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Answer :
Given,
Mass of the body = 20 kg
Initial velocity( u ) =5 m/s
Final velocity( v ) = 2 m/s
The initial kinetic energy
\(= E_i \ = \ \frac{mu^2}{2} \ \)
\( = \ \frac{20 x (5)^2}{2} \ \)
\( = \ 250 \ kgm/s^2 \)
\( \therefore E_i \ = \ 250 \ J \)
Final kinetic energy
\(= E_f \ = \ \frac{mv^2}{2} \ \)
\( = \ \frac{20 x (2)^2}{2} \ \)
\( = \ 40 \ kgm/s^2 \)
\(\therefore E_f \ = \ 40 \ J \)
\(\therefore Work \ done \ = \ Change \ in \ kinetic \ energy \)
\( \Rightarrow Work \ done \ = \ E_f \ – \ E_i \)
\( \Rightarrow Work \ done \ = \ 40 \ J \ – \ 250 \ J \)
\(\Rightarrow Work \ done \ = \ -210 \ J \)
Where negative sign indicates that force acts in opposite direction.