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A man carries a bag hanging it in his hand and he moves horizontally. The bag does not move up or down. What is the work done on the bag? The man gets tired after some time of the movement. Why?

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3 Answers 3

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no mechanical work is being done by the man on the bag. His muscles are notheless getting tired from converting chemical (metabolic) energy into heat.

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Man's muscles are pushing the bag horizontally, applying force that is doing work on the object in the direction of movement. Also, force due to air resistance is doing work in opposite direction of the movement. The man becomes exhausted because he is burning off the calories required by the muscles to do work.

Also, as the bag is not moving up and down , work done by gravity is zero

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Consider the bag at a fixed vertical position as a closed thermodynamic system and apply the first law of thermodynamics to the system. There is no mechanical work done on the bag since its kinetic energy is constant; also there is no other work (in the thermodynamics sense) and no heat transfer to the bag so the internal energy of the bag is constant.

Consider the person as a closed thermodynamic system, and apply the first law of thermodynamics to the system. There is no work done on the person and no heat transfer to the person, and the internal energy of the person is constant. But the person is burning calories for the muscles to keep the bag from falling. The calories burned increase the temperature of the person, but the total internal energy of the person is constant, ignoring heat transfer from the person to the surroundings.

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