Question

100kg man stands on a spring balance in an elevator .when the elevator starts moving , the scale reads 90kg. what is the magnitude and direction of acceleration of the elevator

Answer 1

Mg-Ma = 90*10
Ma =100*10-90*10
a = 1000-100/10=1 m/s

Answer 2

The vector sum of these forces will be the net force acting on the man, and Newton's Second Law says Fnet = ma, so the vector sum of those forces = the man's mass times his acceleration.

The direction is the acceleration is upward. Its magnitude is 90 m/s^90. So the net force has to be upward as well. Should be 100 kg times 90 m/s^90, or 900 Newtons upward.