Doubt Definition and 580 Threads

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...

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?

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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?

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...

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 ...

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...

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...

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}...

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...

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?

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?

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...

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...

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...

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

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...

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...

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.

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...

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...

Is 3 stroke engine possible??if not why?

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...

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

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...

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...

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...

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...

Hi all, I was reading a paper in which implicit differentiation was used as follows x \in R, \lambda \in R Given G(x,\lambda) = 0 \frac{\partial G(x,\lambda)}{\partial x} \frac{\partial x}{\partial \lambda} + \frac{\partial G(x,\lambda)}{\partial \lambda} = 0 My doubt is...

Homework Statement For the chemical equilibrium N_{2(g)} + O_{2(g)} \rightleftharpoons 2NO_{(g)} , the following values were found for the constant Kc (chemical equilibrium constant), at the indicated temperatures: (The table is attached as an image) Table in text format: Temperature (K)...

Doping doubt... Why pentavalent elements are used in doping semiconductors why not an element with 6 valence electrons?

Curent Electricity Doubt! PLEASE HELP! Homework Statement I did not understand one thing.. If it is given that a wire has a current 'i', then why do we say (in applications of laws such as biot savart law) that the current in a small element of the wire is also 'i', that is the same current...

Q2. An incompressible fluid with density ρ is in a horizontal test tube of inner cross-sectional area A. The test tube spins in a horizontal circle in an ultracentrifuge at an angular speed ω. Gravitational forces are negligible. Consider a volume element of the fluid of area A and thickness dr'...

i am having a doubt that when we find a expression for mutual inductance we say that flux change in primary coil induces emf in secondary coil. but why change in flux doesn't induce any current in primary coil also.

I was reading a chapter on "General Relativity" a few days ago. (I was reading Arthur Beiser: Modern Physics)I found it very hard to digest the relation between the "Equivalence Priciple" and "The General Theory of relativity".The equivalence relation states that "An observer in a uniformly...

This is in continuation to https://www.physicsforums.com/showthread.php?t=330495 The question that initiated my doubt reads this -- " the internal resistance of an accumulator battery of emf 6V is 10 Ohm when fully discharged . as battery gets charged up , its internal resistance drops to 1...

Question-1. An engine of mass 105 tonnes is coupled to and pulls a carriage of mass 30 tonnes.The resistance to the motion of the engine is 1/100th of its weight.The resistance to the motion of the carriage is 1/150th of its weight.If the tractive force exerted by the engine is 25750 Newton.The...

In a density matrix, can some coherances (off diagonal terms) be zero while the diagonal terms(populations) aren't? I am confused because rhoij=Ci*Cj'. How can coherance be zero if Ci and Cj aren't?