Q17 / 92 · NTA Abhyas (JEE Main) 2020 · Physics · Units and Dimensions

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 is a dimensionless constant ]

  • Correct answer
    A
    Q=K v−1 F T
  • B
    Q=K v3 F T2
  • C
    Q=2 K v−1 F T
  • D
    Q=3 K v2 F T

Solution

Let the quantity be Q, then,

Q=f(v,F,T)

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

Q = Kvx Fy Tz  ...(i)     

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

        [Q] = [LT-1]x[MLT-2]y[T]z

i.e.,   [Q] = [MyL(x+y)T(-x-2y+z)], ​Now

Q = mass i.e.,   [Q]=[M]

So Equation (ii) becomes

[M] = [MyL(x+y)T(-x-2y+z)]
its dimensional correctness requires 

y=1, x+y=0 and - x - 2y + z = 0

which on solving yields

x=-1, y=1 and z=1

Substituting it in Equation (i), we get

Q=K v-1 F T

Hence, the correct answer is option: 'A'.

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