5,286 reputation
1530
bio website keith-s-thompson.github.com
location San Diego, CA
age 55
visits member for 3 years
seen Oct 8 at 18:08

I'm a programmer and all-around nerd living in San Diego, California.
I work at JetHead Development Inc.

E-mail: Keith.S.Thompson@gmail.com


Aug
25
comment How does gravity work underground?
The ancient Greeks knew about Newtonian gravity?
Mar
12
comment Why would a pendulum experiment give $g > 9.8\ \mathrm{m/s^2}$?
@Kvothe: Yes, but wouldn't applying the formula for larger angles (where the pendulum will take longer to swing back and forth than the simple formula implies) yields a smaller value for the gravitational acceleration -- assuming accurate measurements?
Mar
8
comment How can I destroy earth with physics?
Last December, I received a text message from the USGS which said there had been an earthquake near Polson, Montana with a magnitude of 22.0. My first thought was that the Solar System would have a new asteroid belt, but my calculations indicated that there probably wouldn't be enough left to form one; the energy released would be about 250,000 times Earth's gravitational binding energy, more than enough to vaporize the planet. Unfortunately for your evil schemes, a followup message revised the magnitude to 2.2.
Feb
14
comment Why gravity decreases as we go down into the earth?
The pull of gravity is zero at the center, since the entire planet pulls on you from all directions. It falls off from 1g to 0g (more or less smoothly, but not uniformly) as you go from the surface to the center. But due to the greater density of the core, it actually increases until you reach the bottom of the mantle. See also this question.
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
@MikeDunlavey: I'd say that both Newton's and Einstein's theories (as distinct from what they personally might have said about their theories) are observations about how things behave. Neither addresses the question of "why".
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
Nobody does. We observe how objects behave, and we refer to certain aspects of that behavior as "gravity". That doesn't mean we ultimately understand what, if anything, "causes" gravity. But I see no reason at all to assume that the underlying causes, if any, involve computation or awareness.
Jan
24
comment How physical objects (e.g. Earth and Apple) are aware and do computation about each other?
Why do you assume physical processes (which we may or may not entirely understand) require awareness and computation?
Dec
21
comment Didn't Newton's Corpuscular theory fail to explain reflection?
@VINAY: As I said, the idea is that the corpuscles interact with ordinary matter but not (significantly) with each other. For example, there might be one chance in a thousand of a corpuscle colliding with an atom of ordinary matter, but only one chance in a million of two corpuscles colliding. But we know that the corpuscular theory is wrong, so you can only go go so far in making it consistent with reality.
Dec
21
comment How to initiate fusion explosion without a fission trigger
Please tell me your interest is just theoretical! 8-)}
Oct
31
comment Is self gravitation theoretically impossible?
Wouldn't that imply perpetual motion? I doubt that even GR permits that, but I don't know enough to comment further.
Aug
3
comment Opening the fridge door to cool a room
Opening the door will probably cool the room temporarily, as the cold air in the fridge flows into the room. But over time, the extra work done by the compressor will tend to heat the room.
Jun
29
comment Are there old aged particles?
Another way to think about the experimental evidence. Take two populations of 100 muons each. The first population consists of muons that were just created somehow. The second consists of muons that were created one second ago; as muons go they're old. (Given the 2.2 microsecond mean lifetime, this second population would be very difficult to produce.) Both populations should decay at the same rate; you can't tell the difference between them.
Mar
31
comment What cosmic event would cause Sun rising from the west?
A collision sufficient to reverse the Earth's rotation wouldn't just melt the surface it would disrupt the planet and create a new asteroid belt. (Quite possibly I'm wrong; feel free to do the math.)
Mar
31
comment What cosmic event would cause Sun rising from the west?
"In principle the tidal forces of the Sun will eventually slow the Earth's rotation so it always shows the same face to the Sun." -- Only given certain carefully chosen principles which ignore the existence of the Moon. The Moon's much stronger tides will eventually cause the Earth's rotation to become tidally locked to the Moon, not to the Sun. (Assuming our descendants or successors don't do something about it in the meantime.)
Mar
31
comment Is Feynman's explanation of how the moon stays in orbit wrong?
@mtanti: If you stay in the original frame of reference, the distance the ball moves "down" is larger. But the Earth is no longer straight "down"; it's now below and a little bit "behind" the Moon's new position. Over a short time span, the Moon's motion approximates the parabola it would follow in a uniform gravitational field; the Earth is at the center of the circle that's tangent to that parabola.
Mar
30
comment Is Feynman's explanation of how the moon stays in orbit wrong?
@MarkMitchison: Good point, I've updated my answer accordingly.
Feb
27
comment What's the surface area of a liquid? How does evaporation increase if the surface area of a liquid is increased?
If you have a tall glass of water with a given fixed surface area, it will lose a constant amount of water per unit of time (in an identical environment); the deeper the water, the longer it will last. But for a given volume, the large the surface area that's exposed to air, the more quickly it will evaporate.
Feb
26
comment What's the surface area of a liquid? How does evaporation increase if the surface area of a liquid is increased?
Why would having more surface molecules make the evaporation rate decrease?
Feb
26
comment Density of stars near the center of the Milky Way
@theJollySin: Not really. I've updated my answer with a link to a Wikipedia article that gives different figures.
Feb
17
comment Stresses in asteroid during close flyby
Tide is the difference in (gravitational) acceleration over distance. It falls off with the cube of the distance from the primary, which is why the Moon's tidal effect on the Earth is stronger than the Sun's. For an asteroid near Earth, the (differential) force from the tide and the asteroid's own gravitational cohesion are both fairly small; within Roche's limit, the tide exceed's the asteroid's gravity. The net gravitational force on the asteroid is zero, since it's in free fall, so I don't think "jerk" comes into it, except perhaps as a second or third order effect.