# Search results

• Users: Frodo
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1. ### Two 95 watt light bulbs walk into a bar ....

I have always been meaning to do this - thank you for providing the stimulus for me to do it! I have just measured a 60W bulb for a 240V supply. At room temperature it was 80 ohms. Now, if it consumes 60W when on, then 60W = 240 * 240 / R. The resistance when on is therefore ~960 ohms or 12...
2. ### What is the relationship between refraction and giant waves in Nazaré?

Normal sea waves close to the shore behave exactly like tsunami waves. Both a 'wind driven wave approaching the beach' and a 'tsunami wave during its entire journey' are shallow water waves meaning their wave length is longer than the water depth and their speed is proportional to the depth...
3. ### What is the relationship between refraction and giant waves in Nazaré?

The way in which the Nazerre wave is created is explained in https://www.surfertoday.com/surfing/the-mechanics-of-the-nazare-canyon-wave where the critical importance of the underwater canyon is discussed, as is how the sea floor profile causes the wave to refract. Other factors also contribute...
4. ### Electrical Screw heads: Phillips/Slotted vs ECX

They may be INSULATED ELECTRICIANS SCREWDRIVER SET VDE - slotted Phillips. Note the visible taper on the driving part :-( NB - my error - it's Pozidriv, not Posidrive :mad:
5. ### Electrical Screw heads: Phillips/Slotted vs ECX

From your image, the ECX head looks more akin to a Pozidriv head than to a Phillips head. Does a Pozidriv screwdriver fit it? As I recall, Phillips were introduced in the US for automated manufacturing - the driver was supposed to self align. In reality, the flanks of the driver are slightly...
6. ### Does any classical mechanics textbook solve Kepler's Problem?

See https://www.researchgate.net/publication/26386594_Relativity_in_the_Global_Positioning_System by Neil Ashby which describes in detail the multiple relativistic corrections applied in the GPS.
7. ### B Could Archimedes' problem have been solved a little differently?

I think the site is being a little disingenuous so as to prove its point. It uses the example with the volume of a solid gold wreath being 51.8 cc. The volume of wreath adulterated with 30% silver would be 64.8cc, a difference of 13cc or 25%, which I think would be reasonably easy to measure...
8. ### B Could Archimedes' problem have been solved a little differently?

That is correct and deliberate. I read the poster's question "When I was reading this page to understand Archimedes' 'story'" and answered appropriately by giving the story. The poster is not asking how it would be done today but is trying to understand what Archimedes did or could have done...
9. ### B Could Archimedes' problem have been solved a little differently?

You may be interested in https://thestouracademytrust.org.uk/wp-content/uploads/2020/05/Archimedes-Story.pdf for a description of what apparently occurred - Archimedes realized he could measure the volume of an object by measuring the volume of water it displaced. I don't know how accurate it...
10. ### B Twins and GPS

Neil Ashby's https://www.researchgate.net/publication/26386594_Relativity_in_the_Global_Positioning_System gives an excellent account of the relativistic corrections made in the GPS system. The correction for the Sagnac Effect is amusing. The design team accepted the Sagnac Effect and designed...
11. ### How can you determine if a fan is better at blowing or sucking?

Dave Pax! My "spend a few minutes actually thinking about what I said " was addressed to berkeman after his put down. It was not addressed to you! Your comment was reasonable and, as you see, I agree with you. I have no disagreement with you - my disagreement is with berkeman not with you -...
12. ### How can you determine if a fan is better at blowing or sucking?

I agree - "suck" is a useful concept to work with. But it is as well to understand what is actually happening whenever one resorts to a concept which isn't actually totally true. Archimedes Principle is a useful concept but the unwary will fall l into trap after trap if she uses it blindly...
13. ### How can you determine if a fan is better at blowing or sucking?

Please withdraw that comment before I report you to a moderator. You have no knowledge of my academic qualifications nor of my expertise. Neither is relevant here. I seem to recall Einstein was a patent clerk when he upended the scientific world.
14. ### How can you determine if a fan is better at blowing or sucking?

May I suggest you carefully read what I said before contradicting me. I said " How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force. ". Can you perhaps tell us all how a molecule...
15. ### How can you determine if a fan is better at blowing or sucking?

There is no such thing as "suck" when applied to a fluid or gas. Say a fan is set up to blow air to the right. How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force. What a fan does...
16. ### Endless bridge around the equator and gravity

The engineers are correct. Let's do a simple calculation. Imagine, for simplicity, the bridge is a footbridge and it is 1m wide and 1m deep and made of concrete. Concrete weighs about 2,500 kg/m^3 so a 1 metre length of the bridge will weigh 2,500kg. See the diagram below where the angles...
17. ### B Is gravity a force or not?

And I thought I had anticipated that objection :headbang: First, I'm just following Feynman, someone for whom I have great respect. Secondly, in what sense is "a light inextensible string passing over a frictionless pulley" not a "highly idealized academic example"? It, and a "point mass"...
18. ### B Is gravity a force or not?

We are in violent agreement.
19. ### B Is gravity a force or not?

I apologise profusely :mad: as my fingers did not type what my mind was thinking. I meant an infinite plate of zero thickness and of uniform mass density. The gravitational force is then uniform throughout the region and there are no tidal forces either parallel or perpendicular to the plate...
20. ### B Is gravity a force or not?

Did you see my suggestion of considering a semi-infinite slab of mass? You can have your region as big as you want without tidal forces.
21. ### Relay Failure Possibilities

Please upload the relevant parts of the circuit diagram so sensible suggestions can be made. If it is a DC circuit then you are almost certainly overloading the contacts. DC does not quickly (within 10ms for 50Hz) extinguish any arc by going to zero as AC does, so much more damage is done to...
22. ### B Is gravity a force or not?

May I suggest you start by reading Fictitious force as I think it will greatly assist you in understanding fictitious forces. I gave a few examples in my post above - they may also help your understanding. If you always do your analyses using an inertial frame of reference you never see...
23. ### B Is gravity a force or not?

It does seem to me that a number of posters (I have one, not vanheese71, in mind!) are more keen to demonstrate their depth of knowledge than to put themselves in the shoes of the questioner and answer the question without bringing up minutiae which serve only to confuse and digress. One gives...
24. ### B Is gravity a force or not?

Interesting quibble when one sees the generality of what the OP was asking in a sub-forum entitled Classical Physics and at Basic (High school) level. Quibbling your quibble, does that apply at a point? Don't tidal forces require an extended region?
25. ### B Is gravity a force or not?

No and yes. It is also a matter of semantics and how people decide to describe things. No because the force a body feels when attracted by a massive body is due to the body's interaction with the curved space time created by the massive body as is described by GR. However, describing it as...

Absolutely. How long is a photon? As long as a piece of string. A photon has a frequency and, (simplistically) to have a frequency, you must have an extended wave with something changing and going up and down. The more cycles you have in the wave, the more accurately you can measure the...

Without wading through the thread ... ... my advice would be to get or read a copy of Feynman's Lectures in Physics and all will be revealed. The lectures are mainly written prose as Feynman was a most brilliant teacher who always explained the physics before going on to develop the equations...
28. ### "Power reading" for learning physics?

I recently came across this most apposite ancient Chinese proverb I hear - and I forget I see - and I remember I do - and I understand Attributed to Xun Kuang (c. 310 - c. 235 BC) known as Xunzi (”Master Xun”), a Chinese Realist Confucian philosopher. See...
29. ### Describe the motion of yoyos suspended from the ceiling

It's always useful to solve a similar but simpler problem first to understand what is happening. Make sure you understand the physics of what is happening before blindly applying any equations. I would solve for one yoyo to understand the tension in its supporting string throughout the motion...
30. ### What are the limitations of the Explicit Finite Difference Model?

Ask yourself: What do you think can go wrong if you use it? Will always go right? Will it always converge to a solution? Why? Or why not? How quickly does it converge towards a solution? How much computing resource does it use? If you speed it up using an accelerating factor what may...
31. ### B Single photon detection

A duckduckgo search with single photon detector found much of interest including Invited Review Article: Single-photon sources and detectors. Have a look at C. Photon-number-resolving detectors which discusses several techniques.
32. ### B Single photon detection

Students do the double-slit experiment with single photons these days - see the links above.
33. ### B Single photon detection

Today students do experiments with single photons. For example I refer you to the Advanced Laboratory Physics Association's Single Photon Quantum Mechanics which says If you can accept that an interferometer is equivalent to a double slit I think your objection is answered. You may also care...
34. ### B Single photon detection

Thank you for pointing that out although I credit the readers here with the ability to spot that without needing you to recap it for them. btw, it isn't a quote - it is my recap of a lecture I attended. Each to his own but I read those accounts differently to you. In order to explain the...
35. ### B Single photon detection

I think an interesting thing about the double slit experiment is to do it this way. Experiment A: Get 100 different people located all over the world. Each person does their double slit experiment with just one photon or electron. All the experiments are done at different times so there is...
36. ### What to fill a glass bottle with (a liquid that hardens) to lower its volume?

Sand will work. Add a bit of glue if you want it to stay in place. Glass fibre resin - be careful - it gets hot when setting. Potting compound/resin. Mortar. Molten lead - don't breathe any fumes as they are extremely toxic. Don't use if it will damage the skull. Bore a hole in the base...
37. ### Bohr theory Problem

That statement is wrong for mass as U has lots of neutrons as well as protons. Compare the U and H atomic weights of ~238 and ~1. It is correct for charge when you remove approximately as the U nucleus has 92 protons whereas the H nucleus has only 1 proton. Mass does not come into your...
38. ### Wave optics, double slit

Incidentally, you can see how impractical the question actually is. The path difference is ~5.3 wavelengths of light and the answer is the ~0.3 wavelengths of light. So, to be accurate, the screen has to be perpendicular to a phenomenal accuracy. If it is just the tiniest bit...
39. ### Wave optics, double slit

You have had that Aha! moment when you have understood something. You can now solve any problem like this no matter how much the question is dressed up to make it look more difficult. Remember what Rutherford said: " All of physics is either impossible or trivial. It is impossible until you...
40. ### Wave optics, double slit

The point about the laser is that laser light is coherent - all the light is in step or in phase. So, the phase of the light arriving at the top slit is the same as the phase of the light arriving at the bottom slit. Hence, the phase of the light leaving the top slit and the phase of the light...
41. ### Wave optics, double slit

I didn't check your calculation because I cannot remember if the formula you give is right or wrong. You don't need to apply a formula blindly to solve (ii) if you think about the physics of what is actually happening. The slits are very wide apart compared with a normal double slit experiment...
42. ### Trying to find an algorithm for the Number Mastermind game

Sorry - I didn't make myself clear. If you are at 163 and you have zero correct, and you go to 164, and you have one correct, it must be the 4. You now only have to check the first two digits. It fails at 169 goes to 170. More complexity results in fewer guesses needed to get the number...
43. ### Trying to find an algorithm for the Number Mastermind game

The simplest method is the slowest with a max of 1,000 steps or 500 on average. Start: try all numbers 000 - 999 until you get 3 correct. End: You can speed it up because you know the changed digit(s) was the correct one. The next method probably needs 2 or 3 random guesses at max, followed by...
44. ### 3D model I made of the Three Mile Island control room

Did you include the cardboard tag which, IIRC, was hanging on a wall panel obscuring one of the two Emergency Core Cooling System, ECCS, warning lights saying the ECCS was disabled as the valves were closed? Report Of The President's Commission On The Accident At Three Mile Island is an...
45. ### Optimization problem - right circular cylinder inscribed in cone

1. You know the expression for the volume of the cylinder in terms of r and h - that's dead easy. The problem is it has two variables, h and r so is tricky to differentiate - it needs partial differentiation with respect to both variables which you have probably not yet done. So make it...
46. ### "Power reading" for learning physics?

There is physics and then there is physics. Einstein apparently once sat next to an attractive young lady at a dinner party. She had no idea who he was and began a polite conversation by asking "What do you?" "I study physics", said Einstein. "Gosh!", said she, clearly surprised. "I...
47. ### Comp Sci Matlab report help please (Finite Difference Method)

I would say the essential point is that, as there are no heat sources in the plate, then at steady state, the heat flowing into any bounded region exactly equals the heat flowing out of that region, or Div D = 0 (ie the divergence is zero). Hence the temperature at any grid point is the average...
48. ### Engineering How could a solar farm affect the trains' signalling?

May I dispute "unanswerable"? :cool: It is answerable ... ... but the assumptions which need to be made give an answer which is probably of no use whatsoever. Of course, if you have a solar farm in Afghanistan, if your train is in Venezuela and if you only want the train to run during the...
49. ### Engineering How could a solar farm affect the trains' signalling?

I think other responders have been too cautious. All we need to do is to make some assumptions: You build your solar farm in Afghanistan Your train is in Venezuela The electrical power from your Afghanistan solar farm which you feed into the Venezuelan train infrastructure meets the Venezuelan...
50. ### Gas Laws - pumped tires exploding at the top of a mountain

Me too. But don't forget the increased cosmic ray density at that altitude; nor the increased UV intensity - UV rots rubber; nor quantum mechanical effects as all those tyre gas molecules may move in the same direction at the same time and force the tyre off the rim; nor altitude sickness as it...