What is Doubt: Definition and 591 Discussions

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

    Is a vector space a Set?(Beginner's doubt)

    A vector space is a collection of vectors. So can we say that it is a set (although with special properties) ? Just wanted to confirm this. Is this also true for a vector space consisting of all functions from R to R , i.e can we say that it is also a set , where each member of the set is a...
  2. V

    Understanding Acceleration and Friction in a Block Over Block System

    Hello :D The block over block questions today got on my mind And i got some doubts in which i need some guidance :) Let the two blocks as shown http://s1102.photobucket.com/albums/g448/vissh/?action=view&current=LOM.jpg" . I want to find the accelerations of blocks when :- <1>F = 20 N <2>F...
  3. P

    Fundamental doubt on Acceleration.

    Hi Everyone, I have very fundamental question on Acceleration. I can understand that a moving body accelerates or de-accelerate when its velocity goes up and down. But I couldn't understand; when we say a moving body accelrates or de-accelerates when the direction of moving body changes(with...
  4. Telemachus

    Triple integral doubt, spherical coordinates

    Hi there. I have some doubts about this exercise. It asks me to use the appropriated coordinates to find the volume of the region indicated below. The region is determined by the first octant under the sphere x^2+ y^2+ z^2=16 and inside the cylinder x^2 +y^2=4x The first thing I did was to...
  5. Telemachus

    Understanding Curvilinear Integrals: A Scientist's Dilemma

    Homework Statement Hi. I have a doubt with this exercise. I'm not sure about what it asks me to do, when it asks me for the curvilinear integral. The exercise says: Calculate the next curvilinear integral: \displaystyle\int_{C}^{}(x^2-2xy)dx+(y^2-2xy)dy, C the arc of parabola y=x^2 which...
  6. D

    How Does ID3 Algorithm Determine the Root Node in Decision Trees?

    If this is in the wrong place please move it, i was in doubt where to post this My problem is the following I Have to create an induction tree able to calssify the attributes to this table, being y the class and A,B,C the Attributes, this tree shall be constructed from ID3 Algorithm ID3...
  7. A

    Solving Alice's Shell Script Doubt for Comparing & Generating Scores

    Hi all 30 students have sent me their answers to me in folders, "name1", "name2", ..., "name30". Each folder contains 2 outputs, "output_a.txt" and "output_b.txt" Now I have the correct answer "correct_a.txt" and "correct_b.txt" with me. I know that using the command diff, I can compare...
  8. A

    Beginner's doubt regarding vector spaces.

    I have just started a study of linear algebra and I have a doubt regarding vector spaces. Consider the vector space spanned by the 3 dimensional vectors [1,0,0] and [0,1,0] , this would be a 2-dimensional vector space no doubt.But it also is a subspace of \mathbb R ^3.I have no problem in...
  9. C

    Gauss Divergence Theorem - Silly doubt - Almost solved

    Homework Statement The problem statement has been attached with this post. Homework Equations I considered u = ux i + uy j and unit normal n = nx i + ny j. The Attempt at a Solution I used gauss' divergence theorem. Then it came as integral [(dux/dx) d(omega)] + integral...
  10. I

    A doubt about frictional forces

    Hello I have some doubts in the phenomona of frictional forces. I am studying from the Serway's book. He says that direction of frictional force is always opposite to the relative motion between the two surfaces.For example, for a block of wood moving on the ground to the right, the force of...
  11. O

    Proving Inclusion of Vector Subspaces in W: A Scientific Approach

    If U and V are vector subspaces of W and if U union V is also a subspace of W, how would i show that either U or V is contained in the other?
  12. M

    Indirect band gap semiconductor doubt

    indirect band gap semiconductor doubt its said that indirect band gap semiconductors like silicon and germanium are not used as a light sources because they have more non-radiative recombinations rather than radiative ,which inturn decreases efficiency of optical source. but i want to...
  13. P

    Doubt with centripetal acceleration

    Good afternoon, For a body in a curvilinear motion moving with velocity \vec{v}, velocity can be divided into two components (see http://en.wikipedia.org/wiki/Angular_velocity#Two_dimensions" Wikipedia article): a component parallel to the position vector (the radius), which changes the...
  14. B

    Doubt in defining transfer function

    why in a transfer function it is written by considering the initial condition to zero.. because if we individual write the integro-differential equation and apply Laplace transform to them and obtain a output/input relation,it definitely contains the initial condition terms in it also.. so...
  15. R

    Fluids doubt water falling from a tap

    Fluids doubt...water falling from a tap When water falls from a tap its area of cross-section is supposed to decrease progressively in accordance with the equation of continuity A1V1=A2V2 in order to maintain the conservation of mass. But using another line of thought, every particle in a...
  16. C

    What Are the Color Codes Used in Computers for Outputting Videos?

    Hi! I am so fascinating about the computers and digital technology... If we consider a number or a letter they have specific digital codes and compilers convert them into numbers or letters for humans... In computers each and every signal is digitalized,in that case videos are also...
  17. G

    Newton's 1st Law: Proving Equal Forces Acting on a Box

    Hello friends! I'm new to this site and can see its good. Well I have a doubt which I would like to get clarified. According to Newton's 1st law , Any body continues to be in its state of rest unless and until an external force acts on it. Imagine a box kept on the floor. There are 2...
  18. S

    Doubt with power of lens with 2 media

    This is more of a general question. Suppose there is a biconvex lens(Radii are R1 and R2) with Refractive index =n2,it is bounded on 2 sides by media of RI=n1,n3. Now if I have to find its focal length, 1/f=(n2-n1)/R1+(n2-n3)/R1. Is it correct? Suppose it were correct, If there is a...
  19. T

    Doubt about differential Gauss's law

    We know that the Gauss's law expressed in the differential form is: \mathbf{\nabla}\cdot\mathbf{E} = \frac{\rho}{\epsilon_0}, right? I read at wikipedia that \rho is: the total charge density including dipole charges bound in a material. I don't understand... The left side of...
  20. A

    Doubt regarding constants. Elastic Beam Theory.

    Hello. As a good mathematician, I'm having troubles reading some constants for a PDE. I'm modelling an elastic rod using the equation \rho A U_{tt} - N U_{xx} + E I U_{xxxx} = 0, where "\rho is the beam density, A and I are the area and moment of inertia of the beam cross section...
  21. P

    Doubt about the polar equation of a Kepler orbit

    Good morning, I have a doubt about the differentiation of the polar equation of an orbit: r=\frac{p}{1+e\cos\nu} It represents the relative position of a planet with respect to the central body. Here, p is the parameter, e is the eccentricity and r is the radius of the planet measured from...
  22. P

    What is the proper way to integrate a vector?

    Good morning, I was reading a derivation of equations of the two-body problem and I found the following statement: \mu \frac{d}{dt}\left (\frac{\vec{r}}{r}\right ) = \frac{\mu}{r} \vec{v} - \frac{\mu \dot{r} }{r^2} \vec{r} Where μ is a constant. (If you're interested on where this came...
  23. 5

    Doubt regarding removing of electrons from shells

    Cu - 1s2 2s2 2p6 3s2 3p6 4s1 3d10 Cu2+ - 1s2 2s2 2p6 3s2 3p6 4s0 3d7 This is the configuration of the respective atoms and ions.. According to Aufbau's principle.The subshells with lesser energies get filled first ! Which means 4s sub shell is filled and only after that 3d10 sub...
  24. R

    GPS - Doubt regarding RMC speed value in NMEA files

    I am afraid that this is either too specific and\or that this is in the wrong forum (if the latter is the case, if moderators could change this, I'd appreciate :). I'm trying to do an application that uses the GPS data that comes from NMEA files. On a particular sentence (RMC) there is a value...
  25. R

    Gravitational force quick doubt

    Homework Statement A spacecraft S of mass m moves in a straight line towards the centre of the earth. The earth is modeled as a fixed sphere of radius R. When S is at a distance x from the centre of the earth, the force exerted by the Earth on S is directed towards the centre of the Earth...
  26. TrickyDicky

    Doubt about our spacetime manifold

    I understand that accordingt to GR mass curves the spacetime (I'm not referring to spatial curvature k), so that the universe globally considered is a manifold with constant curvature, is this right? If so, is this curvature positive or negative in the current cosmological model?
  27. S

    What career options are available for mechanical engineering students?

    I am having a big problem.. I don't really know what i can do in my future career although i am going to graduate soon.. I am a mechanical student..some of my friends who are not ME student.. When they are asking me about what is mechanical and what can i do in future.. i always unable to...
  28. M

    Doubt in semiconductor diode theory ?

    doubt in semiconductor diode theory ? hello friends :smile:... in this picture diode is forward biased. positive terminal of battery repel holes n negative repel electrons , finaly electrons n holes recombine n eliminate each other. but its said that current flow is due to electrons ...
  29. M

    Srednicki's Lehmann-Kallen propagator derivation doubt

    Homework Statement Problem with the ordering of integrals in the derivation of the Lehmann-Kaller form of the exact propagator in Srednicki's book. We start with the definition of the exact propagator in terms of the 2-point correlation function and introduce the complete set of momentum...
  30. R

    Fluid velocity in free jets doubt

    Hi friends This isn't a homework poblem. However,I got this confusion when i was solving some problems. Consider fluid flowing out of a tank as shown in figure attached. Fluid moves out of pipe through a nozzle (generally used to accelerate the fluid). Now if we apply bernoulli's equation...
  31. T

    Electric field and electric potential doubt

    Hi =D I know that the electric potential in any given point (considering only a point charge) in a region is defined to be: \varphi = \frac{q}{4\pi \varepsilon_{0} d} I know too that the electric potential generally can be defined as: \varphi = - \int_{P1}^{P2} \overrightarrow{E}...
  32. R

    What happens to precession when torque is removed?

    Hi friends I read about gyros and read their working and analysis based conservation of angular momentum. I understood it as I was made to by the analysis but i didn't find the real reason why gyro shows such a behaviour. I mean what forces it to do the magic it does? Is there any analysis...
  33. A

    Spur Gears: Is Tangential Pitch Circle Necessary?

    I have this doubt on spur gears Is it necessary that the pitch circles of the two mating gears need to be tangential? or will a small gap of 0.3mm matter?
  34. R

    Flow Separation Doubt: Hi PFians

    Hi PFians I saw a fluid dynamics video (classic one) that when a body moves within a fluid then flow separation occurs at or near the rear of a body (like sphere or foil) but when fluid flows over the body i.e body at rest , then flow separation doesn't occurs! Why?
  35. P

    Is the Proof of E=mc² Valid for Accelerating Objects in Special Relativity?

    Good night, I've read the following proof that E=mc², through work and relativistic mass. The expression for work is: W=\int Fds W=\int \frac{d(mv)}{dt}vdt=\int d(mv)v=\int (vdm+mdv)v=\int v^2 dm+mvdv The expression for relativistic mass is: m=\frac{m_0}{\sqrt{1-\frac{v^2}{c^2}}}, where m is...
  36. fluidistic

    Can the amplitude of an EM wave affect its physical size?

    In the case of plane waves, E is orthogonal to B and they're both orthogonal to the direction of propagation, call it k. I'm not sure I'm picturing well what such an EM wave is. For instance I know that E and B oscillates with respect to time. Without looking to quantum electrodynamics that...
  37. R

    Why do mountain bikers choose smaller front sprockets when climbing hills?

    Hi friends I have a little doubt on the gear ratios of a bike like a mountain bike which have front sprockets also. It's clear that we get maximum RPM and min torque out of a bike when the chain is on smallest sprocket at the rear and largest at the front(highest gear ratio). But i read...
  38. N

    Doubt about laser power in laser transmitter

    I'm making a laser torch based audio transmitter n receiver using 2n5777 phototransistor as receiver what must be the min power of the torch
  39. P

    Conceptual doubt about elastic potential energy of a system

    Good afternoon, First of all, this is not a homework question, but I'm not sure whether it should be posted on the Homework section. My problem arose when I tried to think about what happens when a spring does work in two blocks. First, consider two blocks (1 and 2) on a frictionless...
  40. N

    How Does Convolution Work for Analog Signals?

    Dear Experts, For convolution to work in any input signal we should be able to represent the input signal in terms of appropriately scaled and shifted unit impulses. This one holds good for discrete time system in which the input signal can be represented as sum of scaled shifted unit impulses...
  41. S

    Doubt regarding schmitt triggers

    I have a very basic doubt in schmmit triggers...suppose if I connect the required power supplies to the op-amp (ie. V+ and V-) and make the desired connections(see attached circuit). Now, what would be the output when Vin=0, ie. before the signal is applied.
  42. T

    Patentability Doubt: Advice from Forummers

    Dear Forummers, I have a doubt regarding patents. Hope you can provide me some advice. 1. If product A is patented , can product B also be patented if it does the same thing ? For discussion purpose, let's use an example. Lets say a product, A, is patented. Its purpose or function...
  43. K

    Doubt from Georgi(Lie Algebras)

    In Georgi(Lie Algebras-Second Edition), theorem 1.5 states: " The matrix elements of the unitary, irreducible representations of G are a complete orthonormal set for the vector space of the regular representation, or alternatively, for functions of g belonging to G." Now, just before...
  44. V

    Doubt: Is 3 Stroke Engine Possible?

    Is 3 stroke engine possible??if not why?
  45. W

    Wittgenstein & Doubt: Can You Know What You Cannot Doubt?

    Wittgenstein often made the statement that if you cannot doubt that a thing is so, you cannot know it. For example ( the example he gives) , you cannot know that you are in pain because you cannot doubt it. You cannot say," This pain hurts but I doubt that I feel it." I can understand how...
  46. C

    Doubt reagarding magnetic fields

    Is the statement below always true: Work done by a magnetic field is zero. If it is. How do magnets attract iron?
  47. L

    Understanding 110/220V Transformers and Designing Linear 24V 3A DC Power Supply

    I can't figure out how those 110/220v transformer work. Like the ones in cell phone chargers. Say for example, if the primary voltage is 110v, the secondary will be 24v. But if the primary is connected to 220v, the secondary should be 48v. Are the stages after the secondary designed for up to...
  48. U

    Pneumatic Circuit Doubt: Understanding Force and Pressure in Pneumatic Systems

    Hello, I"m designing an pneumatic circuit. I have an doubt regarding the force exerted by the actuator. The maximum pressure which I will be using is 8 bar (approximately 116 PSI). The compressed air will be stored an air tank. The tank will be connected to the regulator, then the valve and...
  49. S

    AC with INDUCTOR IMP DOUBT

    What i know the most fundamental thing is that the emf supplies energy to the charges and these charges have to reach the other end(say for electrons to reach the + end). IN order to do that somehow or the other they have to drop their voltage or energy for this to happen . AC with inductor...
  50. D

    Electric Current basic definition doubt. PLEASE HELP.

    1)I do understand why current must be same at all points in series as charge is conserved. BUT, If a current, say 'I' is created in a loop, then why, exactly, do we say that the current in a part or element of the wire is the same, that is, 'I'?? Why is current independent of the length of wire...
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