# Activity for Istiak‭

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Edit Post #281581 Post edited:
Comment Post #285133 @#8056 I will welcome any answer of my question. Usually, I like derivation rather than he had done it. A derivation can clear up any confusion no matter who done it what way. Everyone doesn’t think same way. The way I was learnt derivation of E= mc^2 from high school's book was not the way how Einst...
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Comment Post #285133 When putting -1 in Minkowskian metric I just get the spacetime invariant which is $ds^2=c^2t^2-dx^2$. Even I get the same spacetime invariant if I put ict into time coordinate. My first question was, how Minkowski found that? Or he just wrote it curiously. Second question was, how imaginary nu...
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Comment Post #285133 Yep! It is about putting -1 in the Minkowski metric and putting factor i into the time coordinate.
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Edit Post #285133 Initial revision 1 day ago
Question Clear up confusion on Minkowski signature
All given metrics are for orthonormal-basis. 2 dimensional spacetime : I saw that Minkowski Metric looks like this : $$\pmatrix{-1 & 0 \\\\ 0 & 1}$$ or $$\pmatrix{1 & 0 \\\\ 0 & -1}$$ I was wondering why it's not written like identity metric. To define a vector in spacetime it's written li...
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To understand virtual work you should understand virtual displacement first, The wiki I have linked in first paragraph describes virtual displacement deeply. Usually we write $\delta$ or $d$ to say infinitesimal, now the question is infinitesimal what? As the OP wrote in question that $\sumi Fi\cd... (more) 4 days ago Comment Post #284890 He was asking why it was happening which is for friction or something similar to it. But if he wants us to say how to repair it than I will remark it as off-topic. (more) 4 days ago Edit Post #285090 Initial revision 4 days ago Answer A: What does Laplace operator represent? Laplacian acts like Divergence but not completely. If you take a function (called$\vec{A}$) and write that laplacian of that function is$0$than it will be flat space. $$\nabla^2\vec{A}=0$$ But if you write Laplacian like this : $$\Delta \vec{A}=\text{c}\frac{\partial^2 \vec A}{\partial ... (more) 4 days ago Edit Post #285089 Initial revision 4 days ago Answer A: What is \mathcal{O} in integration of potential? Actually \mathcal O is some standard point and r is where potential depends on.$$V(b)-V(a)=-\int{\mathcal{O}}^B \vec E \cdot d\vec l +\int{\mathcal{O}}^A \vec E \cdot d\vec l  =-\int{\mathcal{O}}^B \vec E \cdot d\vec l-\int^{\mathcal{O}}B \vec E \cdot d\vec l=-\intA^B \vec E \cdot ... (more) 4 days ago Edit Post #285088 Initial revision 4 days ago Question What is$\mathcal{O}$in integration of potential? I saw following equation in Griffiths EM $$V(r)=-\int\mathcal{O}^r \vec E \cdot d\vec l$$ While the surface was closed but not symmetrical circular. At first I was thinking$\mathcal O$was representing closed surface and$r$was representing radius of that circle. But when I noticed the circle ... (more) 4 days ago Comment Post #285044 I have shared in a WhatsApp group and 3 facebook groups, I had in lots of groups but I left them cause they were unhelpful for me. If I can see new activities here than I will market the site.. :) Interested to do anything for the community (more) 6 days ago Edit Post #285044 Initial revision 6 days ago Answer A: How can we grow this community? I am not a marketer so my idea may not be good. As a physics enthusiast I can give some thoughts. Recently, I was just reading questions and answers in PSE just to learn new things. And what I have seen that is most of questions were asked by undergraduate students and high students or colleg... (more) 6 days ago Edit Post #284761 Initial revision about 1 month ago Question What does Laplace operator represent? I was wondering what's the physical meaning of Laplace operator.$\vec\nabla$actually represent a field. I had seen that Laplace operator is written as$\vec \nabla \cdot \vec\nabla=\nabla^2=\Delta$So Laplace operator is scalar quantity. I had found usage of Laplace operator in a book and Wiki-pag... (more) about 1 month ago Comment Post #284739 I agree with the idea. I believe professional might stop visiting the site when they see that there's no more question. Seems like, the site might be dead. If we don’t get questioners than the site will be dead for less question. Later, when there will be more questioner then the site will be dead fo... (more) about 1 month ago Edit Post #284728 Initial revision about 1 month ago Answer A: Who should the temporary moderators be? Is it the correct time to have a moderator? It's already a year we have the Physics community. But it's really inactive. There's only 42 question in Q&A which proves that we still don't have lots of member to build the community yet. So it's not the better time to have a mod either. Mithrandir w... (more) about 1 month ago Comment Post #284645 Instead of deleting my earlier comment, I am just adding new comment. Since **it's not you who misread**. It's me who misread. Cause the youtuber had wrote Entropy but I read Energy. Sorry again! (more) about 2 months ago Comment Post #284645 Sorry! Did you misread? I said energy. But you said Entropy. There is difference. And I had read it in First law of thermodynamics. (more) about 2 months ago Edit Post #284645 Post edited: about 2 months ago Comment Post #284645 At the current time of the video : https://youtu.be/sPveM04XCxI?t=143 he wrote that "Energy can't be destroyed but it can created". I don't believe it but the person wrote in that video's description that "this video is specially made for the S.Y.Bsc Physics students". My mind can't agree with th... (more) about 2 months ago Edit Post #284645 Initial revision about 2 months ago Question What is "order" and "disorder" in entropy? What is "disorder" in entropy? >Entropy is measurement of "disorder". (Some says it's not "disorder") I had read "disorder" and "order" of entropy in my book. But I was wondering what "order" and "disorder" actually represent. As Boltzmann said, $$S=k\ln \Omega$$ Where$\Omega$is number of... (more) about 2 months ago Comment Post #284434 It is about general.. (more) 2 months ago Edit Post #284434 Initial revision 2 months ago Question Should I always write units in equation no matter if it looks like variable? I am not sure if it is suitable in the site. I am asking the question here cause Olin and Celtschk said it here. They told me to use unit in every "single line" (Nope! They didn't say like it. Olin said to always separate number and units. And Celtschk said to write units in middle lines). I wa... (more) 2 months ago Edit Post #283894 Post edited: 3 months ago Edit Post #283894 Post edited: 3 months ago Edit Post #284073 Post edited: 3 months ago Edit Post #284075 Initial revision 3 months ago Answer A: Find the values of A, B, and C such that the action is a minimum >The Euler-lagrangian equation gives the equations of motion that once solved give you a family of solutions that minimize the action. A unique solution is given by specifying boundary conditions. It is just a case of inputing those boundary conditions. > >Wlog let$ x(0)=0 $and$x(t0)=a $. Integ... (more) 3 months ago Edit Post #284073 Initial revision 3 months ago Question What's the importance of Poisson brackets? $$F=F(q,p,t)$$ $$\frac{dF}{dt}=\frac{\partial F}{\partial q}\frac{\partial q}{\partial t}+\frac{\partial F}{\partial p}\frac{\partial p}{\partial t}+\frac{\partial F}{\partial t}$$ $$=\frac{\partial F}{\partial q}\dot{q}+\frac{\partial F}{\partial p}\dot{p}+\frac{\partial F}{\partial t}$$ $$=\frac... (more) 3 months ago Edit Post #284050 Initial revision 3 months ago Question Find the values of A, B, and C such that the action is a minimum > A particle is subjected to the potential V (x) = −F x, where F is a constant. The particle travels from x = 0 to x = a in a time interval t0 . Assume the motion of the particle can be expressed in the form x(t) = A + B t + C t^2 . Find the values of A, B, and C such that the action is a ... (more) 3 months ago Edit Post #283954 Post edited: 3 months ago Edit Post #283954 Post edited: 3 months ago Edit Post #283954 Initial revision 3 months ago Answer A: What does Lagrangian actually represent? Lagrangian is no energy. It’s just the Lagrangian. It's perhaps more fundamental than energy in a certain sense. In general, you can think of it as a function that minimizes the action. That's the definition of Lagrangian. said by Golam Ishtiak (You can find him right here also) For reference ... (more) 3 months ago Edit Post #283919 Initial revision 3 months ago Answer A: Find initial velocity when a stuntman jump from 1.25 \ m height The method is correct. >when you write s=vt, s is the horizontal distance and v is the horizontal component of the initial velocity (and it happens to be that the initial velocity has only horizontal component but it could have been different), which does not change through the motion, that's ... (more) 3 months ago Edit Post #283918 Initial revision 3 months ago Question Find equation of motion using. Lagrangian given equation is L'=\frac{m}{2}(a\dot{x}^2+2b\dot{x}{y}+c\dot{y}^2)-\frac{k}{2}(ax^2+2bxy+cy^2) >$$L'=\frac{m}{2}(a\dot{x}^2+2b\dot{x}{y}+c\dot{y}^2)-\frac{k}{2}(ax^2+2bxy+cy^2)$$where a, b, and c are arbitrary constants but subject to the condition that b^2 − ac \ne 0. What are the equations of motion? Examine particularly the two cases a = 0 = c and b = 0, c = −a. What is the physic... (more) 3 months ago Edit Post #283894 Post edited: 3 months ago Edit Post #283894 Initial revision 3 months ago Answer A: Book suggestion for Classical Mechanics For an absolute beginner : If you don't have any idea of any theories than, I would suggest to study theories at first. Just practice beginner problems at first which contains no-calculus(It's OK if that book contains few beginner calculus). I suggest to read High Schools Physics Book for studying... (more) 3 months ago Comment Post #283783 added... currently, it's second question in list of Q&A (more) 3 months ago Edit Post #283783 Post edited: 3 months ago Edit Post #283783 Initial revision 3 months ago Question Question is showing closed instead of last_activity or reopened Magic.. :D list The question was reopened but, showing closed instead of reopened (It should show lastactivity according to code). question link (more) 3 months ago Comment Post #283703 I was looking at your profile then, I saw that moderator ability isn't showing in Physics but, showing in CD Meta, Software and others (not visited all communities). (more) 4 months ago Comment Post #283293 Is there something else which I should add? (more) 4 months ago Comment Post #283695 In your last second equation you had \delta q beside dt. I know that dt is used for representing changes respect to something. While integral goes away, than dt goes away either. Hence, dt is no more. But, I wonder where \delta t had gone? It should be right there. If not than, it is because th... (more) 4 months ago Comment Post #283680 Somehow I can see it now... Could you write an answer? So, it might be helpful for future visitor if they see something related to this... (more) 4 months ago Edit Post #283684 Initial revision 4 months ago Question What does Lagrangian actually represent? L=T-U Here, L is Lagrangian. T is kinetic energy. U is potential energy. But, what Lagrangian actually is? I know what Holonomic and non-holonomic is. But, I was thinking what the Lagrangian represent. Like, F represent force applied on some body. To me, Lagrangian is just representing some kind ... (more) 4 months ago Edit Post #283683 Post edited: 4 months ago Edit Post #283683 Post edited: typo 4 months ago Edit Post #283683 Initial revision 4 months ago Question Prove differential form of Lagrangian How to derive the Lagrangian differential force?$$\frac{d}{dt}(\frac{\partial L}{\partial \dot{x}})+\frac{\partial L}{\partial x}=0$$I was trying to do something.$$L=T-U=\frac{1}{2} m\dot{x}^2-\frac{1}{2}kx^2\frac{\partial L}{\partial \dot{x}}=m\dot{x}\frac{\partial L}{\partia... (more) 4 months ago Edit Post #283293 Post edited: 4 months ago Edit Post #283680 Post edited: 4 months ago Edit Post #283680 Post edited: 4 months ago Edit Post #283680 Initial revision 4 months ago Question Why ability doesn't show in every users profile? Why abilities don't show in every user page? Take a look at @Mithrandir's profile according to his profile he doesn't have any ability. But, I don't think that he doesn't have ability. I was checking other users if they also don't have ability. But, I saw they had. What's the reason of it? I had earl... (more) 4 months ago Edit Post #283293 Post edited: 4 months ago Edit Post #283656 Post edited: 4 months ago Edit Post #283656 Post edited: 4 months ago Edit Post #283656 Initial revision 4 months ago Question What is virtual work? $$\sumi Fi \cdot \delta ri$$ is virtual work when internal force is$0. For that reason, $$\sumi Fi \cdot \delta ri = 0$$ Here internal force stands for what? When a object's displacement is imaginary then, that object's work is virtual work. But, how a object's displacement can be imaginary? ... (more) 4 months ago Comment Post #283511 [Done](https://physics.codidact.com/posts/283618) (more) 4 months ago Edit Post #283618 Initial revision 4 months ago Question What happens if an electron collides with another electron? We know that electrons always repel electrons. But, if somehow they collide, then what will happen? I found a related question of above one in Quora. >An incoming electron can instead make them both bounce away. > >Two free electrons will bounce off each other unless perfectly aligned. In whi... (more) 4 months ago Edit Post #283511 Post edited: 4 months ago Comment Post #283573 Gotcha! Thanks. (more) 4 months ago Comment Post #283573 Newton said,g=\frac{Gm}{r^2}$. OK! I am taking$x=r$and,$a=g$.$a=\frac{Gm}{x^2}$. Although, acceleration is positive. I also know that when we push objects upward then, acceleration becomes negative. But, OP doesn't say we are pushing up or down. (more) 4 months ago Edit Post #283293 Post edited: 4 months ago Edit Post #281633 Post edited: 4 months ago Edit Post #283511 Post edited: 4 months ago Comment Post #283511 @#8049 Isn't electron, proton and positron noun? First later of noun is forever capital. But, in your suggestion it was small letter why? (more) 4 months ago Comment Post #283517 That should be$mg\cos \theta$instead of$mg$.. I made the mistake while drawing (more) 4 months ago Edit Post #283517 Initial revision 4 months ago Answer A: Why used$\cos\theta$for$\text{y}$axis or, gravitational force? picture Force is perpendicular to the surface. And, green color "rod" is parallel to the surface. So, $$F = N - mg\cos \theta=0$$ Reference (more) 4 months ago Edit Post #283397 Post edited: 4 months ago Edit Post #283511 Initial revision 4 months ago Question What happens if an electron collides with a proton? What will happen if an electron and a proton collide? They attract each other inside an atom. But, why they don't collide inside atoms? Does the "boundary" of the nucleus push them away? >depends on the energy of the electron. For low energies, a bound state will be formed due to electromagnetic i... (more) 4 months ago Edit Post #283448 Post edited: 4 months ago Comment Post #283410 @#8056 It is well-fitted in Q&A but, I found the problem in "Problems" that's why I asked it here. If you think that Q&A is better than Problems for the question than, you can flag for mod attention. They will move to Q&A (more) 4 months ago Edit Post #283448 Initial revision 4 months ago Answer A: Why answer matched but, unit? The error was on mass. In question : $$m=20 \ g$$ And, I wrote following one in answer $$m=20\times 10^3 \ kg$$ which is completely wrong. Cause,$1 \ kg=1000 \ g$(more) 4 months ago Edit Post #283420 Post edited: 4 months ago Comment Post #283420 Ohh! Actually, I didn't know that writing units in intermediate steps is required. I always write in last steps. So, Thanks for the information. And, I have completed the question also... :) (more) 4 months ago Edit Post #283420 Post edited: 4 months ago Comment Post #283420 I have edited... It was actually$kgm^{-3}$(more) 4 months ago Edit Post #283420 Post edited: 4 months ago Comment Post #283293 @#52997 So, I have to choose a random book which is looking good to me. If I can understand that properly then, I should move to another book. So, I have to read lots of books? (more) 4 months ago Edit Post #283420 Post edited: 4 months ago Edit Post #283420 Initial revision 4 months ago Question Why answer matched but, unit? >Mass of a timber is$20 \ g$. And, density of that timber is$0.27 \ g/cc$. That timber was bind to a metallic materials and, it was released to$0.970 \ g/cc$water. How much the wood was submerged in water? I was trying to solve the problem following way. $$F=Ah\rho g$$ $$=V\rho g$$ $$=V \... (more) 4 months ago Edit Post #283410 Initial revision 4 months ago Question Why used \cos\theta for \text{y} axis or, gravitational force? >figure 3.2figure 3.1 >Mass M1 is held on a plane with inclination angle θ, and mass M2 hangs over the side. The two masses are connected by a massless string which runs over a massless pulley (see Fig. 3.1). The coefficient of kinetic friction between M1 and the plane is µ. M1 is released from r... (more) 4 months ago Comment Post #283293 @#52997 Usually, I am not a school, college or University student. I am interested in learning Physics. That's why I am looking for a classical mechanics book which contains most of Classical topics and, little bit of Lagrangian, GR, SR (But, It's OK if that classical Mechanics book doesn't contain t... (more) 4 months ago Comment Post #283395 Since, photons always travel at constant speed. Hence, photons acceleration is forever 0. speed of photon changes depending on medium. (more) 4 months ago Edit Post #283397 Initial revision 4 months ago Question Find initial velocity when a stuntman jump from 1.25 \ m height >figure >A stuntman jumped from 1.25 \ \text{m} height and, landed at distance 10 \ \text{m}. Find velocity when he jumped. (Take \text{g}=10 \ ms^{-2}) I had solved it following way.$$h=\frac{1}{2}gt^2=>1.25=5\cdot t^2=>t=\frac{1}{2} \ s$$And,$$s=vtv=\frac{s}{t}$$... (more) 4 months ago Edit Post #283386 Post edited: \cdot or, direct cross product is better than asterisk 4 months ago Edit Post #283392 Post edited: The phrase (your moving at a constant speed) isn’t looking good to me 4 months ago Suggested Edit Post #283386 Suggested edit: \cdot or, direct cross product is better than asterisk (more) helpful 4 months ago Suggested Edit Post #283392 Suggested edit: The phrase (your moving at a constant speed) isn’t looking good to me (more) helpful 4 months ago Edit Post #283391 Post edited: 4 months ago Edit Post #283391 Post edited: 4 months ago Edit Post #283391 Initial revision 4 months ago Question Why we can't find a particle accelerating unless there's some other particle accelerating somewhere else? I was reading "Introduction to classical Mechanics" by David Morin. In that book they wrote that >The third law says we will never find a particle accelerating unless there’s some other particle accelerating somewhere else. The other particle might be far away, as with the earth–sun system, but it... (more) 4 months ago Comment Post #283304 Anyone can answer on that post thousand years later also. "That person" just want to find the perfect book available right now... (more) 4 months ago Comment Post #283258 @#8176 Yes! But, there's no way to migrate to another community :( So, we can flag it now (more) 4 months ago Edit Post #283293 Initial revision 4 months ago Question Book suggestion for Classical Mechanics Since, there's no book category currently so, I am asking it here. And, It can be moved to that category later. I want to start learning now so, I don't want to wait. Which book is best for Classical Mechanics? I have studied Classical Mechanics little bit. But, I want a book which explains th... (more) 4 months ago Edit Post #283272 Initial revision 4 months ago Question Book suggestion category proposal I was thinking for a new category called Book suggestion (not only book suggestion either). Usually, most of Science students like to study Science books (their favorite subject) by themselves. Some of them self-study rather than getting to University. And, some of them looks for book suggestion. If ... (more) 4 months ago Comment Post #283258 Seems, like computer science 🤔 (more) 4 months ago Edit Post #283072 Post edited: 4 months ago Edit Post #283072 Post edited: 4 months ago Comment Post #283046 History related question is off-topic (more) 4 months ago Edit Post #283072 Initial revision 4 months ago Question When selecting needs author attention it doesn't reach to moderator I had flagged the post as off-topic. There was a comment automatically created (Post Feedback). When I visited the flag raised page. I noticed it's not available anymore. Seems like, if I choose needs author attention than, it doesn't reach to moderator. But, some kind of questions can't be made on... (more) 4 months ago Comment Post #283046 I don’t think history related question is on-topic (more) 4 months ago Comment Post #283046 The question is off-topic (more) 4 months ago Suggested Edit Post #282727 Suggested edit: improve formatting (more) pending 5 months ago Edit Post #282668 Initial revision 5 months ago Answer A: What is inflation in cosmology? I haven't studied to much. I don't have any idea of Physics. Last few moments, I was just passing my time by reading theories. I have little bit idea of inflationary theory. History of the universe If you think of a balloon than, if it suddenly burst then, it will spread. Something happened... (more) 5 months ago Comment Post #281840 @Canina suppose, if i code a software and, i take the mobile(which have that software) in a "silent" room(where i can't hear anything) and, if i put the dB level 0 or less than 0 somehow than is that correct measurement? (more) 7 months ago Comment Post #281840 @Canina what is actually reference level? (more) 7 months ago Comment Post #281840 http://www.cochlea.org/en/hear/human-auditory-range (more) 7 months ago Edit Post #281840 Initial revision 7 months ago Question decibel level of human hearing(human-auditory-range) Decibel level listed of solfege. | Do | Re | Mi| Fa | So | La | Ti | Do |- | - | - |- |- |- |- |- | | 24 | 27 | 30 | 32 | 36 | 40 | 45 | 80 noise intensity I saw that whispering decibel level is around 40. And, normal voice level is around 60. And, Human can hear 0 - (120-130). So, my que... (more) 7 months ago Comment Post #281647 To reviewer, this post is off-topic in Q&A site. So, i would to request migrate it to problems if possible otherwise close it as off-topic (more) 7 months ago Suggested Edit Post #281649 Suggested edit: (more) declined 7 months ago Comment Post #281649 In my other post I had added main question. I forgot to add it here. EE site will be better for the problem. But, EE is a part of Physics. That's why I ask any Physics related question here. What current flows thru the 4 Ω resistor? I don't have the value. If I think of the circuit without 4 Ω than... (more) 7 months ago Edit Post #281647 Post edited: 7 months ago Edit Post #281647 Post edited: 7 months ago Edit Post #281647 Initial revision 7 months ago Question Delta to Wye conversion ![enter image description here] I have a circuit like this. And, I want to calculate resistance from here. I was following these steps. I was calculating resistance for left side circuit.$$R1=\frac{2 × 2}{2+2+4}=0.5\OmegaR2=\frac{2 × 4}{2+2+4}=1\OmegaR3=\frac{2 × 4}{2+2+4}=1... (more) 7 months ago Edit Post #281633 Post edited: mistook tag 7 months ago Edit Post #281633 Initial revision 7 months ago Question showing a downvote in statistics while I haven't got any downvote statistics In my Physics Codidact profile I am getting that I got 1 downvotes. I am not sure if I asked a question earlier in which I got 1 downvotes. In my current posts I don't have any downvotes or upvotes. Even, my reputation is OK. So, I think it is a bug in Statistics. For better understa... (more) 7 months ago Comment Post #281631 Yes! I had solved those problem. I just added them as sample. I have invited once my friend. That's why I had to ask this type question. (more) 7 months ago Edit Post #281629 Post edited: quoting 7 months ago Edit Post #281629 Initial revision 7 months ago Question Is "homework-and-exercises" off-topic in Codidact? In SE sites, homework-and-exercises question isn't allowed in Physics SE. So, I want to know does Codidact also mark home-and-exercises question as off-topic? I have earlier asked twice question on homework-and-exercises but, I didn't get any response cause, there's just few active people in Ph... (more) 7 months ago Edit Post #281581 Post edited: 7 months ago Edit Post #281581 Post edited: 7 months ago Edit Post #281581 Post edited: 7 months ago Edit Post #281581 Initial revision 7 months ago Question Slipping and rotation >A disk of mass$M$and radius$R$is initially rotating at angular velocity of$\omega$. While rotating, it is placed on a horizontal surface whose coefficient of friction is$\mu=0.5$. How long will it take for the disk to stop rotating? Someone had solved the problem following way \begin{al... (more) 7 months ago Edit Post #281568 Initial revision 7 months ago Answer A: Calculating frequency of oscillation While force is vector. So, we should use vector addition not "direct addition". From vector addition formula $$C=\sqrt{A^2+B^2+2ABcos\alpha}$$ For force, $$F=Eq$$ $$F=mg$$ $$F=\sqrt{(Eq)^2+(mg)^2+2Eqmgcos\alpha}$$$$f=\frac{1}{2\pi} \sqrt{\frac{\sqrt{(Eq)^2+(mg)^2+2Eqmgcos\alpha}}{m... (more) 7 months ago Comment Post #281541 @Olin Lathrop Yes! I understood the question(maybe, 1 or 2 hours ago) after asking the question. (more) 7 months ago Edit Post #281541 Initial revision 7 months ago Question Calculating frequency of oscillation > A 4C point charge of mass 2kg is suspended by a string of length 6m. The charge is placed in the Earth's Gravitational field and a uniform horizontal electric field of strength$\sqrt{11}NC^{-2}\$. If the charge is displaced slightly from equilibrium, what will be the frequency of oscillation? (g=10...