Calculate the acceleration of the elevator

[bpw91284]

Homework Statement

A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads
only 0.78 of the person’s regular weight.
a. Calculate the acceleration of the elevator.
b. Find the direction of acceleration.

Homework Equations

Sum of forces.

The Attempt at a Solution

I know the equation is F_normal-mg=ma but don't understand how it was derived.

 

[Decan]

The normal force points in the y direction and is opposite to the weight of the person (mg). Newtons law says that the sum of all forces = ma. Since the elevator is only moving in y direction, the only component of the net force is the component in the y direction. Put the 2 together and you have F_n -mg = ma

Now that you know this...what are your thoughts on the problem?