# 568 - Physics Questions Answers

Q IF IN THE SAME QUESTn OF BAROMETER THE ELEVATOR IS ACCELERATING UPWARD READS 76cm. THEN AIR PRESSURE IN ELEVATOR IS >76cm PLS EXPLAIN THE CONCEPT BEHIND SUCH CASES.

**Asked By: SARIKA**

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**Joshi sir comment**

when elevator will move in upward direction the extra Hg level will feel a down force and given that in movable condition it is 76 cm. Hg so really it will be more than 76 cm.

Q A CURVE IS REPRESENTED BY y=sinx IFx IS CHANGED FROM PIE/3 TO PIE/3+PIE/100, FIND APPROX. CHANGE IN x?

Q A CAR IS GOING DUE NORTH AT A SPEED =50km/h IT MAKES A 90* LEFT TURN WITHOUT CHANGING SPEED THE CHANGE IN VEL. OF CAR IS=? (ANS 70km/h towards south west ) i want to know that here we are just changing the directn so only directn. shd. b change but here magnitude is also changing HOW?

FLUIDS

Q A BAROMETER KEPT IN N ELEVATOR READS 76cm WHEN IT IS AT REST . IF ELEVATOR GOES UP WITH INCREASING SPEED READING WILL BE <76 cm . IS REASON IS THAT PRESSURE IS DIRECTLY PROPTn TO HEIGHT ?

**Asked By: SARIKA**

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**Joshi sir comment**

I think we have to caculate change in y

since y = sin x

so dy = cosx dx = cosπ/3* π/100 = π/200

the initial velocity of car = 50j and after turn velocity = -50i here i and j are the unit vectors along east and north direction

so change in velocity = -50i - 50j

its mod = 50 √2 = 70

Because in barometer the extra Hg level will experience a downward force so Hg level will decrease.

Q A VECTOR A POINTS VERTICALLY UPWARD & B PTS. TOWARDS NORTH. VEC. PRODUCT ACROSS B IS ALONG WHICH DIRECTION?

Q LET < B/W TWO NON ZERO VECTORS A & B BE 120* & ITS RESULTANT BE C THEN

1) C MUST BE EQUAL TO MOD. OF A-B

2) C MUST BE LESS THAN MOD. OF A-B

3)C MUST BE GREATER THAN MOD. OF A-B

4)C MAY BE EQUAL TO MOD. OF A-B

**Asked By: SARIKA**

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**Joshi sir comment**

Take your right hand fingers along A and curl these fingers along B, thumb will give you an idea for A × B. for the given problem answer will be west.

|A+B|^{2} = |A|^{2} + |B|^{2}+ 2A.B (1)

|A-B|^{2} = |A|^{2} + |B|^{2} - 2A.B (2)

on subtracting (2) by (1)

|A+B|^{2} - |A-B|^{2 }= 4A.B

so C^{2} = |A-B|^{2 }+ 4A.B

or C^{2} = |A-B|^{2} +4|A||B|cos120

since cos120 is negative so C will be less than |A-B|

Q E=ELECTROMAG F, G=GRAVT.F, N = NUCLEAR F B/W 2 e- AT A GIVEN PTN. THEN E>G>N HOW=? BECAUSE G< THEN OTHER FORCES.

**Asked By: SARIKA**

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**Joshi sir comment**

for the distance upto fermi level order will be N>E>G

but the distance between electrons is greater than fermi level so the given order is justified because now nuclear forces will be very weak.

Q A PARTICLE START MOVING FROM REST STATEVALONG A STRAIGHT LINE UNDER ACTION OF A CONST. F & TRAVEL DIST. X IN FIRST 5 sec THE DIST. TRAVELLED IN NEXT 5 sec WILL BE=?

QA CUBE OF A SIDE a IS HEATED TO RAISE ITS TEMP. BY 3*C IF Y IS COEFF. OF VOL. EXPANSION THEN TOTAL INCREASE IN AREA=? (ANS 12Yasqure. )

WAVES & OCILL. Q OBSV. IS MOVING TOWARDS A SOURCE AT REST WITH 30m/s if sound is sounding a sound of freq 500Hz & speed of sound is 300m/s then wavelenth by obsv. =?

Q TEMP. OF GAS INCREASED BY 2K KEEPING PRESS. CONST. SPEED OF SOUND IN GAS=? ans=1.22m/s

Q THE MASS OF ELEVATOR = 4000kg .STARTING FROM REST HOW MUCH DIST. WILL IT RISE IN 2s IF TENSION =56 kN . I want to know why we use ,' a ' for cal. dist. why 'a+g' is NOT use.

**Asked By: SARIKA**

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**Joshi sir comment**

by formula s = ut + 1/2 a t^{2}

u = 0 [given]

so s = 1/2 a (5)^{2}= 25a/2 for first 5 sec.

and s = 1/2 a (10)^{2} = 50a for total 10 sec. from start

so for next 5 sec. distance = 50a - 25a/2 = 75a/2

answer : a'^{3} = a^{3} (1+ γ3) here a' is the side of the cube after heating it

so a' = a (1+3γ)^{1/3}

so area increment = 6a'^{2} - 6a^{2} = 6a^{2} [(1+3γ)^{2/3} - 1] = 6a^{2} [ 2γ] = 12γa^{2}

Please submit all the questions seperately.

λ = v/n = 300/500

i think you are asking about increment in speed

so according to formula v'/v = [(T+2)/T]^{1/2}

or v'/v = 1 + (1/T)

so v' - v = v(1/T) = 332*1/273 = 1.22 here 332 is the speed of sound at 273K

when a machine works , you should not count gravity, for example if we raise our hand upword with uniform velocity then gravity makes no problem in it because our hand is working by our internal capabilitites

Q A PARTICLE STARTS MOVING WITH ACC.=2m/s 2 DISTANCE TRAVELLED BY IT IN 5th HALF SECOND IS=?

Q A PARTICLE MOVES IN A STRAIGHT LINE & ITS POSITION X AT TIME t IS GIVEN BY X RAISED TO P. 2=2+t . ACC. IS GIVEN BY=? (ans . -2/4t cube)

**Asked By: SARIKA**

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**Joshi sir comment**

first calculate distance of 4 seconds by using s = ut + 1/2 a t^{2}

then distance of 4.5 sec. by same formula

difference of these 2 is the distance of 5th half sec.

but some writter consider 5th half as 1/2 +1/2+1/2+1/2+1/2 means 2.5

so they use the same formula first for 2 sec then for 2.5 sec, then their difference.

according to my opinion first is more true way to solve.

Dear Sarika please submit only one question in a box, for a new question use another box so that i could give answers more quickly.

According to the given condition

x^{2} = 2+t

on differentiaing we get 2x dx/dt = 1

or v = 1/2x

now differentiate it again we get dv/dt = -(1/2x^{2} )(dx/dt) = -(1/2x2 )(-1/2x) = 1/4x^{3}

Q HEAT & WORK ARE EQUIVALENT. THIS MEANS

TEMP OF BODY CAN BE INCREASED BY DOING WORK ON IT. PLS TELL HOW THIS STAT. IS CORRECT.Q

Q IF H IS SUPPLIED TO IDEAL GAS IN ISO PROCESS. THEN W= +ve HOW? BECAUSE IF ISO THEN U= 0 & EQN IS Q=W+0 & H IS SUPPLIED SO Q=-veSO W =-ve.so ans shd be -veBUT ANS GIVEN IS +ve PLS TELL ANS WITH EXPL. THANKS FOR PRE SOLUTIONS .

**Asked By: SARIKA**

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**Joshi sir comment**

This means heat can be used to make work and work done produces heat

By doing 4.2 joule work 1 cal. heat will be produced in a body so due to this 1 cal, temp. will increase

In physics heat is supplied means Q will be +ve so W will also be positive hence is correct. A system is placed in refrigrator for absorbing heat from the body, this process will have Q -ve.

Q IF Al DIPPED IN H2O AT 10*C THEN FORCE OF BUOYANCY WILL INCREASE OR DECREASE?

Q TEMP. OF A SOLID OBJECT IS OBSV. CONST .DURING A PERIOD . IN THIS PERIOD

a)HEAT MAY HAVE BEEN SUPPLIED TO IT

b) H MAY HAVE BEEN EXTRACTED FROM IT

c) no H SUPPLIED TO IT

d)NO H EXTRACTED FROM IT

more than one opt is correct . pls explain your each opt with expln.

**Asked By: SARIKA**

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**Joshi sir comment**

It depends that either Al or water will change its density more rapidly, because Bouyancy is based on densities of solid and liquid

Answer : a, because it may be an isothermal process

b, because in isothermal process, surrounding can also give heat to the system.

c, may be the process is adiabatic

d, may be adiabatic.

Q gas of certain mass at 273K is expanded to 81 times its vol. under adia. condition. if gamma = 1.25 for gas then its final temp. = ?

Q for a const. vol. gas thermometer one should fill gas at high temp & low pressure WHY ?

Q iF TEMP of uniform rod is slightly increase by del T its moment of inertia about 90* bisector increase BY = ? (ANS 2alpha I del T ).

Q Al sphere is dipped into H2O at 100*C if temp is increased forse of buoyancy will inc. OR decreased WHY =?

DENSITY OF A SUB. IS 10g/cc at 0*C &at 100*C d= 9.7 g/cc . coeff. of linear expantion is =?

**Asked By: SARIKA**

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**Joshi sir comment**

Answer : For adiabatic process TV^{γ-1 }= K

so 273*V^{0.25 }= T*(81V)^{0.25}

so 273/3 = T

Answer : Because real gases perform ideal behaviour in this range.

Answer : Inertia about perpendicular bisector = 1/12 ML^{2}

on increasing temperature by dT new inertia = 1/12 M[L(1+αdT)]^{2}

so increment in inertia = 1/12 ML^{2} [(1+αdT)^{2}-1^{-}] = 1/12 ML^{2} [ (αdT)^{2} + 2αdT ] now neglect (αdT)^{2 }because it is small term

Answer : Since temperature is already 100 degree centigrade so on increasing it further density of Al will increase rapidly in comparision to density of water so Buoyancy force will be increased.

Answer : use the formula d_{100} = d_{0 }/ (1+γ100) here γ=3α calculate α

Q Agraph having temp *C along xaxis& *F along y axis. if graph is straight then it intercept +ve y axis. WHY ?

**Asked By: SARIKA**

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**Joshi sir comment**

(F-32)/9 = t/5

so F = 9t/5 + 32

on taking t = 0 we will get F = 32

thats why it makes an intercept of 32 in y axis