Measurement MCQs for NEET — Physics Questions with Answers

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While measuring the acceleration due to gravity by a simple pendulum, a student makes a positive error of 1% in the length of the pendulum and a negative error of 3% in the value of time period. His percentage error in the measurement of g by the relation g=4π2(l/T2) will be

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Explanation

Percentage error in g = (%error in l) + 2(% error in T) = 1% + 2(3%) = 7%

The length, breadth, and thickness of a block are given by l = 12 cm, b = 6 cm and t = 2.45 cm The volume of the block according to the idea of significant figures should be 

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Explanation

Volume V = l × b × t

=12×6×2.45=176.4cm3 

V =1.764×102cm3

since, the minimum number of significant figure is one in breadth, hence volume will also contain only one significant figure. Hence, V=2×102cm3.

A physical quantity of the dimensions of length that can be formed out of c,G and e24πεο is [c is velocity of light, G is universal constant of gravitation and e is charge]

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Explanation

(a) As force F=e24πεor2

  e24πεo=r2.F

putting dimensions of r and F, we get

      e24πεo=ML3T-2      ...(i)

Also force. F=Gm2r2

       G=MLT-2L2M2                      ...(ii)

    G=M-1L3T-2                            ...(iii)

     and   1c2=1L2T-2=L-2T2

Now, checking optionwise. 

               =1c2Ge24πεo12

             =L-2T2 L6T-412=L

            

Planck's constant (h),speed of light in vacuum (c) and Newton's gravitational constant (G) are three fundamental constants. Which of the following combinations of these has the dimensions of lenghth?

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Explanation

(a) In forms of h,c and G length can be expressed as L=ha cb Gc

writing dimensions on both sides,we get MoLTo=ML2T-1a  LT-1b M-1L3T-2c=Ma-c L2a+b+3c T-a-b-2c

On comparing powers of M,L and T on both sides, we get 

a-c=0, 2a+b+3c=1 and -a-b-2c=0

On solving we, get 

   a=c =12 and b=-32  

 Dimensions length  is 

L=h1/2  c-3/2 G1/2=hGc3/2

If energy (E), velocity (v) and time (T) are chosen as the fundamental quantities, the dimensional formula of surface tension will be

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Explanation

We know that  Surface tension(S)=Force[F]/Length[L]
So, [S]=[MLT-2]/[L]=[ML0T-2]

Energy[E]=Force x displacement
=> [E]=[ML2T2]

Velocity(v)=displacement/time

=>[v]=[LT-1]

As, S∝EavbTC

where, a,b,c are constants.

From the principle of homogeneity  
[LHS]=[RHS]

=>[ML0T-2]=[ML2T-2]aLT-1Tc

=>[ML0T-2]=[MaL2a+bT-2a-b+c]

Equating the power on both sides, we get 

a=1,2a+b=0,b=-2

=>-2a-b+c=-2

=>c=(2a+b)-2=0-2=-2

So [s]=[Ev-2T-2]

If force (F), velocity (v) and time (T) are taken as fundamental units, then the dimensions of mass are

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Explanation

We know that
F=ma

=>F=mv/t

=>m=Ft/v

[M]=[F][T]/[v]=[Fv-1T]

The density of material in CGS system of units is 4g/cm3.In a system of units in which unit of length is 10cm and unit of mass is 100g, yhen the value of density of material in this system will be-

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Explanation

 

Density, ρ=MV                ρ=4gmcm3If unit of mass = 100gm1gm = 1100unit of massAnd, if unit of length = 10cm1cm = 110unit of lengthThen,Density, ρ =4100unit of mass1103(unit of length)3=40unit of mass(unit of length)3

The dimension of 12ε0E2, where ε0 is permittivity of free space and E is electric field, is

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Explanation

Using Coulomb's law

F=kq1q2r2 where k=14πoF=14πoq1q2r2Separating o,o=14πFq1q2r2Now dimensions of o,[o] =AT2MLT-2X L2 =M-1A2L-3T4

Dimensions of ε0=M-1L-3T4A2

Dimensions of E=MLT-3A-1

Dimensions of 12ε0E2=M-1L-3T4A2×M2L2T-6A-2=ML-1T-2

If the dimension of a physical quantity are given by MaLbTc, then the physical quantity will be

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Explanation

(i) Dimensions of velocity =M0L1T-1

     Here, a=0, b=1, c=-1

(ii) Dimensions of acceleration  =M0L1T-2

      Here, a=0, b=1, c=-2

(iii) Dimensions of force =M1L1T-2

       Here, a=1, b=1, T=-2

(iv) Dimensions of pressure =M1L-1T-2

         Here, a=1, b=-1, c=-2

  The physical quantity is pressure.  

 

Which two of the following five physical parameters have the same dimensions ?

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Explanation

 

Energy density= Energy Volume=ML2T-2L3=ML-1T-2

Refractive index hole no  dimensions.

Dielectric constant has no dimensions.

Young's modulus,

               Y=FlAl=MLT-3LL2L=ML-1T-2

(5) Magnetic field, 

                    B=FIl=MLT-2AL=MT-2A-1

Therefore,option (c) is correct  

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