# Search results

1. ### I am confused about the meaning, and value, of kinetic energy

Thanks for the replies everybody! If I understand it correctly, kinetic energy is self-consistent provided you stay in the same frame, and that my example was flawed in the sense that I disregarded the "accelerator" which would have made the total energy stay constant when taken into account.
2. ### I am confused about the meaning, and value, of kinetic energy

This confusion has lingered in the back of my mind for years now, would be good for me to finally get a grasp on this. Say I have an object currently at rest, and I use energy X to accelerate it to speed v. According to the standard formula, it now has a kinetic energy 1/2mv^2. Now I use the...
3. ### Fluid going through pipe: what is the influence of speed on exchanging heat?

I'm doing a fun home project, and it involves water flowing through a metal pipe, where the surrounding is significantly lower temperature than the water in the pipe. The point of the exercise is to cool the water in the pipe as it flows through it. The question is, what influence does flow...
4. ### B Calculate, not measure the value of c

I am taking your question to mean, "can c be derived from first principles, purely on paper, not relying on measured quantities?", in which case the answer is no. It is considered a fundamental physical constant that is established empirically.
5. ### I If all of spacetime was created with the Big Bang...

Disregarding the Big Bang event for a moment, I find that topic incredibly interesting from a standpoint of causality. At least to me, causality implies the requirement of time. So, if it is nonsensical to ask "what was before spacetime", then isn't nonsensical to ask "what casued/created...
6. ### I If all of spacetime was created with the Big Bang...

Well ... but doesn't that usage imply the assumption that normal physics prevailed during that process, i.e. that spacetime was "intact" enough to give a notion of a "before"? It's probably a good assumption ("barring contrary evidence, assume everything stays the same"), but at the same time...
7. ### Angular frequency of a non-sinusoidal pulse

Doesn't the whole concept of describing arbitrary waveforms as a sum of sinusoids fall apart when you consider nonlinear systems? The reason you can do all these convenient acrobatics of angular frequencies etc is because sinusoids are the solution to the *linear* wave equation. The moment...
8. ### I If all of spacetime was created with the Big Bang...

...why do people like Stephen Hawking talk about things/events "before" the Big Bang? If time (as part of spacetime) was created with this event, it also marks the point in time that had no "before", doesn't it?
9. ### B Shadow of clouds over the Sea

I think you misunderstood my point. I wasn't saying that the emitted light has some kind of directionality to it (I agree with you, it doesn't), but instead that if you section the "sun disk" into vertical slices, the slices towards the edge of the sun are shorter and thus contribute less to the...
10. ### B Shadow of clouds over the Sea

Keep in mind that the sun is a sphere, so you are getting almost no light from the left edge of it or the right. You can probably halve your estimate to 8m because of that.
11. ### Regenerating electricity on an e-bike

This has probably popped into every cycling engineer's head at some point (inckuduin mine). The problem is that most energy is lost during riding by friction and wind resistance, none of which can be recouped. Because any device will add weight to the bike, thus requiring additional energy by...
12. ### B Warm air goes up...reason on a microscopic scale?

What you are describing is heat conduction. However, air actually being a pretty good insulator, it could not account for the often drastic and fast changes. Nah, hot air rising is likely to be due to convection instead. Even in the atmosphere, that's how wind comes about after all.
13. ### B Warm air goes up...reason on a microscopic scale?

Drakkith, I think your intuition about the pressure differential is right. In zero gravity hot air doesn't "move", it only slowly equalizes in temperature. I suspect the higher average kinetic energy gives the "hot" molecules more opportunity to escape both up and down, whereas the "cold...
14. ### Why does egg float upwards in stirred bottle of water?

For the second video I think it might also have something to do with the fact that the vessel bulges out in the middle. The centrifugal force will naturally make it go to the widest part of the vessel.
15. ### A theoretical flotation device using a vacuum (for a novel)

Wow, that's crazy, I was just yesterday googling exactly this, i.e. how feasible suspending an object purely by vacuum would be. One interesting thing I found was that most calculations I saw online presumed a spherical shape, and then calculating the necessary compression strength of the...
16. ### Detailed simulation or SlowMo of elastic collision?

Dagnabbit, I'm in no position to make an educated criticism of what you posted :) I easily understand the parts of what you posted, but I can't say whether the conclusion you draw is logically binding. At the very least it will give me something to do on an upcoming 3-hour flight of mine. I...
17. ### Detailed simulation or SlowMo of elastic collision?

That's all true. Somehow I still have a hard time imagining even a theoretical setup in which *any* object, entirely within Hooke's Law's bounds, would come out with zero ringing. Negligible amount, maybe. Entirely? Somehow I doubt that.
18. ### Detailed simulation or SlowMo of elastic collision?

That's interesting though, because the golf ball exhibits exactly that "ringing" I was talking about (it keeps deforming after it separated from the wall). Clearly this wasn't a purely elastic collision because part of the energy is now used in that ringing. Interesting question: usually energy...
19. ### Detailed simulation or SlowMo of elastic collision?

Actually, when I think about it, this reflection at the end of the rods is probably the classic "closed end" wave reflection where the phase reverses. Meaning, the compression turns into expansion and travels backwards through the rod, as you mention. I'm still a bit surprised that no energy...
20. ### Detailed simulation or SlowMo of elastic collision?

I was thinking about Newton's Cradle the other day, and I wondered how those collisions actually look like in detail. Which then got me thinking that my understanding of even a basic elastic collision of two macroscopic objects is weak to say the least. Simplified, the two objects could be two...
21. ### How can we melt ice in a microwave efficiently?

I find the "ice doesn't heat easily because the molecules can't move as much" theory unconvincing. After all, this isn't too unlike a spring system; even if the spring is more rigid it can still absorb just as much energy. Just because something moves less doesn't mean it contains less energy...
22. ### How can we melt ice in a microwave efficiently?

As you mention, the inhomogeneous thawing is always an issue though, right? That is, if you put frozen food into a microwave, some pocket will thaw first, and then you have a runaway effect in that pocket because it absorbs the microwaves much more efficient. So, some part will always be...
23. ### How can we melt ice in a microwave efficiently?

Here's an idea: given how the lack of liquid water is the problem in the beginning, maybe you could take a thin piece of cloth, soak it in water, and place it on top of the ice. The cloth would get hot very fast, melting the ice below it, thus jump-starting the overall heating process.
24. ### Straight line plot of bipolar transistor - temperature?

Is there a reason why you think you can transform it into a straight line?
25. ### How can this interesting set of mirrors be constructed?

From the Wikipedia page: That was going to be my guess also, that it's two mirrors at an angle to each other, and you're looking head on.
26. ### Forces on magnets in a uniform magnetic field

Unless I am mistaken, a completely uniform field, impossible as it is to create, would exert no force. You can derive the resulting field easily, since you can just add the magnetic field of each coil (Superposition Principle). They would just counteract each other, probably resulting in...
27. ### Suspending a Magnet or Ferrous rod in a Tube

Given the frequency with which the question comes up, I think we should be a bit more discerning in our answer. Earnshaw's theorem is not as restrictive as most people assume. For example, you can very easily levitate a block of Bismuth in a static magnetic field because it is diamagnetic.
28. ### I Self balancing stick based on flywheels

Your question comes down to, can you indefinitely accelerate a motor? I think the answer is kinda obvious.
29. ### I Self balancing stick based on flywheels

Given how the motor has a limited amount of torque it can exert through acceleration, I would think it's rather obvious that there will be an amount of impact above which the motor can not counteract.
30. ### I 1s orbital overlaps with the nucleus, why don't they interact?

I would think the previous discussion in this thread indicates that I was indeed talking about interaction between the electron's wave function and the proton's, not just a chemistry consideration. I was wondering why, if the the highest probability of an 1s electron is smack in the middle of...