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gneill
#6
Apr16-11, 10:57 PM
Mentor
P: 11,685
Think in terms of the net acceleration. g is an acceleration. The elevator is providing another acceleration acting along the same axis as g. If the elevator is accelerating upwards, then it adds to g (inertial forces generated by its action will act downwards). If the elevator is accelerating downwards, it subtracts from g. The result is a new net acceleration, call it g', that will act on an object contacting the elevator floor.

For your example, and taking g to be 10 m/s2, if the elevator is accelerating upwards at 2m/s2, then the 'weight' of the man on the elevator floor is

(10 + 2)m/s2 * 100kg = 1200N

If the elevator is accelerating downwards,

(10 - 2)m/s2 * 100kg = 800N