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We don't have even 15 days for preparing .Hence i think we should prefer just discussing only few important topics via questions.

----Note

---1. Pls ensure that ques deals more with physics and less with maths(though i love such questions)

---2. Before posting Ensure that ques is really of INPHO level.

---3.Ques which involve dealing with major Topics are Welcomed.

---4.The one who solves any particular q needs to post the next q within 3 hrs.

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## Comments

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TopNewesthere is another question!!

Natural uranium consists of mainly two isotopes, \(U^{238}\) and \(U^{235}\),whereas the relative concentration of the latter is 0.7%.Uranium is "enriched"(ie., concentration of \(U^{235}\) is increased) by implementing a multi-stage process,where at each stage,evaporated chemical compound \(UF_{6}\) is led through a porous wall.the porous wall can be considered to be a thin film having microscopic holes in it.How many stages are nedded to double the concentration

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iska to bta do

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Take a look at the thin rod(of length \(a\)) above.

The mass density along the rod varies as \(\sigma=\sigma_{0}(1+\frac{x^{n}}{a^{n}})\) where \(n\) is a positive integer.

While calculating the co-ordinates of the centre of mass of the rod, we assume(wrongly) the COM to be at the geometric centre ie., \((\frac{a}{2},0)\)

Now, we know that's wrong and find the actual co-ordinate.

We note that the actual \(x\) co-ordinate is \(\frac{a}{2}+\delta X\) where \(X\) is the actual co-ordinate of the COM.

Define \(100\frac{\delta X}{X}\) as percentage error

Approximately what maximum percentage error will we have made in assigning the COM to rods from \(n=1\) to \(n=20\) ie.,\(\displaystyle \frac{100}{20} \sum_{n=1}^{n=20} \frac{\delta X_{n}}{X_{n}}\) where \(X_{n}\) is the COM when \(n\) is the exponent of \(x\) in the surface density expression

Details and Assumptionsthe \(21^{st}\) harmonic number \(H_{21}\) = 3.645

\(1/23 + 1/24 + 1/25 = 0.12514\)

give your answer to the nearest integer

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u are wrong coz u probably took ratio of masses as 238/235

take ratio of masses of uf6

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you are off by 53 stages

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110 stages ??

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Hey try this. Asphere of radius r is projected up an inclined plane of @inclination with initial speed u and omega In a direction in which it will roll up.the coefficient of friction is tan@/7.and v>omega*r.find the total time of motion of the sphere up the plane before it stops.as this is quite a old q I don't have any source to match my ans.post yours so that we can check.....

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(123vi-18wr)/(147gsin@)

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And this is a typical inpho q.not many will notice the fact that friction reverses its direction.I hope you liked it.thanx.

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I got my ans as (17v+4wr)/18gsin@.

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how did u solve it??

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so how to achieve 0.4mgsin@??

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Comment deleted Jan 27, 2017

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Another such seemingly simple problem simple trick

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Nice problem it is .

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but it isn't INPHO level, is it ?

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No it is as it is not that much tricky Though by the name it may suggest it uses a trick but actually it is a widely used technique which may be asked in inpho.

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Isn't it \(\frac{2xg}{H}\) ?

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No.

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is it 4xg/H ?

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I am also getting 2xg/H.

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What are your INPhO marks ?

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43 :(

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u?

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pls mention here the marks u know in inpho here are some expected marks which i know 46 37 [44,49] [42,51] 60 65 pls also do mention nos u know

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i am getting around 48-50

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Ok finally its over. Pls post the QUES of inpho and provide link here

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what value u got N and \(\omega_{0}\)

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omega = 4.57 * (10^15)

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i think i wont make it to OCSC

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I got -ve value for refractive index

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what value u got for N??

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Comment deleted Jan 29, 2017

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N kya hai bhai :(

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https://brilliant.org/profile/aryan-2xcg8y/sets/that-is-why-i-love-physics/462021/problem/inpho/

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We will add it on 31st. How was your paper ? Solved Q5 and Q6 ? I was getting -ve refractive index for that X-ray

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yep did q5 and q6 a,b part

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can you uplod the questions ?

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quit pho , cbse is more imporant , lol !

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Comment deleted Jan 25, 2017

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can we discuss ur problem it has certain ambiguities :| , if the motor was detached , what is the role of fringes , light and stefan's constant ? if not, write it there , also you din't tell the initial temp. as the light source was not playing any role at that time , i think the problem needs slight changes .

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no leave it

its far too convoluted there are far too many ambiguities to bother correcting

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i dunno unimodulr constnt would you tell bout it ?

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a unimodular constant means a quantity with magnitude1

for example, 1m is a unimodular constant

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isn't anybody participating ?

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nobody dares to do my problem ? :P

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that connecting moves through the wedge??

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if u could add what kind of displacement are u giving

are you shifting it kepping string inclined at same angle as that of equillibrium position

or are you displacing only one mass kepping the other non disturbed

or ?

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both down

or one up other down

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try with spring too ..:)

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the bottom one depicts equilibrium position

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the wedge is fixed .

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isn't anybody participating ?

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Let L1 be the present total angular momentum of the Earth-Moon system. Now, make the following assumptions: i) L1 is the sum of the rotation of the Earth around its axis and the translation of the Moon in its orbit around the Earth only. ii) The Moon’s orbit is circular and the Moon can be taken as a point. iii) The Earth’s axis of rotation and the Moon’s axis of revolution are parallel. iv) To simplify the calculations, we take the motion to be around the center of the Earth and not the center of mass. Throughout the problem, all moments of inertia, torques and angular momenta are defined around the axis of the Earth. v) Ignore the influence of the Sun. 1a Write down the equation for the present total angular momentum of the Earth-Moon system. Set this equation in terms of E I , the moment of inertia of the Earth; ωE1 , the present angular frequency of the Earth’s rotation; M1 I , the present moment of inertia of the Moon with respect to the Earth´s axis; and ω M1 , the present angular frequency of the Moon’s orbit

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Isn't it L1 = (E I)

(wE1) + (M1 I)(w M1) ?Log in to reply

i think that must be so .pls post the next q.

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Ya that's quite apparent.you can access the full prob.it gets more interesting after.ipho 2009/10(most prob)

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@shubham dhull @Spandan Senapati @Gauri shankar Mishra @Akarsh Singh @Samarth Agarwal @CH Nikhil @neelesh vij @Nihar Mahajan @Abhineet Nayyar @Abhisek Mohanty @avi solanki @Aditya Chauhan @Kishore S Shenoy @Somyaneel Sinha @Jyotisman Das @Svatejas Shivakumar @Ayush Agarwal @Sarthak Singhal and ...........

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@Archit Agrawal @Aniket Sanghi @Prakhar Bindal @Rajdeep Dhingra @Harsh Shrivastava @Swapnil Das @Ankit Dhindhwal @puneet mangla and ......

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OK guys here is my first question

1--- A small ball moves at a constant velocity v along a horizontal surface and at a point A falls into a vertical cylindrical well of depth H and radius r.The velocity v of the ball forms an angle alpha with the diameter of the well drawn through point A. Determine relation b/w v,r,H and alpha so that ball gets out of well after elastic impact.(take all collision to be elastic +friction=0)

(u don't need to solve any ques to post your ques if you have one please post it and share this note with as much people as you can)

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OK I have solved this.we can take the projection of the motion on a horizontal plane.the problem then reduces that the velocities get reflected along a normal to a point at which the ball stikes in the plane.so we can conclude mathematically that in a circle I take a line with @ angle to the horizontal and every time it strikes the circle it gets reflected.then the length of that segment is 2rcos@ and of I keep on doing so at some time I must return to the original point.the total length is then 2nrcos@.and this must be v

k2√2h/g.as v in the projected plane is constant.and the constant 2with √2h/g for the cyclic processLog in to reply

nrcos(alpha)=kv√(2H/g) where n and k belong to integers.

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yes , that's what i get if alpha is 0, so i guess u r right :)

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can you attach a a pic ?because you say the velocity of ball is parallel to hori. and the well is vertical so in this case alpha is 0 , right ? by the way in this case i get answer as r=v*root(2h/g) is it right ?if alpha=0 ?

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It means that the balls velocity was initially in a horizontal plane with an angle alpha to the actual horizontal line.

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Krotov question I guess??

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Yes

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