Doubt Definition and 591 Threads

Doubt is a mental state in which the mind remains suspended between two or more contradictory propositions, unable to be certain of any of them. Doubt on an emotional level is indecision between belief and disbelief. It may involve uncertainty, distrust or lack of conviction on certain facts, actions, motives, or decisions. Doubt can result in delaying or rejecting relevant action out of concern for mistakes or missed opportunities.

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  1. A

    What happens when objects travel at the speed of light?

    Hello , I have this question in my mind from long time. There are two travelers in two trains and one observer on ground(Stationary). One train is traveling at 99.99% speed of light and another train is traveling at speed of light . My question is that the person who is at light speed train...
  2. S

    Differential Geometry, easy question, weird hint making me doubt myself

    Homework Statement Homework Equations From my notes: (\psi_{*}v)_{k}(x)=\sum_{i=1}^{n}v_{i}(x)\frac{{{{{\partial}}}}{\psi_{k}(x)}}{{x_{i}}} The Attempt at a Solution Okay so i) is fine (ignoring the typo in the question) but I'm a bit confused about ii) I don't see any need...
  3. S

    Why Does Id Increase with Constant Vgs Despite Body Effect?

    I found on my notes that for an NMOS with increase in Vsb voltage Body Effect increases, Vt also increases. But it is mentioned that with Vgs constant id increases which I feel is wrong since id is proportional to (Vgs - Vt)2. Hence I think id should decrease. Please show me the light :-)...
  4. P

    Doubt can actually be essential to our living

    Doubt, as odd as this may sound, can actually be essential to our living. We all make decisions and later have questions on whether we made the right choice or not so doubt will help influence our next decision when it comes to the same question or choice. While doubt is a natural part of the...
  5. R

    Understanding Microphone Sensor Sensitivity: Explained by Experts

    Hi, I have a microphone sensor and when i am reading that datasheet, they have written it in as SENSITIVITY -12mV/Pa. I am not sure, what does it mean. Can someone clear me?
  6. L

    Canal rays experimental set up - a doubt in this

    Canal rays experimental set up -- a doubt in this Homework Statement Please go through my attachment. It has been mentioned that luminous stream of particles were observed behind the cathode. Homework Equations Behind the cathode means , Whether right of the cathode or left of the...
  7. Z

    Constant acceleration kinematics doubt

    Hi, I hope it is a very simple mistake and easy to spot. Suppose you start at rest so initial velocity is 0 and you accelerate constantly over a certain distance. An acceleration vs. distance graph (not time) would be like a rectangle. The area under the graph would be A=as=(s/t2)*s = v2...
  8. N

    To write x as a function of time doubt.

    Hello! Homework Statement It is more a conceptual problem. If all the variables are known, and I am asked to write x as a function of time, why the equations below are not equivalent when equation (b) is derived from (a) after substituting "v_f = v_i + at" ? (a) x_f = x_i + 1/2(v_f + v_...
  9. R

    Understanding FFT Power Spectrum, Phase and Magnitude: Clearing Doubts

    Hi, I have some silly doubts and i read some articles about FFT but could not able to conclude my self. What are the difference between 1) FFT power sprectrum and Power sprectrum density 2) FFT Phase and magnitude 3) FFT Real and imaginary Can some make it clear to me.
  10. H

    Doubt about Relativity: Is it Possible?

    Doubt about relativity! I am a noob! Forgive me if it sounds too stupid! Let me say, I am in CERN right now at the LHC. I fire a proton at 0.9c in one direction. While this happens, I also fire another proton at 0.5c in the opposite direction. Both the measurements were done considering...
  11. K

    Why do we need an m between -m2 and m1 to prove Theorem 1.20?

    https://www.physicsforums.com/showthread.php?t=253563 There is this post on this theorem. But the issue I wanted to discuss isn't discussed there fully(so far as I could follow the thread). So I am creating this thread for that issue, so as to avoid necro-post in that thread. My doubt is...
  12. R

    Why is my sensor output only 1v when using a 2.5v reference voltage?

    I have sensor that produces differential output. I saw in the sensor schematic that they have used 2.5v DC as reference in the output minus terminal. I am taking sensor output as differential that both positive and negative(2.5DC ref terminal)in count. I got output as 1v of DC when the sensor is...
  13. R

    Understand Rail-to-Rail Output Swing & Multi-Channel Data Acquisition

    I am just reading an op-amp datasheet. In that, they have written in features as "RAIL-TO-RAIL OUTPUT SWING" and in Applications as "MULTI-CHANNEL DATA ACQUISITION". What does they mean? Can someone explain me in simple way.
  14. S

    Doubt in Ammeter: Shunt Resistor or Inductance?

    The other day I happened to open an ammeter just out of curiosity. What I saw was that across the positive and negative terminals a wire of considerable thickness(almost equal to the thickness of wires we use for household joints) was wound..It just had 4 or 5 turns. I guess it should be the...
  15. P

    Calculate Lowest Frequency for X&Y: Constructive/Destructive Interference at Q

    Hi, Question: X and Y are two identical sources of sound which emit in phase. Calculate the lowest possible value of frequency of the sources for there to be (a) constructive (b) destructive interference at Q. So, X is at 1.8 m from Q and Y is 1.2 m away from Q. a) If they leave in...
  16. N

    How can I find two points where a function has the same tangent line?

    Hello people! Well, I am doing some excercises for fun. Picked some Precalculus stuff, and found this excercise: "Construct a function that has the same slope at x = 1 and x = 2. Then find two points where y = x^4 - 2x^2 has the same tangent line (draw the graph)." I have found a solution, but...
  17. R

    Infinite Series Doubt: Last Step Explained

    On the provided attachment, I have problem understanding the last step of the 1st sum of that page. Can anyone explain to me what is being done after the second-last step?
  18. jaumzaum

    Understanding Centripetal Force: The Confusion Behind Car Skidding Explained

    Hi everyone, Maybe it could be an idiot question but I've never know why. If centripetal force points to the center, Why a car making a turn tend to skid to away of the road? I mean, there would have to be a force opposite to centripetal, that is in fact, greater than it, for the...
  19. B

    A basic doubt (about inertial frames)

    Hi This question is taken from the book Classical mechanics by Gregory. Please clear this doubt TIA ------------- Suppose that a reference frame fixed to the Earth is exactly inertial. Which of the following are then inertial frames ? A frame fixed to a motor car which is (i) moving...
  20. D

    Help Solve Dennis's Doubt in Circuits: Battery Charger Circuit

    Hello, my name is Dennis; and I am rookie engineer. I have a doubt in the above circuit and I was wondering if any of you could help me. I have attached a schematic for a power/ battery charger circuit. Right now My concern is on the battery charger circuit marked with a red box. According to...
  21. M

    A doubt about Indian EE aspirant physicists

    No, not doubt as in questioning the capabilities or fact of the matter (as connoted in the western usage), but doubt as in general uncertainty / question (as I see from many Indian / Pakistani users, and grad students around my department!) Maybe this is just confirmation bias, but I notice a...
  22. H

    Doubt on Depletion Layer Width in Biased pn-Jnction

    " Depletion Layer is the region in diode which consists of ions( formed due to formation of octet configuration )of p-type and n-type dopants" What happens to the ions present in the depletion layer when a diode is biased? why the depletion layer width changes??
  23. D

    Are there exceptions to adding h parameters in feedback problems of amplifiers?

    Homework Statement In feedback problems of amplifiers we often use H feedback in the voltage controlled voltage sources... The good thing about that is that they are connected in series parallel form and they are then to find the net h parameters we simply add the h parameters of the...
  24. B

    Doubt on this problem from David Morin's book - Intro to Classical Mech

    Will someone please explain the foll. doubts in this solved problem from David Morin's book. TIA sree ------------ DOUBTS : 1. What is the principle of eqn. 3.78 ? If V is block speed and Vy is its component in y dirn., why should V + Vy be constant ? 2. In Why should...
  25. M

    Big doubt about movement and friction

    I have a doubt that I cannot resolve: With a 10kg stop object on a surface I apply a 100 N force to move this object. The static friction coefficient is 0.5 and kinetic coefficient is 0.3. I would like to know the maximum speed of this object and the distanced covered till stop. I know the...
  26. Z

    Is arg\xi(1/2+is) an Increasing Function of 's'?

    given the function arg\xi(1/2+is) is this an increasing function of 's' ?? , i mean if its derivative is always bigger than 0 here xi is the Riemann Xi function http://en.wikipedia.org/wiki/Riemann_Xi_function could we define the 'inverse' (at least for positive s) of...
  27. S

    What is the role of rotation in mechanics?

    well if i understand relative motion quite well but while thinking about it i had this doubt- if A is in center of circle and if B is rotating in that circle, A would see B rotating rite? now , how would B see A?... there are more questions to follow up based on the answer...
  28. R

    Friends,A doubt in piezoelectricity. I heard that when pressure is

    Friends, A doubt in piezoelectricity. I heard that when pressure is applied in a quartz crystals(or other similar items), a high voltage is produced and this is called piezoelectricity. My question is, is it necessary a pressure cycle should go on? i.e. first a high pressure, then low, then...
  29. B

    Doubt on this worked example from book by David Morin

    Attached is a worked problem in the Classical Mech book by Morin. I don't understand how eqn (2.57) is the net moment about the pivot. Isn't it only the moment of the weight of the portion of the stick to the right of the pivot ? What about the moment due to the weight of length 'l' on left...
  30. TrickyDicky

    Doubt about gravitational waves

    The usual derivation of the wave form equations from the GR field equations is done in the weak field, linearized approximation of the GR theory. In this limit, that ignores non-linear contributions and that gives accurate results when used to predict solutions for problems in the Newtonian...
  31. R

    Solving Programming Doubts: Logic, Shifts & Situations

    I have been working on programming and trying to learn fast all the things. I have some doubts about following logics that when to use and where. Please explain me in short and undestandble way. 1) when to use exactly AND, OR and reamining logic functions. 2) when to use arthimatic and...
  32. S

    Inductor Doubt: Instantaneous EMF=Back EMF?

    Doubt on Inductor!pleasez help Imagine an AC source and a pure inductor...if the Instantaneous EMF=Back EMF due to changing Magnetic field in Inductor then how does the current flow?
  33. P

    Another doubt in Peskin Schroeder Sec 4.2

    This doubt is about a text in Peskin Schroeder Pg 86. I reproduce it here. -------------------------------- U(t,t') satisfies the same differential equation (4.18), i \frac{\partial}{\partial t} U(t,t') = H_I(t) U(t,t') but now with the initial condition U=1 for t=t'. From this...
  34. J

    Latest results from the LHC are casting doubt on the theory of supersymmetry

    Latest results from the LHC are casting doubt on the theory of supersymmetry http://www.scientificamerican.com/article.cfm?id=beautiful-theory-collides "I'm wouldn't say I'm concerned," says John Ellis, a theorist at CERN, Europe's particle-physics lab near Geneva, who has worked on...
  35. Telemachus

    How to Derive the Simplified Form of Modified Euler Equations?

    Homework Statement Hi there. I'm not sure if this question corresponds to this subforum, but I think you must be more familiarized with it. The thing is I don't know how to get from: M_x=(I_0-I)\dot\Psi^2\sin\theta\cos\theta+I_0\dot\Phi\dot\Psi\sin\theta to...
  36. J

    Doubt regarding uncertainty principle

    I had a thought going in my head today about the uncertainty principle. Forgive me if it sounds too silly. Consider an electron in motion. Now, suppose that I'm measuring it's position with infinite accuracy. So, by the uncertainty principle, it's velocity is blurry. But what if I measure the...
  37. R

    Why Did I Feel a Shock from a High Voltage, Low Current Transformer?

    A small doubt in transformer... Hi friends A transformer (say step up one) increases the output voltage. However, power input is equal to power output so output current should be small. Now yesterday I took a spark circuit(from an electric gas lighter using a 3 volt battery) and allowed the...
  38. K

    A doubt from the book Galactic Dynamics from Binney and Tremaine

    Hi... I was reading through the second edition of the book Galactic Dynamics from Binney and Tremaine. Chapter 3 :eqns 3.8 and 3.9. 3.8 says r^2 \frac{d \phi}{dt}=L=constant. Then 3.9 writes \frac{d}{dt}=\frac{L}{r^2}\frac{d}{d\phi} But is this correct? For example, if I act this on r...
  39. D

    Is There a Hidden Factor in the Definition of [A,A]?

    Hi everyone, Homework Statement I've been studying a paper in which there is a connection given by, A = f(r)\sigma_1 dx+g(r)\sigma_2 dy, where \sigma's are half the Pauli matrices. I need to calculate the field strength, F = dA +[A,A]. Homework Equations A = f(r)\sigma_1...
  40. D

    What is the factor in the definition of [A,A] in differential forms?

    Hi everyone, I've been studying a paper in which there is a connection given by, A = f(r)\sigma_1 dx+g(r)\sigma_2 dy, where \sigma's are half the Pauli matrices. I need to calculate the field strength, F = dA +[A,A]. I have computed it, but a factor is given me problems. I would say, dA =...
  41. C

    Laurent Series doubt about the Sum limits

    Hi everyone! I'm studying Laurent Series, and I thought I understood it but after solving one exercises I get confused. The problem was to get the Laurent series for the following function: \frac{1}{\left(z-1\right)^2\left(z+3\right)} I do it this way: \frac{1}{\left(z-1\right)^2}\cdot...
  42. R

    Doubt regarding Noise in the system

    Hi, I am new to circuit design. Although, I am designing CSA&SA(charge sensitive @ shaping amplifier) for our detector system to detect the radon gas emission. We soldered every thing and we observe the output that is bit noisy. I learned some concepts in my course about...
  43. R

    What Is Planar Technology in Semiconductor Sensors?

    what is planar technology?
  44. R

    Doubt regarding silicon waffers and silicon diodes

    Hi, basically i am confused about the characteristricts of silicon diodes and waffers. As i know the VI @ CV characterstricts of the diode. But i tested the same characterstricts with silicon waffers which behaves perfectly same as like diodes. Mainly i need to know about the difference between...
  45. R

    Doubt regarding silicon waffers and silicon diodes

    Hi, basically i am confused about the characteristricts of silicon diodes and waffers. As i know the VI @ CV characterstricts of the diode. But i tested the same characterstricts with silicon waffers which behaves perfectly same as like diodes. Mainly i need to know about the difference between...
  46. S

    Doubt regarding Planck's Quantum Theory

    Hello We all see this statement everywhere : "Planck's quantum theory states that the radiant energy emitted or absorbed by a body is in the form of discreet packets of energy called Quanta, and the energy of the quantum depends on the frequency by the formula E = hv" Now, my question is...
  47. E

    What Is the Role of Displacement Current in an RC Circuit?

    In an RC circuit shown below, with the switch closed after a long time, when the capacitor is charging, there is a current across the capacitor given by Initial current x e ^ (-t/RC)... Is this current...the current supplied by capacitor to the rest of circuit, or simply the current across...
  48. A

    Theoritical doubt in aeronautical engg

    Homework Statement is the drag on an airplane dependent on its weight? Homework Equations The Attempt at a Solution it was told that the drag depends on lift and the lift,on weight,in turn.but i am unable to comprehend it physically.
  49. A

    Cauchy Riemann Equations (basic doubt)

    Lets say we have a function of a complex variable z , f(z). I read that for the function to be differentiable at a point z0 , the CR equations are a necessary condition but not a sufficient condition. Can someone give me an example where the CR equations hold but the function is not...
  50. R

    Doubt in Fourier's Law of Heat conduction

    Hello friends Fourier law of heat conduction states that: Q ~ A.(Dt/Dx) where A= area normal to direction of heat flow. Dt/Dx= temp gradient in same direction. Now, obviously rate of heat flow will depend upon temp gradient but my doubt is how/why...
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