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NTA Abhyas 2020
Exam
NTA Abhyas
Year
2020
Subject
Physics
Chapter
Units and Dimensions

NTA Abhyas 2020 · Physics · Units and Dimensions

If velocity, force and time are taken as the fundamental...

If velocity, force and time are taken as the fundamental quantities, then using dimensional analysis choose the correct dimensional formula for mass among the following. [K[K is a dimensionless constant]]

Options

  1. AQ=Kv−1FTQ = Kv^{- 1}FT
  2. BQ=Kv3FT2Q = Kv^{3}FT^{2}
  3. CQ=2Kv−1FTQ = 2Kv^{- 1}FT
  4. DQ=3Kv2FTQ = 3Kv^{2}FT

Correct answer

A. Q=Kv−1FTQ = Kv^{- 1}FT

Step-by-step solution

Let the quantity be Q, then,

Q=f(v,F,T)Q = f ( v, F, T )

Assuming that the function is the product of power functions of v,Fv, F and TT,

Q=KvxFyTz...(i)Q = Kv^{x}F^{y}T^{z}... ( i ) & &

where K is a dimensionless constant of proportionality. The above equation dimensionally becomes

[Q]=[LT−1]x[MLT−2]y[T]z[ Q ] = [ LT^{- 1}{]}^{x} [ MLT^{- 2}{]}^{y} [ T{]}^{z}

i.e., [Q]=[MyL(x+y)T(−x−2y+z)][ Q ] = [ M^{y}L^{( x + y )}T^{( - x - 2y + z )} ], Now

Q = mass i.e., [Q]=[M][ Q ] = [ M ]

So Equation (ii) becomes

[M]=[MyL(x+y)T(−x−2y+z)][ M ] = [ M^{y}L^{( x + y )}T^{( - x - 2y + z )} ]
its dimensional correctness requires

y=1,x+y=0y = 1, x + y = 0and −x−2y+z=0- x - 2y + z = 0

which on solving yields

x=−1,y=1x = - 1, y = 1and z=1z = 1

Substituting it in Equation (i), we get

Q=Kv−1FTQ = Kv^{- 1}FT

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