IIT-JEE problems

[pauldmps]

^^i know it will be beacuse of unpaired electrons!!. but i am asking for electronic config or some explanation to write in exam.

I thought it was a thread meant for IIT-JEE & other competition exams in which you'll have objective questions.

Anyways, here are some questions from me:

1. Chemistry - What is the minimum number of carbon atom an alkane molecule must have in order to show optical activity ?

A. 5 B. 6 C. 7 D. 8

2. Maths - Suppose f(x) = x^3 + ax^2 + bx + c, where a,b,c are chosen respectively by throwing a die three times. Then the probability that f(x) is is decreasing in [1,3] is:

A. 5/9 B. 4/9 C. 0 D. 2/5

3. Physics - A solid sphere rolls on a smooth horizontal surface at 10 m/s and then rolls up an inclined plane of inclination 30° with horizontal. The mass of the sphere is 2kg. Find the height attained by the sphere before it stops in metre. (g= 10 m/s^2)
[Answer will be a digit between 0 to 9]

Let's see who can crack these IIT-JEE level question on his own. No cheating please. I'll answer them after anyone replies their answers.

 

[abhijangda]

I thought it was a thread meant for IIT-JEE & other competition exams in which you'll have objective questions.

Anyways, here are some questions from me:



Let's see who can crack these IIT-JEE level question on his own. No cheating please. I'll answer them after anyone replies their answers.

this thread is for asking doubts and not for taking tests or testing abilities of individuals. Just ask ques. If u have any doubt.

 

[Jaskanwar Singh]

Well is the ans to physics question - 7m?

 

[nims11]

guyz i need Tips for studying organic Chemistry. i am good at Physics and Mathematics but my Chemistry is poor due to organic Chem and my overall performance gets affected due to it. teachers say that Chemistry is skoring as well as less tym taking. My friends too score good in Chem. i am really comfortable with basic reaction mechanisms, basics of org chem like isomerism, optical activity,etc. i am also comfortable in hydrocarbons, alkyl halides and alcohols. but as the number of reagents increase, i lose track and am unable to grasp stuffs.

i need to get Computer Science as branch in a very good college so i think its very imperative for me to increase my overall performance in any possible ways.

@paul
answer to the chemistry one -> 7
answer to the maths one -> 0
answer to the physics one ->its coming 7 after assuming that it "pure rolls" on the smooth horizontal surface.
i hope these are right

 

[pauldmps]

^^ Absolutely Right all three of them.
Where are you taking your coaching BTW ?

@Jaskanwar Singh
Yes the physics answer is right. The chem & maths were on the tougher side, right ?

@abhijangda
I think such surprise question really help in preparation. Isn't it ?

 

[Jaskanwar Singh]

Actually buddy these chem and maths if from 12th i dont know as i in 11th. But if 11th i need to study more

but that was phy rotation level of iit?

 

[nims11]

^^ physics rotation question was of IIT's easy level questions as it could have been made quite complex by introducing few conditions.

@paul i take my coaching at FIITJEE. the classes are now over and i am preparing for 12th boards along with my revision for IITJEE.
i think that these surprise questions comes as a challenge and helps us to revisit the past topics which is necessary because the syllabus is vast and its difficult to keep track of the past topics while absorbing the new topics. for eg, i solved a question of rotation after many days.

 

[pauldmps]

@Jaskanwar Singh
IIT does not ask you very difficult questions. From last 3-4 years, the questions have been comparatively easier. You solved the question because you knew the concept. Give the same question to some of your friends & then see what they do. Some may not be able to identify that this question requires energy conservation & some may forget to add the rotational KE term.

More questions from class XI to come.......


@nims11
My classes at Prerna Classes are also over. Why don't you post some questions from your side here ?

 

[abhijangda]

^^ Absolutely Right all three of them.
Where are you taking your coaching BTW ?

@Jaskanwar Singh
Yes the physics answer is right. The chem & maths were on the tougher side, right ?

@abhijangda
I think such surprise question really help in preparation. Isn't it ?

i didnt said that these are not helpfull. Presently I m in NIT and also I have qualified JEE. I know the importance of surprise questions. But you should not write that 'lets see who can solve these problems' etc. Because if some one is not able to solve these problems his confidence will go down. Also the questions you have asked are of JEE levels for sure.

guyz i need Tips for studying organic Chemistry. i am good at Physics and Mathematics but my Chemistry is poor due to organic Chem and my overall performance gets affected due to it. teachers say that Chemistry is skoring as well as less tym taking. My friends too score good in Chem. i am really comfortable with basic reaction mechanisms, basics of org chem like isomerism, optical activity,etc. i am also comfortable in hydrocarbons, alkyl halides and alcohols. but as the number of reagents increase, i lose track and am unable to grasp stuffs.

i need to get Computer Science as branch in a very good college so i think its very imperative for me to increase my overall performance in any possible ways.

@paul
answer to the chemistry one -> 7
answer to the maths one -> 0
answer to the physics one ->its coming 7 after assuming that it "pure rolls" on the smooth horizontal surface.
i hope these are right

see if you are good at Maths and Physics then do not study for CBSE exams now. Study for exam only in March. I was also good at PCM and mostly at P and M. I prepared for exam in March and got 97 in P and 95 in M. For chem you have to study hard. Because ques. In chem are more theory oriented. So, prepare for chem only. Also at present do Organic only. Those reaction based questions are frequent in JEE. Good luck!!

guyz i need Tips for studying organic Chemistry. i am good at Physics and Mathematics but my Chemistry is poor due to organic Chem and my overall performance gets affected due to it. teachers say that Chemistry is skoring as well as less tym taking. My friends too score good in Chem. i am really comfortable with basic reaction mechanisms, basics of org chem like isomerism, optical activity,etc. i am also comfortable in hydrocarbons, alkyl halides and alcohols. but as the number of reagents increase, i lose track and am unable to grasp stuffs.

i need to get Computer Science as branch in a very good college so i think its very imperative for me to increase my overall performance in any possible ways.

@paul
answer to the chemistry one -> 7
answer to the maths one -> 0
answer to the physics one ->its coming 7 after assuming that it "pure rolls" on the smooth horizontal surface.
i hope these are right

see if you are good at Maths and Physics then do not study for CBSE exams now. Study for exam only in March. I was also good at PCM and mostly at P and M. I prepared for exam in March and got 97 in P and 95 in M. For chem you have to study hard. Because ques. In chem are more theory oriented. So, prepare for chem only. Also at present do Organic only. Those reaction based questions are frequent in JEE. Good luck!!

 

[asingh]

Really nice thread. Feels good to see a platform like TDF being used so constructively. J.Singh, great start up, and hope you all maintain the rigor. Heck brings back old memories -- am B.Sc. in Physics (Electronics) -- but cannot even attempt these questions now.

Kudos to you all.

 

[pauldmps]

Questions from class XI syllabus:

Chemistry: For the reaction : CO(g) + H2O(g) <------> CO2(g) + H2(g) at a given temperature, the equilibrium amount of CO2(g) can be increased by :

A. Adding a suitable catalyst
B. Adding an inert gas
C. Decreasing the volume of the container
D. Increasing the amount of CO(g)
[One or more than one option may be correct]


Physics: Two blocks of masses 3kg & 6kg respectively are placed on a smooth horizontal surface. They are connected by a light spring of spring constant k= 200 N\m. Initially the spring is unstretched. A velocity of 1 m\s is imparted to the 3kg block & 2 m\s to the 6 kg block in the opposite directions which is also opposite to the side on which the spring is attached on both blocks.

The maximum extension in the spring would be:

A. 30cm B. 25cm C. 20cm D. 15cm [Only one option correct]


Maths: If x,y,z are in H.P. ,then ln(x+z) + ln(x-2y+z), (where "ln" is natural logarithm) is equal to :

A. ln(x-z) B. 2ln(x-z) C. 3ln(x-z) D. 4ln(x-z) [Only one option correct]

 

[nims11]

^^

Spoiler

Chemistry : seems like 'D' is the only correct answer
Physics : 15cm
Maths : ln(x-z)
are these correct?

I will also try post some good questions i come across.

 

[Jaskanwar Singh]

ok nims11 and pauldmps. i got that.

thanks asingh. yes it tends to forget if you are not in touch. so not a problem for you now. and we all will keep this up and ready with questions.

i am solving those questions. but please all of you from now on put answers in spoiler tags. it spoils the fun.

 

[nims11]

see if you are good at Maths and Physics then do not study for CBSE exams now. Study for exam only in March. I was also good at PCM and mostly at P and M. I prepared for exam in March and got 97 in P and 95 in M. For chem you have to study hard. Because ques. In chem are more theory oriented. So, prepare for chem only. Also at present do Organic only. Those reaction based questions are frequent in JEE. Good luck!!

Thanx for the suggestions!! i am currently putting my all into organic chem and i have decided to fill in all my gaps and holes in it by 15th.
i am not taking too much worry about boards until 20th. And the CBSE preparations will be along with my JEE preparation. CBSE Chemistry and Physics syllabus is easy and i have 11 days gap for maths which is more than enough. Also my 5th subjct is C.Sc in which i dont have to give much efforts.

ok nims11 and pauldmps. i got that.

thanks asingh. yes it tends to forget if you are not in touch. so not a problem for you now. and we all will keep this up and ready with questions.

i am solving those questions. but please all of you from now on put answers in spoiler tags. it spoils the fun.

Sorry. i will take care of that.

 

[Jaskanwar Singh]

Spoiler

Chemistry D
solving physics now
i have to do log still in mathematics.

 

[pauldmps]

Solutions:

Spoiler

Physics: A. 30cm
At max. extension, the velocity of both the blocks are same.

Applying conservation of linear momentum,

(3+6)v = {(6X2) - (3x1)} = 9
=> v = 1 m/s
Let 'x' be the max. extension in the spring:
Applying conservation of Energy,

(1/2)(3)(1)^2 + (1/2)(6)(2)^2 = (1/2)(200)(x)^2 + (1/2)(9)(1)^2
=> x= 0.3m = 30cm




Maths: B. 2ln(x+z)

x,y,z are in H.P => 1/x,1/y,1/z are in A.P
=> 2/y = 1/x + 1/z
=> 2xz = y(x+z)

Now, ln(x+z) + ln(x-2y+z) = ln{ (x+z)^2 - 2y(x+z)}
= ln{(x+z)^2 - 4xz}
= 2ln(x-z)

Chemistry: Option D. only
Since Δn = 0, addition of inert gas does not change the equilibrium.

 

[Jaskanwar Singh]

i too got 15m in physx

ok tell me whats wrong with my method in physx -

consider extension produced be x. then force that pulls back the bodies will be Kx. let retardation of 3kg mass be r and 6kg mass be r'. and let extension produced by 3kg be a and by 6kg be b. then 2ra = 1^2 and 2r'b = 2^2. (v^2 - u^2 = 2as) and final velocity after reaching max extension = 0 and moreover Kx = 3r and Kx = 6r'. x=a+b solve these equations. i got 15m.

 

[nims11]

OH NO....I had interchanged the initial velocities of both the blocks, its now coming to 30m. Reading the question properly is very important.

 

[pauldmps]

i too got 15m in physx

ok tell me whats wrong with my method in physx -

consider extension produced be x. then force that pulls back the bodies will be Kx. let retardation of 3kg mass be r and 6kg mass be r'. and let extension produced by 3kg be a and by 6kg be b. then 2ra = 1^2 and 2r'b = 2^2. (v^2 - u^2 = 2as) and final velocity after reaching max extension = 0 and moreover Kx = 3r and Kx = 6r'. x=a+b solve these equations. i got 15m.

You cannot use v^2 - u^2 = 2as as retardation is not constant.

 

[nims11]

consider extension produced be x. then force that pulls back the bodies will be Kx. let retardation of 3kg mass be r and 6kg mass be r'. and let extension produced by 3kg be a and by 6kg be b. then 2ra = 1^2 and 2r'b = 2^2. (v^2 - u^2 = 2as) and final velocity after reaching max extension = 0 and moreover Kx = 3r and Kx = 6r'. x=a+b solve these equations. i got 15m.

you are ignoring the fact that the retardation is not constant and it varies with x.
Also the the final velocity of the two blocks wont be zero.
At max extension, the velocity of the blocks will be same and equal to the velocity of the center of mass(1 m/s for the given question) of the system. velocity of center of mass remains the same throughout the motion, so calculating it is easy.
in other words, the relative velocity of the center of mass and the two blocks is zero with respect to each other.
using conservation of momentum will also lead to the final velocity of the blocks as the velocity of the center of mass. energy and momentum conservation is more handy in such cases. if you want proceed your way, you will need to do some calculus which is undesirable in objective-questions based exams.

BTW do you guyz know a shortcut(Cutting the spring method) for solving these kinds of questions? its really handy and allows us to observe each block independently and then combine the result. i use it frequently and is really helpful in saving time.