Q125 / 150 · MHT CET 2020 (13 Oct Shift 2) · Physics

A block of mass 'm', kept on a horizontal surface, is moved through a distance 's' by applying a horizontal force (F) to it. What is the work done by the normal reaction?

  • A
    $\frac{\mathrm{F}}{\mathrm{s}}$
  • B
    $\mathrm{Fs}$
  • Correct answer
    C
    zero
  • D
    $\frac{\mathrm{S}}{\mathrm{F}}$

Solution

In the first case, the centre of gravity is at a height $=\frac{b}{2}$ $\therefore$ Work done $=\mathrm{mgh}=\mathrm{mg}\left(\frac{b}{2}\right)$ In the second case, the height of the centre of gravity $=\frac{L}{2}$ $\therefore$ Work done $=m g \frac{L}{2} \therefore L>b$ $\therefore$ Extra work done $\quad=m g\left(\frac{L-b}{2}\right)$

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

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