Why Does the Scale Reading Change in an Elevator?

[joe215]

Homework Statement

A person with a weight of 500N stands in an elevator on a scale.
from 0-5s his weight reads 500N.
from 5-10s his weight reads 700N.
from 10-15s his weight reads 500N.
from 15-20s his weight reads 300N.

Find his acceleration (in each time interval) and his velocity at 5s, 10s, 15s, and 20s.

The Attempt at a Solution

I think my accelerations are correct:
(Fn-mg) /m=a

0-5s: (500N-500N) / 51kg= 0m/s^2
5-10: (700-500)/51=3.92
10-15: (500-500)/51=0
15-20: 300-500/51= -3.92

Now, I think my velocities seem a little unreasonable.
vf=vi+at

at 5: v=0
10: v=0m/s + (3.92)(10)=39.2m/s <- that is really fast!
15: v=39.2m/s + (0)(15)= 39.2m/s
20: 39.2m/s+(-3.92)(20)= -39.2m/s

thanks for any help

 

[Doc Al]

Now, I think my velocities seem a little unreasonable.
vf=vi+at

That time is the time in the interval for which the acceleration is "a", not the time from the beginning. I'd write it as: Vf = Vi + aΔt.