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  1. Merlin3189

    Intro Physics Physics textbook recommendation for young gifted child

    Sounds to me as if they need to do some biology in order to understand physics here. IMO the reason you need to do work to carry the <heavy object> horizontally, is not so much to accelerate it horizontally to slow walking speed, nor to overcome friction (where?), rather the biological fact...
  2. Merlin3189

    B Magnetizing a Compass Needle

    But if you did that, the torque due to an external field would be much less. I would be inclined to have a plastic disc with an arrow E-W or NE-SW or whatever you want, but keep the long magnet beneath aligned N-S.
  3. Merlin3189

    Electrical Quartz Clocks (Battery Operated)

    All battery-operated equipment must run off a range of voltages, since batteries vary over their useful life. Both Zn-C and alkaline batteries are nominally 1.5 V and spend most of their life between about 1.5 - 1.3 V NiCd and NiMH have a lower voltage, starting around 1.3 V and spending most...
  4. Merlin3189

    Medical Question About blood pressure

    My understanding of BP is that it is measured by stopping the blood flow with a cuff - like a tourniquet. Then as the cuff pressure is reduced, you listen for the resumption of some flow and the resumption of flow for the whole cycle. The first of these would seem to me to represent purely the...
  5. Merlin3189

    Trying to Understand the Polarization of light

    Glad to see this video helped a bit. I think it is brilliant and really helped me a lot. What I think may be hanging you up now, is that a vector can always be replaced by a pair of orthogonal vectors, which totally represent it and can in turn be replaced by a single vector (their sum)...
  6. Merlin3189

    Why does less dense air rise and more dense air come down?

    Not sure I understand what is being said, so I drew a diagram (as always for me!) Now, I'm as troubled as you (or whoever) that when there is no solid box, there can't be two different pressures in the same place. So I guess that as soon as there is the smallest difference in density...
  7. Merlin3189

    I Vacuum force factors (vacuum created by a "flow through" liquid)

    In hydraulics, life is very easy because the pressure varies within the liquid simply in proportion to the depth. So in your case, as liquid flows out the tank, the air layer gets expanded, its pressure reduces and water is "sucked" up the tube. This can continue until the depth of water in the...
  8. Merlin3189

    Help understanding this velocity-time question

    Isn't this just Galilean relativity? And you just sit on the lunchbox watching the boat drive away and come back again, and just ignore the banks sliding by.
  9. Merlin3189

    Understanding the effect of duty cycle on the brightness of LEDs

    As DaveE and AverageSupernova seem to be saying, you've come to the wrong forum here. The physics IMO is relatively simple, but it is the physiology and psychology which you need to understand. When any light is on it emits light and when it is off it doesn't! Talking of duty cycle just...
  10. Merlin3189

    EMF produced across one side of a coil

    An unnecessary sidenote, but I find that a strange question. They are indeed the same here, but I always think that the emf is induced and the current IF ANY is caused by that emf. Were the coil to be incomplete - with a break in AB or CD, say - then the induced emf would be the same, but...
  11. Merlin3189

    Engineering Pneumatic Basic Circuit

    Apologies for this. I did not notice it was an old question. Please consign it to oblivion.
  12. Merlin3189

    Engineering Pneumatic Basic Circuit

    These are spool valves which change state when pressed (the left valve) or energised pneumatically (the top/right valve.) All are spring return and are initially shown in the unoperated state (spool to the left.) When the manual valve is operated, its spool moves to the right. In both cases...
  13. Merlin3189

    Velocity Ratio and Efficiency

    VR is the constant: it is MA which is reduced by friction/inefficiency. You can't calculate the VR from the inefficient MA , other than by correcting for efficiency. So you calculated the VR as 1/4 ie. the output moves 4x faster than input which gives an ideal MA (at 100% efficiency) of...
  14. Merlin3189

    Voltage of DC Generator

    Your idea sounds right to me. So what answer does it give? Uload <> 188.8 because it is the generator voltage, U, that is proportional to speed. So at 800 rpm U = 188.8 V Then Uload = U - Uw as you said. So what is the given answer for (b)?
  15. Merlin3189

    B Battery Internal Resistance

    Your graph suggests you are getting errors less than 1%, which is better than any electronic meter I've used. So are you confident that the errors are not in the meter?
  16. Merlin3189

    Find the mode, median and mean of the number of children

    Mode is, the most common number of children in a house (see below): 1 child in a house, occurred in 7 houses; no other number of children per house occurred more often. So mode =1 child in a house. And I think all of this comes down to understanding the actual "experiment". 20 houses...
  17. Merlin3189

    Do LED lights flicker?

    Nice. But I wonder how you know it is the Leds flickering rather than camera strobe? (I don't actually know whether digital cameras have a strobe effect like the old movie cameras did.)
  18. Merlin3189

    I Energy Transfer Between Inertia Wheels

    Conservation of angular momentum.
  19. Merlin3189

    Why in this short circuit geometry can I not sense a photocurrent?

    Maybe you are forgetting the internal resistance of the photodiode (even though you show it in the picture.) The diode generates a voltage and a current flows through the short circuit. But if you think about the resistances, the voltage source is in series with the internal resistance and the...
  20. Merlin3189

    Jumble Word Game

    Can you say what the problem is? For now, some more thoughts: I assume your problem is understanding arrays on the left of = Array[x] means element x of the array When it is on the left, that's where the result is put. When it is on the right, that's the element that is used in the...
  21. Merlin3189

    Jumble Word Game

    This may not appear as I intended - I can't find a Preview button? But I've saved and edited it a few times and it's starting to look ok! theWord = PHYSICS.......index1 index2 ... temp ... jumble...
  22. Merlin3189

    Find the car's speed at point when deceleration is increased - Mechanics

    The question is, why does the line HAVE to intersect ##v=-0.5t+30## and ##v##=##\frac {1}{3}t## ? You only know that after you have the answer. If the two accelerations had been say -0.5 m/sec/sec and -2 m/sec/sec, where your "scale factor" is 4, would you draw the line ##v##=##\frac...
  23. Merlin3189

    At what time will bike and car be side by side again - Mechanics

    Since you have a v-t graph, I don't see how you can tell by inspection the region where this is true. I think you mean, your coordinates span the time when the bike is going faster than the car. The bike remains in front for a further 8.667 seconds, while the now faster car is catching up...
  24. Merlin3189

    Find the car's speed at point when deceleration is increased - Mechanics

    I too was puzzled by the logic, even after I checked the result and found it correct. I have tried to follow the method for other situations, particularly where m1 = -0.5 and tried various values of m2 such as -1, -1.5, -2, -2.5 just to keep the sums easy for my poor old brain. It only seems...
  25. Merlin3189

    B Create an artificial eddy/tornado

    What are the fans for? Wouldn't the fire form a natural vortex?
  26. Merlin3189

    Engineering Having difficulty finding the direction of the induced currents

    I would think about Lenz's law. If you're more mathematically inclined, as I think is the favoured practise, show your calculations and say what conclusions you reach from the sign of the results.
  27. Merlin3189

    Can this comparator accept negative voltage as its negative supply?

    LM319 can accept negative supply, eg power rails up to ±18 V or +11 V to -25 V (with ground at 0V) Within those (and other) parameters, it doesn't matter what absolute values you assign the three voltages (at least until you connect to another circuit!) As far as I can see from the datasheet...
  28. Merlin3189

    Why bulbs and LED diodes keep glowing after being switched off?

    I looked at 10m cable, 1mm diam wires, 2mm apart, PVC insulation and got about 1nF capacitance, for about 3MΩ impedance at 50 Hz and about 80uA current, neglecting the bulb impedance. A lot of 'about's in there and perhaps I shouldn't neglect the bulb's (complicated) impedance. Maybe I'll...
  29. Merlin3189

    Why bulbs and LED diodes keep glowing after being switched off?

    Coming right back to this, you seem to have drawn a bulb in series with a capacitor connected to the AC power. The switch is in parallel with the capacitor and supplies extra current when closed. But even without the switch a small current can flow via the capacitor, which might be enough to...
  30. Merlin3189

    Why bulbs and LED diodes keep glowing after being switched off?

    If you go back to CRTs, then oscilloscopes could have long persistence phosphors lasting seconds (they fade gradually, so depends where you call it cut off.) Similarly radar displays needed the image to remain reasonably visible for one rotation of the antenna, which would have been seconds...
  31. Merlin3189

    I Observations on the electromagnetic spectrum with a simple DIY spectroscope

    I don't know the answer, nor TBH what the question is! But as you've been asking for a week, I'll offer a few comments. First, if something emits light, it has an emission spectrum - that's just tautology isn't it? If you are looking at an LED screen, the emissions are from LEDs (obviously)...
  32. Merlin3189

    B Why does light diffract into only seven colours?

    If 000 is black, would not 111 be white, leaving 6 colours ? Which is what most people see. I suspect a protanope might still see six colours in a rainbow, as do normal people, but not necessarily the same ones. That is based on the notion that 7±2 is the number of categories we can...
  33. Merlin3189

    B Why does light diffract into only seven colours?

    And I think George Miller might suggest that we generally perceive seven gradations of anything.
  34. Merlin3189

    I DDWFTTW: Looking for the least confusing explanation

    [Moderator Note -- sub-thread merged into this main thread] Having seen some discussion of FTTWDDW in the past on PF, I thought people might be interested in Veritasium's latest videos: Original (land car) experiment Enhanced "proof" to settle a $10k bet with physics prof No doubt it's a bit...
  35. Merlin3189

    The first and second derivatives at various points on a drawn graph

    @Orodruin Thanks very much. Now I can also post in What I learned Today.
  36. Merlin3189

    The first and second derivatives at various points on a drawn graph

    Apologies. I hadn't remembered this was a HW Q. I've deleted it for now. I see some people can hide things with "Spoiler Alert" but I don't know how to do that.
  37. Merlin3189

    The first and second derivatives at various points on a drawn graph

    I'm no mathematician, but we used to do curve sketching. You just mark some obvious points then join them up in approx the right sort of way. Here green is 1st deriv, then blue is 2nd deriv obtained as 1st deriv of green sketch. This is a quick first attempt in Paint, so not very accurate, but...
  38. Merlin3189

    Electrical Junction Boxes

    Yes. In my experience that is the most common way of doing mains power junction boxes in the UK. Connections to switches and sockets use similar connectors. I've never seen "wire screws" in power wiring, only, occasionally, inside equipment. Also in UK we have the terminal strips, commonly...
  39. Merlin3189

    Conservation of Linear Momentum of Rigid Body

    TSny deals with the first method and the conservation of momentum issue of the second. Another problem with the second approach is, I think, that you take ##v_{CG}## as the velocity of all the masses immediately prior to impact. On impact, C stops and A & B do not increase their vertical...
  40. Merlin3189

    Stages Two and Three: Voltage Derived

    I'm not all that clear on what you are asking and I'm no expert, but since you have no replies, I'll have a stab. In your first diagram, I can see voltage series feedback around the first transistor, but no two stage FB. Similarly in your second I can see single stage voltage series FB by R3...
  41. Merlin3189

    Measuring ripple reduction in LTSpice

    Apologies Baluncore, yes it was I who was confused. If the base current is supplied by the capacitor and the collector voltage falls below the base voltage, indeed the current in the resistor must reverse.
  42. Merlin3189

    Measuring ripple reduction in LTSpice

    Assuming you're interested from a theoretical PoV rather than practical (when you'd better follow other people's advice), my observations: The transistor does not look to be in saturation, because the Vce from the data printed at the side is 2.3 V or 3.3 V, both much more than I'd expect in...
  43. Merlin3189

    Waves - General question about Damping and how it affects a propagating wave

    It depends on whether you are talking about the wave, or the oscillator which generates the wave. An undamped wave ##A sin(\omega t + \phi )## becomes a damped wave when multiplied by an exponential decay term to give ## \exp ^{-kt} A sin(\omega t + \phi )## Then the frequency and...
  44. Merlin3189

    Why do we not need to convert from degrees Celsius to Kelvin?

    You seem to understand that for heat capacity, it is the change of temperature that is significant. Since degrees Celsius and Kelvin are the same size, it doesn't matter which you use to measure a change in temperature. A change, on any scale, is determined by taking one measurement from...
  45. Merlin3189

    I What is the astronomy reason for no UHF channel 37?

    I read that too and also found a textbook on RA to look for any mention of specific signals in that range. I assumed, as the OP, that there might be some reason for that specific frequency. But as far as I can see, there was nothing that couldn't be equally served +/- 100 MHz or +more. If...
  46. Merlin3189

    Real life example of the composition of two SHMs with same angular frequency

    If the two SHMs have the same frequency, then you just get a constant amplitude at the same frequency with a different phase. And it's not very interesting. You can't tell it's not just a single SHM. If you want different frequencies, whether harmonically related or not, you could have two...
  47. Merlin3189

    Left Hand Rule Applied to a Winding

    This is an interesting sort of motor. I only came across it quite recently, when looking for a low inertia motor. It is very definitely a "wire in magnetic field" because there is no rotating iron/magnet. The field is generated by the fixed core magnet, which is inside the rotor, but doesn't...
  48. Merlin3189

    Left Hand Rule Applied to a Winding

    ? Well, AFAIK monopoles don't exist, but if you had monopoles at each end which were always opposite polarity, that would be a dipole, which does exist ! Putting such niceties aside, you seem to be describing our little commutated DC motor, except that you want the rotor magnet to reverse...
  49. Merlin3189

    Left Hand Rule Applied to a Winding

    It should be something like your graph, a full wave rectified sinewave. I'd reckon a bit peakier, with the sine shape being distorted by non-uniform field and the nodes being distorted by a dead spot as the brushes are either in a gap or are shorting both sectors. " It's the only hand crank...
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