BHU PMT BHU PMT (Screening) Solved Paper-2005

  • question_answer
    A force F is given by\[F=at+b{{t}^{2}},\]where t is time. What are the dimensions of a and b?

    A)  \[[ML{{T}^{-1}}]\]and\[[ML{{T}^{0}}]\]

    B)  \[[ML{{T}^{-3}}]\]and\[[M{{L}^{2}}{{T}^{4}}]\]

    C)  \[[ML{{T}^{-4}}]\]and\[[ML{{T}^{1}}]\]

    D)  \[[ML{{T}^{-3}}]\]and\[[ML{{T}^{-4}}]\]              

    Correct Answer: D

    Solution :

                      Key Idea: Every equation relating physical quantity should be in dimensional balance. Given, equation \[F=at+b{{t}^{2}}\] Only similar quantities can be equated, hence dimensions of the terms on both sides of the equation must be same.                 \[[ML{{T}^{-2}}]=a[T]\] \[\Rightarrow \]               \[a=[ML{{T}^{-3}}]\] Similarly                 \[[ML{{T}^{-2}}]=b{{[T]}^{2}}\] \[\Rightarrow \]               \[b=[ML{{T}^{-4}}]\] Note: The physical quantities separated by the symbols +, -. =, >, < etc. have the same dimensions.


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