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Equation st=u + at - 1/2 a does not seem...

Equation `s_t=u + at - 1/2 a` does not seem dimensionally correct, why?

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The correct Answer is:
A
`S_t=[ut+1/2 at^2]-[u(t-1)+1/2a(t-1)^2]`
`=(u)(1)+1/2 (a)(2t)-1/2(a)(1)^2`
The first term is (u)(1), which we are writing only u.
Dimensional of u are `[LT^-1]` and dimension of 1
(which is actually 1 second) are [T]
`:. [(u)(1)]=[LT^-1][T]`
`=[L]=s_t`
Therfore dimension of (u)(1) are same as the
dimensions of `s_t.` Same logic can be applied with
other terms too.
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