# JEE:: (PCM questions ) Help Please !

Q-1)[IIT JEE 2012] - A person blows into open end of a long pipe.As a result , a high pressure pulse of air trevels down the pipe . when this pulse reaches the other end of the pipe.Then

A)- a high pressure pulse starts treveling up the pipe , if the other end is open.

B)- a Low ...................................................................................... , if the .......................open.

C)- a Low....................................................................................... , if ...............................Closed.

D)- a High...................................................................................... , if ...............................closed.

Q-2)- the equation(s) of two sides AB and AC of an isoscles triangles ABC are are $x+y=5$ & $7x-y=3$ respect. Then equation(s) of sides BC if Area(ABC)=5 is/are: ?

Q-3)- does ortho hydroxy aniline shows Steric inhibition effect ?Please Explain

Q-4)- there are 5 different circles are in placed in incresing order of their radius and each circle touches it's adjacent circle and Two lines L1 and L2 are common tangent to all circles . Radius of smallest is R1=8 and largest is R5= 18 . Then find R3= ? (middle circle)

Thanks

Note by Karan Shekhawat
4 years, 5 months ago

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1) It's B and D.

The air in the tube vibrates in one of it's resonant frequencies. If the other end of the tube is open, the air particles near this end are free to vibrate and thus we have a displacement antinode, i.e a pressure node here, giving B.

The opposite is true if the pipe is closed and thus option D is also correct.

(Also note that the word 'high' in 'high pressure wave' as given in the question may seem ambiguous to some people but it only means that the wave has enough energy to reach the other end)

2)The calculations are plain boring, so the steps would be:

i)Note that the angle between sides AB and AC is $45^{o}$.

ii)Write the area as $\Delta=\frac{1}{2}bcsinA=\frac{1}{2}b^2sin45^{o}=5(given)$

iii)Hence find the length of altitude from A onto side BC(you will need to know $cos(22.5^{o})$).

iv)Write equations of angle bisectors and find the points on these lines that are at a distance equal to the length of the altitude from A.

v)Write the equation of BC using these points and the fact that BC will be perpendicular to the angle bisectors(as it is isosceles)

3)In a lot of cases, the ortho effect is often categorised as a steric effect and this does indeed operate in ortho-hydroxy anniline and leads to a decrease in basicity. Thus, I think they are talking about ortho effect here.

The -OH group is too small to contribute to the conventional steric hindrance.

4)Using trigonometry it is easy to prove that:

$\frac{r_{2}-r_{1}}{r_{2}+r_{1}} =\frac{r_{3}-r_{2}}{r_{3}+r_{2}}$

Apply the componendo-dividendo rule (or cross-multiply) to get: $r_{2}^2=r_{1}*r_{3}$

Thus the radii are in G.P and thus,

And thus, $r_{3} =\sqrt{r_{1}*r_{5}}=12$.

- 4 years, 5 months ago

Thanks a lot ....

But Please I didn't understand 1st Question (Physics one) ...... I did not understand question .... what it is mean by " High pressure pulse ..... andhow you conculude that low pressure pulse is moving from bottom to top if other end is open ? .... I did not understand .... Please explain it more ... thanks ! I 'am greatful for your help.... @Shashwat Shukla

Also in Q-3) - Answerr which I have is that Ortho hydroxy doesn't show steric inhibition effect .....

Also Can you please tell priorty order of SIP & SIR & M-effect while comparing bascity ....

- 4 years, 5 months ago

You're welcome :)

In question 1, 'high pressure pulse' simply means that the pulse carries enough energy to reach the other end of the tube. (The energy of a wave is proportional to the square of the pressure amplitude.) It does not mean that the pressure near the mouth of the man or anywhere else is always high.

Also I concluded the answers based on the assumption that the oscillations in the tube are of a $resonant$ frequency. (this is always valid after a little time from the beginning as all other frequencies die away). We know the results for resonance in an open and closed tube (it's also there in physics NCERT class XI) and that gives the answer.

In question 3, the answer you have is correct. As I said, the -OH group is too $small$ to contribute to steric hindrance. The ortho effect is not the $same$ as steric hindrance and is only related to it. Therefore I guess my initial reasoning was incorrect.

Can you please give full forms for SIP and SIR?

- 4 years, 5 months ago

Thanks again shashwat .. now I got it ...!

And I mean [SIP] --- Steric inhibition in protination

And By [SIR] ----- Steric inhibition in Resonance

- 4 years, 5 months ago

i dont think so ortho hydroxy aniline shows Steric inhibition effect as the incoming electropile will approach the other positions. Steric inhibition effect is showed only when a neucleophile attacks the compound between two large functional groups. I hope my logic is right. If wrong please do correct me.

- 4 years, 5 months ago