I'm afraid that HallsoOfIvy's post has two fundamental errors: the force when the elevator is rising is m(g+a), and when descending is m(g-a). He wrote ma + g and ma-g, but you can tell that's not correct because the units don't work out: ma is units of force (i.e. Kg-m/s^2), and g is acceleration (m/s^2). The two equations you should be starting with are:
m(g+a) = 1.2g
m(g-a) = 0.8g
Regarding the right hand side of these equation, note that the spring balance gives readings in Kg, which strictly speaking is incorrect - a scale or balance measures force, not mass. When it gives a reading of 1.2 Kg what that really means is the spring balance is experiencing a force equivalent to 1.2 Kg under normal Earth gravity; hence the magnitude of that force is 1.2g.
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Last edited by ChipB; Aug 17th 2016 at 08:08 AM.
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