Elevator Question: Calculating displacement from weighing scale readings

[jisbon]

Homework Statement

Bob has a mass of 50kg. He stands on a weighing scale inside an a elevator which moves up from stationary position. Reading on scale shows 60kg for first 10 seconds and 45kg for the next 20 seconds. Assuming g is 10m/s^2, what is displacement of elevator after 30 seconds?

Relevant Equations

Fnet= F-mg
F=ma

EDIT: As per @Orodruin 's suggestion(s), I have edited my problems and equations, sorry for the confusion as I typed this out hasty.

So from this question, I understand I have to find the net force for the first 10 seconds and deceleration for 20 seconds.

The scale shows the force with which the elevator acts on Bob, hence for first 10 seconds,

$F = 60kg * 10m/s^2= 600N$
$F_{net} = 600N- (50)(10) = 100N$
$Acc_{elevator 10seconds} = 100N/50kg = 2m/s^2$
$Velocity_{elevator 10seconds} = at = (2)(10) =20m/s$

On the other hand, for next twenty seconds
$F = 45kg * 10m/s^2= 450N$
$F_{net} = 450N- (50)(10) = -50N$
$Acc_{elevator 10seconds} = -50N/50kg = -1m/s^2$

Displacement = Displacement of first 10 seconds + Displacement of next 20 seconds
= $(ut+\frac{1}{2}at^2) + (ut+\frac{1}{2}at^2)$
= $(0(10)+\frac{1}{2}(2)10^2) + (20(20)+\frac{1}{2}(-1)20^2)$
= 300m

 

[Orodruin]

Homework Statement: Bob has a mass of 50kg. He stands on a weighing scale inside an a elevator which moves up from stationary position. Reading on scale shows 60kg for first 10 seconds and 45kg for the next 20 seconds. Assuming g is 10m/s^2, what is displacement of elevator after 30 seconds?
Homework Equations: Fnet= F-mg
F=ma

For first 10, Fnet = 60*10=600

This is incorrect. First of all, you are missing units, which is a cardinal sin in physics and natural science in general. Second, this is not the net force. The scale shows the force with which the elevator acts on Bob. This is not the only force acting on Bob.

 

[jisbon]

This is incorrect. First of all, you are missing units, which is a cardinal sin in physics and natural science in general. Second, this is not the net force. The scale shows the force with which the elevator acts on Bob. This is not the only force acting on Bob.

Hi there.

Sorry for kind of breaking the rules :( Was typing this in a haste, so didn't put in those steps.

Have corrected as per your suggestion, is it correct for you? Thank you :)