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Question

# Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.(A). Suma is swimming in a pond.(B). A donkey is carrying a load on its back.(C). A wind-mill is lifting water from a well.(D). A green plant is carrying out photosynthesis.(E). An engine is pulling a train.(F). Food grains are getting dried in the sun.(G). A sailboat is moving due to wind energy.

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Solution

## Work done W=FS cosθ where θ is the angle between F and S(A) : While swimming, Suma pushes the water in the backward direction and as a result of Newton's 3rd law of motion, the water applies an equal force on Suma in forward direction. Hence Suma swims in the forward direction. ∴ In this case, θ=0o ⟹W=FScos0o=FS(B) : Donkey applies a force in the vertical direction to carry the load and its displacement is in the horizontal direction, thus θ=90o in this case. ⟹W=FScos90o=0(C) : Wind-mill applies a force in the vertical direction to lift the water from well and the water also moves in the vertical direction. ∴ In this case, θ=0o ⟹W=FScos0o=FS(D) : During photosynthesis, all the metabolic reactions take place inside the leaf of the plant, but there is no displacement either of the plant or the leaves. ∴ S=0 ⟹W=0(E) : While pulling, the engine applies a force in the forward direction on the train and the train also moves in the forward direction. ∴ In this case, θ=0o ⟹W=FScos0o=FS(F) : When the food grains are getting dried in the sun, they remain still at their position i.e the food grains do not get displaced. ∴ S=0 ⟹W=0(G) : Wind applies a force on the sailboat in the forward direction making the boat to move in the direction of force. ∴ In this case, θ=0o ⟹W=FScos0o=FS

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