Question
A book with many printing errors contains four different formulae for the displacement $y$ of a particle undergoing a certain periodic function:
$i) \ y = a \sin \frac{2 \pi t}{T}$
$ii) \ y=a \sin v t$
$iii) \ y=\frac{a}{T} \sin \frac{t}{a}$
$iv) \  y =\frac{a}{\sqrt{2}}\left[\sin \frac{2 \pi t}{T}+\cos \frac{2 \pi t}{T}\right]$
Here, $a$ is maximum displacement of particle, $y$ is speed of particle, $T$ is time period of motion. Rule out the wrong formulae on dimensional grounds.

Answer

The argument of trigonometrical function, i.e., angle is dimensionless. Now,
$i)$ The argument, $\left[\frac{2 \pi t }{ T }\right]=\frac{[ T ]}{[ T ]}=1=\left[ L ^0 M ^0 T^0\right]$
which is a dimensionless quantity.
Hence, formula $(i)$ is correct.
$ii)$ The argument,
$[vt] = [LT^{-1}] [T] = [L] = [L^1M^0T^0]$
which is not a dimensionless quantity.
Hence, formula $(ii)$ is incorrect.
$iii)$ The argument,
$\left[\frac{ t }{ a }\right]=\frac{[ T ]}{[ L ]}=\left[ L ^{-1} M ^0 T^1\right]$
which is not a dimensionless quantity.
Hence, formula $(iii)$ is incorrect.
$iv)$ The argument,
$\left[\frac{2 \pi t }{ T }\right]=\frac{[ T ]}{[ T ]}=1=\left[ L ^0 M ^0 T^0\right]$
which is a dimensionless quantity.
Hence, formula $(iv)$ is correct.

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