchronised. But for those who don’t spend their working day checking atomic clocks, why is knowing the time so important? Think for a moment about how the GPS satellite navigation system works.
There is a network of over 30 satellites orbiting earth that broadcast a high-precision time-stamp down to the GPS system in your car.
These signals travel at the speed of light, which is very nearly one foot every thousand-millionth of a second – or one nanosecond (for the more metrically minded, that’s around 30cm, which is far less elegant. If there is a God, he built the universe using imperial measurements).
If that last part is true, God has a hell of a sense of humor.
The was one part of the embedded video that made me cringe, and that was the depiction of the Bohr-ish atom (with wavy orbit lines — is that supposed to make it all better?) and the electron making a transition between them. But in that representation, those are the levels described by the principle quantum number, and the transition of microwave clocks is in the spin state of the electrons, oscillating between spin-up and spin-down (whose energy degeneracy is broken because of interactions with the nucleus, which also has spin, and thus a magnetic moment) And the notion that you’re looking at radiation emitted by the atom is true in an active maser but not a passive standard like a cesium or rubidium clock — in those you make a separate measurement of the atom to tell you what state the electron is in.
(I don’t know if it’s a permanent link, but in the “In Today’s Magazine” column there’s Call him Mr Time . Hence the title, though I can’t actually envision Dennis saying that to anyone)
Funny and Not Funny at the Same Time
(Really, it should have a legend like blue = dead, yellow = alive)