General Physics Homework Help...please?

Question

A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale breifly reads only 0.75 of the person's regular weight. Calculate teh acceleration of the elevator, and find the direction of acceleration.

Answer 1

Force acting on the body besides gravity
F=ma

Force of gravity or weight is
W=mg

F(scale)=W+F=
also
F(scale)=0.75W
0.75W=W+F
F=0.75W - W= - 0.25W=ma
a=-- 0.25mg/m=-0.25g

The minus sign means that acceleration is in apposite direction than acceleration of free fall.

Answer 2

Have you been in an elevator that goes up quickly? Don't you feel in increased pressure on your feet, ankles, legs? You suddenly weigh more when it accelerates upward. If it accelerated at 9.8m/sec^2, you would weigh twice (100% more) as much as you do when the elevator is at rest.

Since the person weighed less, the elevator has to be going down. If the elevator accelerated down at 9.8m/sec^2, you and the elevator would fall at the same rate, and you'd become weightless. Since the weight is reduced by 25%, the acceleration must be 25 % of G, or 0.25x9.8m/sec^2.

Think about this and make sure it makes sense to you.