## Search found 132 matches

Tue Mar 19, 2019 1:56 am UTC
Forum: Individual XKCD Comic Threads
Topic: 2125: "Luna 2"
Replies: 9
Views: 1327

### Re: 2125: "Luna 2"

Frankly if they couldn't make it explode just before impact leaving a hammer and sickle shaped impact crater, THEN THEY WEREN'T EVEN TRYING
Tue Jul 25, 2017 12:08 am UTC
Forum: Individual XKCD Comic Threads
Topic: 1867: "Physics Confession"
Replies: 182
Views: 23776

### Re: 1867: "Physics Confession"

We do have a good answer for the hair/balloon/lightning thing ... it's called the tribolectric effect. What's wrong with that? Too empirical? Beware ontic dumping; If you answer "Why does static charge build up when rubbing a ballon on hair?" with "because of the tribolectric effect&...
Mon Jul 11, 2016 12:15 am UTC
Forum: Science
Topic: How do satellites not crash into each other?
Replies: 94
Views: 15577

### Re: How do satellites not crash into each other?

What about encasing them in some kind of viscous goo, so that anything that hits them (up to a certain limit), sticks? Yes, you then have the problem that that will change their orbit, but better one big sticky slightly-out-of-place agglomeration of satellites than endlessly multiplying debris, no?...
Thu Jun 16, 2016 4:49 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

While I'm not up to speed on U(1) I'm saying since you get the same results form a single electric potential, there's not really a 4 vector, that's just a mathematically clean way of describing how all the charges are interacting, but it is ultimately fictitious. Another analogy, my house's power su...
Thu Jun 16, 2016 2:29 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

I'm not sure what you're claiming. That there's a preferred frame for looking at electromagnetic fields? There's no such thing as a magnetic field, there's only electric fields, that produce magnetic effects. There's no 'magnetic field' dimension of space, but there is an electric field. You can co...
Tue Jun 14, 2016 10:07 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

Fμν is just a restatement of Maxwell's equations. And it's not in any sense wrong, it just includes the fictitious magnetic forces, in a similar way that changing to a rotating reference frame includes fictitious inertial forces. It would just be an astonishing coincidence if there actually was anot...
Tue Jun 14, 2016 6:34 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

The B/H field doesn't exist, it's just the E field. You know the phrase 'electromagnetism'? That's why that's phrase is really used, there's only one field, the electromagnetic one, and really it's the electric field, we know that due to the quantum mechanics underneath running the whole show, and t...
Tue Jun 14, 2016 2:33 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

The magnetic field has a vector potential associated with it. Why is a vector potential somehow less fundamental than a scalar potential? In principle, yes. In reality, no. Which direction does the magnetic vector point in? Why is it at 90 degrees to the electric field that is generating it? Unders...
Mon Jun 13, 2016 10:48 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

Not really, the E field has a scalar potential associated with it, and we're knee deep in electric monopoles.

I think the apparent symmetry between the E field and the B field is just because they're the same underlying field.
Mon Jun 13, 2016 7:00 pm UTC
Forum: Science
Topic: Relativistic Lorentz force
Replies: 38
Views: 7453

### Re: Relativistic Lorentz force

FWIW my understanding of this is that magnetic fields don't really exist, there's only electric fields with changes that propagate at the speed of light. In other words the equation that defines the curl of B in terms of the current and the rate of change of E; is this the definition of the relation...
Sat May 07, 2016 9:52 pm UTC
Forum: Mathematics
Topic: Seating problem
Replies: 6
Views: 2804

### Re: Seating problem

Woah. Very nice, I'm impressed. I didn't think it was possible; but I couldn't definitely prove it, but it looked suspiciously impossible. I feel a little better though that the answer looks so scrambled.
Sat May 07, 2016 5:24 pm UTC
Forum: Mathematics
Topic: Seating problem
Replies: 6
Views: 2804

### Seating problem

This is a real world problem, I think I have an answer, but I'm not sure the proof is entirely solid, and I thought it was vaguely amusing. We need to seat ten people at a table for a meal, for 4 courses, the people are to be reseated at each course, but this must be done so that no two people sit n...
Fri May 06, 2016 2:53 am UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

OK, so if we mean 'thermal equilibrium' in the sense of wikipedia: https://en.wikipedia.org/wiki/Thermal_equilibrium then no, you can't heat stuff up, at all, without some source of low entropy/work etc. But if we take 'passive' in the sense of what the questioner asked, where he specified that a tr...
Thu May 05, 2016 9:40 pm UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

Is this a question of the rhetorical type?
Thu May 05, 2016 9:21 pm UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

But in a conventional sense, it will be in thermal equilibrium; the surface of the sun will be thousands of degrees, the laser will be at (say) 80C or whatever, and the object will be tens of thousands of degrees, and everything will be stable. Sure, the heat from the laser, will be radiating to fre...
Mon May 02, 2016 3:17 am UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

Sorry, I thought I was interacting with people who had some understanding what they are on about; the last two posts have completely disproved that. "TLDR: lasers are always out of equilibrium." ^ this is absolute, total bullshit, continuous wave lasers you just shine pumping light in the ...
Fri Apr 29, 2016 10:52 pm UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

I just mean it's a different scenario, you can't use it to prove that lasers wouldn't work.
Fri Apr 29, 2016 10:49 pm UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

Using 1% of our current energy budget for the next thousand years seems a *lot* more practical What we have at your end is a complete lack of understanding of the scale of the problem. The calculation we just did showed that 100% of the human primary energy use for 10 years would increase the speed...
Tue Apr 26, 2016 9:12 pm UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

The question clearly is not about the heat death of the universe I wasn't talking about the heat death of the universe, I said "a hypothetical scenario where the Sun, the target, and the laser are all enclosed within a perfectly reflective chamber with no energy flowing in or out, and the whol...
Tue Apr 26, 2016 1:26 am UTC
Forum: Science
Topic: How Far Can A Person Walk Into The Desert?
Replies: 27
Views: 9088

### Re: How Far Can A Person Walk Into The Desert?

I really want the answer to be some formulation of the rocket equation, but that seems unlikely. I think it kind of is, as long as the person is allowed multiple trips and establishing caches along the way. ^ this The answer is indefinite, since you can walk out into the desert, somewhat less than ...
Tue Apr 26, 2016 1:16 am UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

At equilibrium the temperature must be the same anywhere, and any local departure from this temperature would represent a reduction in entropy, The question clearly is not about the heat death of the universe, so: so something must go wrong with trying to use the laser to heat the target to hotter ...
Tue Apr 05, 2016 6:58 pm UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

For me a sunlight pumped laser is passive Sure, if you redefine "passive" to mean something no one else uses it to mean, then you can "passively" do all sorts of crazy things. The question is, what wouldn't be passive under this definition? I literally can't imagine a more activ...
Tue Apr 05, 2016 2:19 am UTC
Forum: Science
Topic: Can you (passively) heat an object hotter than your source?
Replies: 98
Views: 25326

### Re: Can you (passively) heat an object hotter than your source?

Xanthir wrote:Yes, the question is about passive lenses. Lasers or spacecraft are neither passive nor lenses.

It depends how you define passive. For me a sunlight pumped laser is passive, and can heat hotter than the source.
Sat Apr 02, 2016 10:18 pm UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

Using 1% of our current energy budget for the next thousand years seems a *lot* more practical What we have at your end is a complete lack of understanding of the scale of the problem. The calculation we just did showed that 100% of the human primary energy use for 10 years would increase the speed...
Sat Apr 02, 2016 5:39 pm UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

The Earth seems to have no particular lack of reaction mass. You can sinter together rock of all sorts and lob it into the sky; but 100% definitely, mankind has practical limits on the amount of energy it can access.

That's why f'ing big guns do not seem to be a viable plan for moving the Earth.
Sat Apr 02, 2016 10:48 am UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

Really, though, if you're going to treat Earth like a rocket then you should be using the rocket equation instead, and concerning yourself with how momentum gets transferred. Not exactly. Not exactly what? We're not talking about momentum, we're talking about energy. And even with the most mass-eff...
Sat Apr 02, 2016 3:29 am UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

That's how much energy would need to be input from a source at rest relative to the Sun. But from the reference frame of something moving at 30km/s already, you're just going from 0m/s to 1m/s, which is a difference of 3e24J for something as massive as Earth. Oh. Doh! Yes of course, that's Oberth e...
Fri Apr 01, 2016 7:48 pm UTC
Forum: Science
Topic: Does potential energy have mass?
Replies: 78
Views: 15447

### Re: Does potential energy have mass?

You can if you want, but, you shouldn't. That was my original point, that ascribing "mass" to "gravitational potential energy" is a kind of double counting. Agreed. If potential energy has mass, at the very least, in the case of gravity, it's a negative mass; you would be losing...
Thu Mar 31, 2016 3:33 pm UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

The reason it can work is because the solar system is a chaotic system, so with great care, and not quickly or easily either, it's possible to move stuff around; by moving the small stuff you can move the bigger stuff, and by moving the bigger stuff you can move the even bigger stuff. It would still...
Wed Mar 30, 2016 11:44 pm UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

OK, you're going to force me to Math you. Mass of Earth = ~6e24. Orbital speed = ~30km/s Orbital energy = 0.5*m*v^2 = 0.5 * 6e24 * 30000^2 = 2.7e33 J Let's say we want to add 1m/s to the orbital speed. How much energy would the Earth then have? Orbital energy2 = 0.5 *m*v2^2 = 0.5 * 6e24 * 30001^2 = ...
Wed Mar 30, 2016 10:50 pm UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

I don't care how many guns you make, how big, or how small you make them, the economics isn't going to work out. Energy costs money, where would you get the money from to make the energy to move the Earth? The whole point of using asteroids is that you can effectively exchange energy/momentum betwee...
Tue Mar 29, 2016 2:30 am UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

No, it's extremely un-doable. The gun is economically impossible to build at a scale big enough to move the Earth. Although you'd think that moving moons isn't possible either, we actually already know how to move small asteroids using gravity tractors, and if you think about it, that means that by ...
Sat Mar 12, 2016 6:26 pm UTC
Forum: Mathematics
Topic: Greatest Mathematician of all time?
Replies: 65
Views: 19212

### Re: Greatest Mathematician of all time?

But this animal invented the entire field of mathematics from scratch.
Sat Mar 12, 2016 3:43 pm UTC
Forum: Mathematics
Topic: Greatest Mathematician of all time?
Replies: 65
Views: 19212

### Re: Greatest Mathematician of all time?

There's a subtle difference between the greatest mathematician of all time, and the greatest mathematician in history. Some animals are able to count up to low numbers. Presumably an animal must have evolved the whole idea of counting stuff, and we've inherited this ability. GREATEST MATHEMATICIAN E...
Wed Mar 02, 2016 4:06 pm UTC
Forum: Mathematics
Topic: Greatest Mathematician of all time?
Replies: 65
Views: 19212

### Re: Greatest Mathematician of all time?

Sun Feb 28, 2016 2:22 am UTC
Forum: Science
Topic: Engines big enough....
Replies: 28
Views: 6075

### Re: Engines big enough....

It's theoretically possible to move the Earth, but it would be dangerous and take a long time. Basically the trick is to use small bodies to deflect bigger bodies using multiple passes around other even bigger bodies and lots of fiddling around to keep everything on course. In effect, you can use Ju...
Wed Feb 10, 2016 1:11 am UTC
Forum: Science
Topic: Landing rockets upright is unnecessary?
Replies: 61
Views: 12876

### Re: Landing rockets upright is unnecessary?

A rocket's efficiency is at a maximum when all of the heat of combustion and all of the pressure have been fully expanded to ambient temperature and pressure. No. That's a finite length nozzle, except in a perfect vacuum. And even there, there's a optimum nozzle length where increasing the length a...
Tue Feb 09, 2016 5:06 pm UTC
Forum: Science
Topic: Landing rockets upright is unnecessary?
Replies: 61
Views: 12876

### Re: Landing rockets upright is unnecessary?

I think what some people are suggesting is essentially a scramjet/ramjet where the flame-holder is a rocket engine that provides the initial startup thrust to get it up to ramjet operational speeds then throttles back as the ramjet portion starts providing more thrust. Not exactly. The concept pres...
Fri Feb 05, 2016 11:28 pm UTC
Forum: Science
Topic: Landing rockets upright is unnecessary?
Replies: 61
Views: 12876

### Re: Landing rockets upright is unnecessary?

For comparison, static structures like buildings and bridges are usually built to withstand five or more times the expected stress. Steel bridges are typically designed for something like 1.2 times the permanent loads, plus 1.5 times the expected "live" loads. With (large scale) plastic d...
Sat Jan 23, 2016 1:06 am UTC
Forum: Science
Topic: electromagnets motors and resistance question
Replies: 3
Views: 1924

### Re: electromagnets motors and resistance question

The stall current is much higher because there's no "back emf" when it's stalled. Look it up. The nail is always effectively stalled, and you just need to use the right amount of current/voltage to stop it burning out. When a motor is running the back emf reduces the effective voltage acro...

Go to advanced search