Question 21How much energy is gained by a box of mass 20 kg when a man(a) carrying the box waits for 5 minutes for a bus?(b) runs carrying the box with a speed of 3 ms-1 to catch the bus?(c) raises the box by 0.5 m in order to place it inside the bus?(g=10 ms-2)

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Accepted Answer

Given,Mass = 20 kg(a) As there is no displacement of the man, so work done is Zero.(b) We know,Work done = Kinetic energy of man = 1 / 2 × mass × (velocity)2Work done=Kinetic energy of man=12×mass×velocity2=12×20×32=12×20×9=90J\text{Work done} = \text{Kinetic energy of man} \[0.5em] = \dfrac{1}{2} \times \text{mass} \times \text{velocity}^2 \[0.5em] = \dfrac{1}{2} \times 20 \times 3^2 \[0.5em] = \dfrac{1}{2} \times 20 \times 9 \[0.5em] = 90 J \[0.5em]Work done=Kinetic energy of man=21​×mass×velocity2=21​×20×32=21​×20×9=90J∴ Energy Gained = 90 J(c) We know,Potential energy = mghPE=20×10×0.5⇒U=100J\text{PE} = 20 \times 10 \times 0.5 \[0.5em] \Rightarrow U = 100 J \[0.5em]PE=20×10×0.5⇒U=100J

Work, Energy and Power — Question 21

Question 21