A person stands on a scale inside an elevator at rest. The Scale reads 800N. a) What is the person's mass? (b) The elevator accelerates upward momentarily scale read then? (c) The elevator then moves with a steady speed of 5m/s. What Does the scale read?

mass m=${\displaystyle \frac{W}{g}}$
= weight/acceleration due togravity
${\displaystyle =\frac{800}{9.8}=81.6326kg}$

Key idea is the scale reading is equal to the magnitude of normal force N on the passenger from the scale.
${\displaystyle N-F_g=ma}$
${\displaystyle N=F_g+ma}$
This tells us that the scale reading , which is equal to N depends on the vertical acceleration a of the cab. Substituting mg for Fg gives us N = mg +ma = m (g+a)
For any choice of acceleration. a. if it accelerates up the weight will be more because a +ve
if it accelerates downward then weight will decrease because a is - ve then.
If moving with constant speed a =0 so the weight will remain the same as 800N
When suddenly accelates up let us say acceleration = a
Then scale reads m(g + a) = 81.83( 9.8 +a)
When it is moving with constant speed means not moving with any additional acceleration . a = 0
Scale reading = mg = 800N