A girl of mass 50 kg stands on a measuring scale
in a lift. At an instant it is detected that the reading
reduces to 40 kg for a while and then returns to
original value. It can be said that
(A) the lift was in constant motion upwards
(B) the lift was in constant motion downwards
(C) the lift was suddenly started in downward
motion
(D) the lift was suddenly started in upward motion
Answers
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c as a-(+g) for down motion
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(c) the lift was suddenly started in downward motion.
According to D'Alemberts principle, For a system of mass of particles, the sum of difference of the force acting on the system and the time derivatives of the momenta is zero when projected onto any virtual displacement.
Thus as the lift suddenly started moving downwards, the inertial force due to the girls mass was acting upwards opposing the state of motion and thus, the the reading on the scale was reduced.
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