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. dexterdev

    To solve ISRO paper, Can I post my doubt questions here in this thread

    I have a multiple choice question paper from ISRO 2012. http://www.isro.org/pdf/EC-2012.pdf Can I post doubtful questions in this thread. Is there someone to help me. -Devanand T
  2. R

    Lagrangian mechanics - derivation doubt.

    In the attached snip, the last few steps of the lagrangian equation is shown. I don't understand how the \frac{\delta V}{\delta\dot{q_j}}= 0. As an example let me take gravitational force. With change in velocity ( along the downwards direction obviously), there sure is a change in gravitational...
  3. dexterdev

    Doubt regarding Transit time in electro magnetics

    Hello PF, I was watching a video from NPTEL (transmission lines), where the professor was explaining transit time. Ok now my doubt is that is the transit time delay in transmission lines due to distributed capacitors and inductors alone. Do resistors contribute to delay. For a...
  4. Government$

    Is It Normal to Feel Self-Doubt in Math?

    Hello friends. It seems to me that i have reach a wall with my math education. In 3.5 months i will be doing tests for mathematics faculty(college), something like SAT but a bit more difficult i think. Anyway i have been doing more difficult problems from problem set and i simply can't figure...
  5. dexterdev

    A signal sampling related doubt

    Hi PF, I have a signal which is sum of 2 sinusoids having frequencies 6KHz and 12KHz. Now if I sample it at a rate of 16KHz and pass it through a ideal Low pass filter having 16KHz cutoff frequency, what is the signal I obtain. What frequency contents does it have. Please help me...
  6. F

    Is the 2-Norm Always Less Than or Equal to the 1-Norm?

    Homework Statement Show that 2-norm is less equal to 1-norm But I've found this proof http://img825.imageshack.us/img825/5451/capturaklt.jpg Which basically shows that if p=1 and q=2 then 1-norm is less equal than 2-norm, i.e. the opposite hypothesis Homework Equations NoneThe Attempt at...
  7. V

    Dumb integral question, can be answered (or hinted) in 2 seconds no doubt.

    Homework Statement Ok, so I am doing areas between curves, but one of the functions is y=50/(25+x^2) This is stupid, but I THINK I am having trouble finding the anti-derivative (I just know my final answer is wrong, but I'm pretty sure it is related to this.) The Attempt at a...
  8. F

    Doubt in hidden variable concepts

    Hi, I am trying to read the chapter 2 of the book "Speakable and Unspeakable in Quantum Mechanics" from J. Bell. I have got some doubts and I would be glad if someone could help me. I see that λ is a vector that makes the role of the hidden variable. All the tricks about the angles between...
  9. H

    Depression, doubt in my mathematical abilities

    Hello all, I am currently an undergraduate student (just finished my first semester) at Stony Brook University. Basically, I'm fairly certain that I want to go into pure math or theoretical physics, it's just that I feel like I may not be up to par with everyone else who will be competing...
  10. F

    Existence of the derivative: a quick doubt

    Homework Statement Determine for which real values of a,b,c,d this function is differentiable ##\forall x \in \mathbb{R}##: ##f(x):=## ##ax+b ## ## for x\leq1## ##ax^2+c ## ## for 1\leq x \leq2## ##\frac{dx^2 +1}{x} ## ##for x>2.##The Attempt at a...
  11. F

    Doubt about two limits (short)

    Homework Statement Find the limit of ##1): \displaystyle \lim_{n \to +\infty}(\frac{f(a+\frac{1}{n})}{f(a)})^{\frac{1}{n}}## ##2) \displaystyle \lim_{x \to a} (\frac{f(x)}{f(a)})^{\frac{1}{ln(x)-ln(a)}}(=1^{\infty})## I am not quite sure if i can solve it the way I did, it has been to easy...
  12. N

    Doubt regarding derivation of bound charges in dielectric

    In Griffiths, for deriving the bound charges for a given polarization P , the formula used is the general formula for dipoles .i.e ( equation 4.9) {Here the potential at r is calculated due to the dipole at r' ) V(r) = ∫\frac{x.P(r')}{X^2}d\tau' Here X = r - r' , and x = unit vector in...
  13. V

    Gravitational Potential & Field: Doubts & Solutions

    Can anyone give me a situation when the gravitational potential due is 0 but gravitational field is non-zero.
  14. V

    Doubt in the calculation of the age of the universe?

    Hi guys! I have a small doubt in the concept of age of our universe.Everyone say that our universe is 14.4 billion years old.Now I can't understand with what frame as reference this calculation is made. Is there any global reference frame with which these calculations are made?
  15. jaumzaum

    Where does interference occur in light waves?

    I'm really confused about the way the light can suffer interference. I'll try to explain the way I think all this occurs and the question I have if this explanation was correct. I would like you guys to correct me if anything I say is wrong and to try to explain me the final question. Light is...
  16. P

    How to Calculate Thermal Expansion in a Rod of Length 'l'?

    Consider a rod of length 'l'. When heated, its temperature got increased by 't'. Increase in its length = l\alphat. total length be (l1) = l + l\alphat. Again the rod is heated and the temperature further got increased by 't'. Total length be l2. l2 can be calculated in two ways, l2 = l1 +...
  17. I

    Doubt in plotting Vector Fields

    Hi, I have a doubt in plotting the vector field. In the post https://www.physicsforums.com/showthread.php?t=155579 it is mentioned that a vector field could be plotted for F (x,y) by, marking the (x,y) as the tail and F(x,y) as the head portion. If so, then consider the function...
  18. S

    [Doubt] Chemical Potential of an Ideal Gas

    Homework Statement Basically, find the chemical potential of an ideal gas knowing its heat capacities.Homework Equations P V = n R T \ \ \ \ (1) U = n c_V T + U_0 \ \ \ \ (2) S = S_0 + n c_V ln (T) + nR ln (V) = S_0 + n c_V ln (T) + nR ln \left ( \frac{nRT}{P} \right ) \ \ \ \ (3) \mu = \left...
  19. D

    Can Energy Conversion Ever Reach 100% Efficiency?

    Hi there people, I have a doubt, if you convert all energy has an expense which makes it is always converted less than 100 percent for example kinetic energy turning electrical energy turning 90 percent of electrical energy, there was a loss of the energy that I'd like to know and if for example...
  20. V

    Basic doubt about the gradient in spherical polar cordinates.

    Let's say we have a scalar function U in terms of r,theta and phi. why cannot this be the gradient at any point P(r,theta,phi)- partial of U wrt. r in the direction of r+partial of U wrt. theta in direction of (theta)+partial of U wrt. phi in the direction of (phi)?
  21. Einj

    Doubt on Ademollo-Gatto theorem proof

    I'm studying the semileptonic decay of the kaon and I'm currently reading about the Ademollo-Gatto theorem. The question is probably silly. However, here it is. The proof starts by just considering that [V_-,V_+]=V_3, where V is the V-spin. Now, we consider the matrix element \langle...
  22. R

    What is the Set Intersection of Successive Midpoint Triangles in R^2?

    Hi some one please help me with the following problem Suppose that T_0 is the interior of a triangle in R^2 with vertices A,B,C. If T_1 is the interior of the trianlge whose vertices are midpoints of the sides of T_0, T_2 the intrior of the triangle whose vertices are midpoints of sides of...
  23. P

    Why Is Iron a Better Conductor of Heat Than Wood?

    We know, heat transfer in a material happens due to vibration or vigorous to and fro movement of particles. Also, the density of wood is much more less than the density of iron i.e, distance between particles is more. Thus, particles in wood has more space to vibrate vigorously than the...
  24. S

    Wave Function Doubt and Derivation

    Homework Statement I was reading up on the Wave Function used in the Schrodinger Wave Equation. However one source said that ψ(x,t)=e^(-i/hbar*(px-Et)) Another source had this ψ(x,t)=e^(i/hbar*(px-Et)) Which one of these is true and could someone give a derivation for the correct...
  25. iVenky

    Doubt on P.I of [itex]e^{ax} V [/itex]

    I have a doubt in finding out the Particular Integral of eaxV, where 'V' is a function of x. I saw the book but they seem to be somewhat unclear regarding this- Here's the derivation from the book: If u is a function of x, then D(eaxu)= eax Du + aeaxu= eax (D+a)u Similarly you can do for...
  26. dexterdev

    A doubt on stastical indeependence , orthogonality and uncorrelatedness ?

    A doubt on statistical independence , orthogonality and uncorrelatedness ? Hi friends, I wanted to make my concepts on statistical independence, uncorrelatedness and orthogonality clear. Suppose I have 2 random variables x and y. I have 2 pictures on the above concepts, is it...
  27. S

    Taylor Series and Maclaurin Series Doubt

    Homework Statement If I take a function f(x) and its taylor series, then will the infinite series give me the value of the function at any x value or will it only give proper values for x≈a? For example, If I take a maclaurin series for a function will it give me proper values for all x...
  28. P

    Doubt regarding current flow during lightening.

    You know current will flow only when the path is complete and closed. During lightning, that doesn't happen. Lightning dies after some time? Its not a complete or closed path. How can you tell that charges are flowing or current is flowing?
  29. A

    Doubt reagarding denseness of a set in (0,1)

    I have been doing a basic math course on Real analysis...I encountered with a problem which follows as" Prove that na(mod1) is dense in (0,1)..where a is an Irrational number , n>=1... I tried to prove it using only basic principles...first of all i proved that above defined sequence is...
  30. P

    Doubt regarding direction of current during lightning

    In which direction do current flow during lightning? If it is from cloud to earth, which charge gets accumulated in below part of the cloud?
  31. A

    A Doubt On Working on Heart And Brain System

    Hello! The other day, i was referring to some comic book and it said something about telepathy which made me wonder if the heart and brain really emit signals thaat are powerful enough to be able to be caught or processed? if yes, how... Elaborate help shall be acknowledged... Thanks
  32. S

    Simple Integration Doubt regarding integral of dy

    Simple Integration Doubt regarding integrals Homework Statement What is the result of ∫dx? is it x or x+C I thought about this two ways: 1) Through indefinite integration, it gives x+C 2) If I take a geometric interpretation, this integral gives me the area under the [f(x) and x] graph...
  33. P

    Doubt in myopia and Hypermetropia

    Will a person suffering from nearsightedness, couldn't see objects far since his eyeball is longer than the usual? (or) Because when he reads more books daily and always focus his vision on objects nearer, lens in his eye ball becomes stub burn and doesn't adjust its focal length when he watches...
  34. E

    Doubt on snubber circuit for an scr for dv/dt protection

    The main aim of turning on/off an scr is to disconnect source from load. I read in dv/dt protection of scr using snubber circuit, the scr may turn on when dV/dt is very high as the charging current i=cdV/dt will be high enough to turn on scr. So we are using a snubber circuit to divert this high...
  35. Ryuzaki

    Can Ʃ (anbn) be divergent if Ʃan and Ʃbn are both convergent?

    Homework Statement Show by example that if Ʃan and Ʃbn are two convergent series, then Ʃ (anbn) may be a divergent series. Homework Equations N/A The Attempt at a Solution I've been trying long and hard to find an example for this particular case, but I'm unable to find one. I've...
  36. P

    Solar Constant: Earth's Receiving Intensity

    Hi, So, I understand the concept that the power radiated by the sun spreads out as a sphere with a radius equal to 1 AU. Therefore, in order to find the intensity, we have to divide the power radiated by the surface area of the sphere. I thought the result of this calculation would yield the...
  37. jaumzaum

    Solving Thermodynamics Doubt - João

    I was solving the question below : "A cylinder have a piston whose mass is insignificant and moves without friction. One mole of an ideal gas is confined into the cylinder. If we expand isotermally the gass at temperature 298K, against a constant external pressure of 0.1 atm, the gas...
  38. jaumzaum

    Solving Solubility Doubt: Sodium Benzoate vs. Toluene

    I was solving an exercise where I had to judge (true or false) 5 statements. One of them said: "Sodium benzoate is more soluble in water than in toluene" The book answer is true, but I don't know how to compare it. I mean, sodium benzoate is ionic, and water is very polar, so the...
  39. dexterdev

    Solving Audio Sources Doubt - Devanand T

    Audio sources doubt... hi all, I don't know where to post this. sorry for that. My doubt is that if there are 2 audio sources like one mp3 music file playing from a cd player and another speech signal coming from a radio. If I can hear both signals, what is the resultant audio...
  40. jaumzaum

    Why aren't the distances of the images the same in this problem?

    I was solving the below question "An object is 30 cm above a container filled up with water. The lower end of the container is coated silver and acts like an ideal spherical concave mirror of radius 60cm. Find the distance of the image to the surface of water."...
  41. S

    Integration related to physics doubt

    Homework Statement I am a high school student and I am very fond of calculus. I know we get the distance by calculating the area under the v-t graph. But this is my take on another way to get an equation The original differential equation is dx/dt=v(t) multiplying both sides by dt, dx=v(t)dt...
  42. V

    I have a basic doubt in the theory of stress and elasticity. Please help me

    A Book says, " When a load is acting on a member, it undergoes deformation. An internal resistance is developed against deformation by the member and the intensity of this internal resistance is called stress." I also read, "When a member is in its elastic range, it offers resistance against...
  43. Ryuzaki

    Bounded Sequence: Thomas-Finney Definition Explained

    Thomas-Finney defines a bounded sequence as follows: - A sequence an is said to be bounded if there exists a real number M such that |an| ≤ M for all n belonging to natural numbers. This is equivalent to saying -M ≤ an ≤ M So, if all terms of a sequence lies between, say -1 and 1, i.e...
  44. atomqwerty

    Doubt with Ampere and Biot-Savart

    Hello, I think I'm terribly wrong by supossing Ampère-Maxwell and Biot-Savart are referred to the same concept of magnetic field B. For example, for calculating B near an infinite line, I used both, as I understood them, obtaining different expressions (see image). What is that that I don't...
  45. Ryuzaki

    Doubt regarding the sequence (-1)^n

    Hello everyone! I was wondering whether the sequence (-1)^n is converging or diverging. According to Wikipedia, a sequence that doesn't have a limit (i.e, which doesn't converge), is automatically divergent. (-1)^n doesn't have a limit, yet all its values oscillate between 1 and -1, and...
  46. R

    Solving Center of Mass Doubt: What is 'y' in Integration?

    A silly doubt regarding center of mass... As we know for bodies having continuous distribution of mass we can know their center of mass by the method of integration... like, Xcm = 1/M∫x.dm but what is x here? in many cases... like in finding the COM of a ring Xcm = 0 and Ycm = 2r/∏...
  47. A

    Is it Possible for a Closed Disk to be Contained in an Open Set?

    Sir , we say that in an open set U for every point (x0,y0) there exists some r>0 such that B((x0,y0),r) lies in U.. where B stands for open disk around (x0,y0) with radiuus r... My doubt is does there exist some "p" such that closed disk around (x0,y0) with radius "p" lies in the open set...
  48. A

    Is Real Numbers an Ordered Field Under <?

    Sir,i have read in wikipedia that for a relation to be ordered it should be transitive,antisymmetric,total...however we know that Real numbers is an ordered field under relation "<" but antisymmetric property is not valid with "<" relation..how is this justified..rectify me...
  49. I

    I doubt you guys are interested in highschool geometry

    http://memorial.jacksonsd.org/www/jacksonsd_memorial/site/hosting/Summer_Math_11/GeometryHonorsSummerAssignment.pdf But it's the summer and I haven't done this in a couple of months. I need pages 10-16 done, I have already finished them but I know most of it isn't correct. I also know math...
  50. M

    Doubt in resolving forces of a man in an elevator(normal force and weight)

    (Before I start, due to the use of vectors let me state I consider up to be positive and down to be negative) Imagine a scenario, of a man in an elevator. And the elevator is accelerating upwards. Now in this case the Normal force > weight. So Normal Force - Weight =mass X acceleration. If...
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