Work done= force x displacement in the direction of force. force=mg=20 x9.8 but displacement  is zero since dispacement of the bag is not in the direction of force .  then work done=zero
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The work done by a force is measured by the (dot or scalar) product of magnitude of the force and the displacement of the object.

The muscles give the force to hold the body and its direction is vertically upwards and is equal to the force of gravitation (weight of 20 kg) on the bag.  The bag is in static equilibrium.

Since the person is holding the object stationary for 30 minutes, the displacement is zero.   Hence the work done is zero.


Actually the muscles stretch a bit to generate the restoration force required to balance the weight of the bag.  When the muscles stretch like a spring does, there is some work done by the muscles.

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