question_answer

Find the work done by a person in carrying a box weighing 30 kg over his head, when he travels a distance of 20m (i) on a plane ground (ii) vertically up (\[\text{g}=\text{9}.\text{8 m}{{\text{s}}^{-\text{2}}}\])

Answer:

Given,   \[m=30\,\,kgS=20m\]                 \[g=9.8\,\,m/{{s}^{2}}W=?\] (i) On a plane ground the weight of the box acting vertically downwards and displacement is in horizontal. Here, the force and displacement are perpendicular \[(90{}^\circ )\] to each other. We know, \[W=F\,\,\,S\,\,\cos \theta \] \[\Rightarrow W=F\times S\cos \,90{}^\circ \] \[\Rightarrow W=0[\because cos\,\,90{}^\circ =0]\] \[\therefore \]On a plane ground the work done is zero. (ii) Vertically upwards We know,\[\Rightarrow \] \[\Rightarrow W=30\times 9.8\times 20=5880J\]