Weight in N with change in acceleration

[lajekahr]

2kg weight is suspended on a tension scale that measures in Newtons.
The elevator is moving at constant velocity up of 10m/s. Its velocity is then uniformly reduced to zero in 4 seconds. So that it comes to rest in 4 seconds. What does the scale read during those 4 seconds.

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Using frame of reference with positive down: (That way gravity's acceleration is positive.)
Using Kinematics:
Vf=Vi*t+1/2a*t^2
Therefore:
a=(-2Vi)/t=-5m/s^2
Applying F=ma:
Ftension+Fgravity=ma
-Ft+mg=ma
Ft=mg-ma=2*9.8-2*(-5)=29.6N

My books says: 14.6N

What'd I do wrong?

 

[Meir Achuz]

2kg weight is suspended on a tension scale that measures in Newtons.
The elevator is moving at constant velocity up of 10m/s. Its velocity is then uniformly reduced to zero in 4 seconds. So that it comes to rest in 4 seconds. What does the scale read during those 4 seconds.

---------
Using frame of reference with positive down: (That way gravity's acceleration is positive.)
Using Kinematics:
Vf=Vi*t+1/2a*t^2
Therefore:
a=(-2Vi)/t=-5m/s^2
Applying F=ma:
Ftension+Fgravity=ma
-Ft+mg=ma
Ft=mg-ma=2*9.8-2*(-5)=29.6N

My books says: 14.6N

What'd I do wrong?

Your first equation is wrong.
It should be Vf=Vi+at.

 

[lajekahr]

Now i feel kinda dumb. I knew that I swear...that first equation is for displacement...doh!