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About Halc

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  1. You need to actually read the post to which you are replying, which gave a fine example of change without speed of change. Another one: Air pressure changes with altitude, which is change without time or 'speed of change'. It has a rate of change which is the first derivative of the atitiude/pressure relationship, but that rate has no time associated with it, and thus cannot be expressed as a 'speed of change'.
  2. Hubble helped provide evidence for the expansion, but did not discover it. He certainly didn't discover the acceleration of the expansion, which wasn't known for over 4 decades after his death. Yes, but that kind of motion (peculiar motion) is caused by local mass distribution, just like Earth's motion is largely effected by nearby masses, notably the sun. With peculiar motion, every change in momentum in one direction must be countered by some other opposite momentum change in the opposite direction. The peculiar motion of the Milky Way is actually more or less away from Andromeda, being largely influenced by the VIrgo supercluster, the Great Attractor, and ultimately the Shapley Attractor, all of which are vaguely in the same direction somewhat away from Andromeda. That will of course change when Andromeda passes us and gets on the other side in its slow dance to eventually consume us. None of this motion has anything to do with accelerated expansion of space. Accleration is observed by noting that Hubble's constant (rate of change in proper distance as a function of comoving distance) is not constant, but has been increasing since a minimum was reached about have the age of the universe ago. Before then it was considerably higher, and decelerating, mostly due to gravity predominating over dark energy during a time when the mass density of the universe was sufficiently high.
  3. It did this from day 1, as do all free standing front load washing machines of which I am aware. The laundromat ones don't do it because they're bolted to the floor and to each other. The imbalance comes from laundry not being perfectly distributed. Nothing I can do stops that since the machine redistributes it, and tries quite hard to do it evenly. My point is that it does it most at a moderate frequency, which is similar to what the OP described. While I'm replying to your posts then: This is a plausible problem. If I take a disk and mount it at a slight angle relative to the axis of rotation, then it will wobble/vibrate as it spins despite not being off center. This is why it takes at least 2 weights to balance a car tire, which in combination yield a static balance (adjust the center of gravity to be on the rotation axis) and a dynamic balance (redistribute the mass of the wheel to a plane perpendicular to the spin axis). Or put it inside. It is a 'washer' after all.
  4. It's the same effect as my washing machine where the resonance really shows. It is never balanced well, despite its effort to distribute the load evenly. At very low speed, the imbalance has little effect as the forces are not large enough to move the machine. At medium speed (when it first attempts a moderate spin) it shakes like crazy, sometimes throwing the dryer next to it off the pedestal they're on. At high speed (1200 RPM), it's smooth as can be, but when it coasts back down to a stop, it has to pass through that moderate RPM place that makes it shake everything to death. I had a ceiling fan that shook badly at higher speed due to imbalance (as opposed to not being aligned with its axis). I duct-taped a thick metal washer about 2/3 of the way out from the axis and it ran flawlessly. Took a bunch of tries to find the sweet spot. So we're all sitting there one day and the thing lets go at high speed. I'm 5 meters away and get this washer smacking into the furniture 20 cm from my head. Never put it back. We just run the thing at medium speed tops.
  5. Comment on the picture "In case you missed it, we are here" puts Earth at a location in the night sky, and at the arbitrary direction chosen to center the image at that. "We are here" is a self contradiction in this case. We are not at a specific direction (some constellation?) in the night sky of where we are.
  6. You should at least attempt to show some effort in your OP so we know where you're getting stuck. It is a simple problem that might appear in a middle-school algebra test, made all the simpler by the fact that it is multiple choice, so the answer is already sitting there in one of the 5 answers, most of which are already absurd.
  7. They're known as Moiré patterns
  8. I hope you realize that spoilers on race cars do not create lift, but rather the opposite, preventing the car from flying. Lift would slow the cars.
  9. Rightly so. The purpose of the sub is not these things. It can use GPS if navigation is a goal at all. We probably want to know our depth at least. But you're making the same mistake with the first two statements. You're assuming a purpose of the craft is to keep a human alive, or to float when unpowered. We just want it to go fast on its own power. That's all. I also assure you that if it experiences structural failure at 1500 m/sec, the survival odds of a hypothetical occupant is not going to be a function of the internal pressure of the life support system.
  10. Why should it be for our racing submarine? Like a fish, there's no particular requirement to keeps a certain pressure inside. By all means, pressurize it if it helps. Even humans can take that so long as they don't mind the time it takes to depressurize. Similarly, given the power our sub is going to need, the buoyancy of the thing (be it positive or negative) seems a drop in the bucket compared to the sorts of forces we plan for it. Likewise, an airplane going fast need not worry about the fact that it has greater density than the air it displaces. Propulsion on the other hand is a serious issue, as it was for the early supersonic aircraft. I think fish tails would work better than a propeller, and in the worst case, we can always fall back to our rocket. Nobody said we had to sustain the speed for a long time. The shock wave is also a serious issue as you point out. The thing will be shaped like a needle, splitting the water 'gently' to the side rather than compressing it in a shock wave. Minimum sonic boom. This reduces the problem to one of the increased friction resulting from the greater surface area presented. Imagine an amoeba, injecting its skin into new territory, and then moving all its interior guts into that new expanded volume, closing it in behind. Just do that a lot faster. They have creatures that word this way. No propulsion since the skin is effectively stationary relative to the water and need not even be particularly slippery. All the motion (movement of its center of gravity) takes place internally.
  11. The original statement has multiple interpretations, left ambiguous by your translation. This being a topic concerning translation of English to logic, I approve the translation. The two interpretations seem to generate mutually exclusive statements. Each has implications seemingly opposite of each other. The interpretations break into logical statements involving two of three variables: S(sunny), G, and E for weddings in garden and elsewhere respectively. Note that neither interpretation is a function of all three variables. Both G and E can be false if no weddings are held that day, and can both be true if weddings are held in both places. Interpretation 1: On sunny days, all weddings are held in the garden. S => ~E This implies that if there is an indoor wedding, it is not a sunny day. It does not imply that if it is a sunny day, there is a wedding. Interpretation 2: On sunny days, there are weddings held in the garden. S => G This implies that if there are no weddings today, it is not a sunny day. It does not imply that if there is an indoor wedding, it is not a sunny day.
  12. Are the requirements such that the object moving has to A) maintain its speed, B) be self propelled and air-propelled, and C) survive the event. If A) is a requirement, the shuttle coming in doesn't count since it is slowing down, as is a meteor coming in at mach 100. If B) is a requirement (probably the most interesting question), then it becomes a question of how much thrust you can generate in a medium using that medium for reaction mass. This is reminiscent of my favorite Bonneville event: Fastest wheel-powered vehicle, far more challenging than simply strapping a cockpit and skateboard to a missile. If C) is a requirement, the meteor is out. This seems a reasonable requirement since anybody can just go out in space, come back and skim the atmosphere at 0.5c. Earth might even survive a graze like that. Related question I've posed to the dinner table: How fast must a ping-pong ball come in from space to destroy a submarine at 100m depth? (We have interesting dinner table conversations)
  13. Sure it is. Bob (owner of Bob's solar farms) was known to sink all his profits into expansion of the business, and thus Bob's solar farms increased its energy production capacity at an average rate of 40 megawatts per year over the last decade.
  14. Halc


    Gravity for a fixed mass object scales with the inverse square law (shown first by Newton, late 1600's), not the square-cube law, which instead relates area to volume, and also orbital periods (Kepler, around 1600). We're not talking about a fixed mass object here, so yes, the square-cube law is relevant, but only when Newton's work is applied. A planet of half the radius would have 4x/8x the gravity (8th the mass, half the radius squared). Ghideon gives a good answer to the OP. A basketball sized Earth would exert as much gravity as a basketball sized rock, which is negligible in comparison to what we're used to.
  15. Not so. I'm functionally a cyclops, and the 3D glasses they give you in the cinema do nothing for me, but I get my 3D vision from motion (moving myself) rather than from binocular vision. It doesn't work as well, but enough that I don't need to resort to groping to perceive depth.
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