8,647 reputation
1730
bio website en.wikipedia.org/wiki/…
location United States
age 71
visits member for 3 years, 10 months
seen 9 hours ago

BS Mechanical Engr.
PhD CS(AI)
CS Prof (4yr)
Numerous consulting jobs.
15 yr at http://www.pharsight.com
Published book on CS & several articles
4 kids, 2 grand
Pilot(student)

P.S. The picture is a Beta-prime distribution. It shows the program speedup factors you can get if you see a problem twice in 2, 3, 4, and 5 samples.


2d
comment What stops giant cruise ships toppling over in rough seas?
@Acid: Every such boat is designed by engineers and you can be absolutely sure they've thought of that. You can be sure they have written a user's manual saying things like: putting more weight than XXX on decks YYY will "void the warranty". They know what they're doing.
2d
comment What stops giant cruise ships toppling over in rough seas?
They look top-heavy, but they are not. Suppose you took a row-boat, and put a few hundred pounds of cement bags in it. Then pile cardboard boxes on top several feet high. Same idea. It might look top-heavy, but if you try to push it over, the weight in the bottom will bring it back up.
Jul
27
comment Why is not the entropy of earth increasing?
@Acid: I disagree with your last paragraph. The second law applies to closed systems, which the earth is not.
Jul
27
comment Why is not the entropy of earth increasing?
@pentane: Entropy can be defined as the integral of $dQ/T$, so it's not unreasonable to say sunlight at high temperature is energy at low entropy, and radiational cooling is the opposite. No?
Jul
27
comment What's the max speed a man-made satellite can travel in space before its circuitry stopped working?
Nice, but I would have said something like 1 - 2e-14 or 1 - 2e-7, rather than string a bunch of 9s and make me count them :)
Jul
26
comment Breaking the sound barrier underwater
Google supersonic torpedoes.
Jul
23
comment What is (theoretically) the most efficient shape for an aircraft, assuming you don't have to carry any cargo?
What matters is the lift/drag ratio. If drag is zero, the plane can't even come down! (They can point the nose down, but all that does is make them go faster, increasing lift, making them go back up.) Airplanes with high L/D are sailplanes (30:1). They minimize drag by traveling slowly and having long skinny wings (as well as being aerodynamically "slippery"). Efficient aircraft need spoilers and/or speed brakes, without which their glide is simply too shallow to be useful when landing. They have to get to the landing zone while missing the trees.
Jul
23
comment Finding minimum time to raise a bucket
It's good that you tried to solve it yourself and showed your approach.
Jul
21
comment Why and how strong are currents near ship side?
Good question. I assume the ship is anchored and aligned with the current. People often swim near large boats that are anchored, but maybe not in a river with current.
Jul
21
comment rotational springs
My favorite example - torsion bar suspension.
Jul
20
comment rotational springs
Torsion spring.
Jul
19
comment What's wrong with this argument that aerodynamic lift really does rely on Bernoulli?
I have to disagree with your last two paragraphs. You don't have to do piecewise analysis to understand that the wing pulls the air into a downwash because of Bernoulli's principle. The answer from BillOer, while he should have said more, points to the best explanation I've ever seen. (And I'm a pilot.)
Jul
16
comment What is the maximum speed of an object moving trough air?
Wasn't the Chelyabinsk meteor traveling at about 100k kmh?
Jul
15
comment Will the water go inside the moving water bottle?
@Vaggelis: Forget drag. Suppose the water is flowing and the bottle is stationary. Then the water outside the mouth of the bottle has to come to a stop. In order to do that, some pressure has to stop it, and that is the stagnation pressure. Then, you get into questions of how big is the nozzle, can some more water flow in while some water/air flows out, etc. That's a different question, depending on how big the nozzle is, and so on.
Jul
15
comment Will the water go inside the moving water bottle?
@Russell: The issue is: where does the air go? Sure, once there is pressure equilibrium at the orifice, stuff can flow out and in at the same time, as long as the net volumetric flow is zero.
Jul
14
comment How to find time when temperature crossed certain threshold?
A parabolic spline needs three points. If you have four points, they describe a cubic. You're not doing least squares unless you are fitting more points than your spline has degrees of freedom.
Jul
12
comment Which one to learn first: Special or general relativity?
Get a copy of Sam Lilley: Discovering Relativity for yourself Lilley taught relativity in continuing-ed classes full of math-phobia to begin with. It's a beautiful book.
Jul
10
comment Is there a simple equivalent to Ohm's law for gas (pressure=R*throughput)?
Electrical resistance (in a resistor) is just a fluid (free electrons) flowing through a lattice under pressure difference (voltage gradient) and bouncing off atoms, making them hot. Seems to me it's practically the same thing as flow in pipes at low speed. At higher speeds you get pressure relative to speed squared, so that's different. The latter is Bernoulli's venturi equation, so you probably want to stick to pipe flow.
Jul
9
comment Does rolling friction depend on area of contact?
Again, I'm not a downvoter, but this is not a very careful answer. Rolling friction has no intrinsic relationship to sliding friction. "true and false at the same time"? Please try to learn from others on this site, who try to give real answers.
Jul
9
comment Force vs. Momentum
What's a "non-preservative" force? What do you mean "continue to hold that force within them"? If a 1kg mass is floating in space at original velocity 0, and a 1 newton force is exerted on it for a span of just 1 second, no longer, then at the end of that second its velocity is 1 meter per second, and at later times its velocity remains 1 meter per second.