A girl is carrying a school bag of 3 kg mass on her back and moves 200 m on a levelled road. The work done against the gravitational force will be (g=10ms −2 )

Answer 1

Because the force of gravity is perpendicular to the displacement of the girl, the work is 0. Formally it is true. Then the question arises: why are we tired after some walk on the leveled road? The answer is simple - when we walk, our center of gravity is not on one level. It is a little bit goes up or down. Then the force of gravity produces either positive or negative work. It means we also perform work against force of gravity by lowering or rising our center of gravity, and after some time sure will be tired.