Forces [ Finding displacement ]

[makemefly]

Homework Statement

Assume you are on a planet similar to Earth where the acceleration of gravity is 10m/s^2 and the positive directions for displacement velocity and acceleration are upward. At t=0sec., an elevator is at a displacement of x=0 with a velocity of v=0m/s. A student whose weight is 400N stands on a scale in an elevator and records the scale reading as a function of time. The data is shown below:

0-5s--------400N
5-10s-------500N
10-15s------600N
15-20s-----0N

g) What is the displacement of the elevator
above the starting point at the end of the
fourth 5 s interval (at 20 s)?
Answer in units of m.


I was only completely sure about a and b, for a the direction is up and for b the mass is 40kg. I obviously don't want just answers, but steps and hopefully explanations to go along with it.

Homework Equations

Vf^2 = Vi^2 + 2a X
X = ViT + 1/2 a t ^2

The Attempt at a Solution

Let
a = acceleration
X = displacement
Vi = Initial Velocity
D(X) = Displacement for time interval

Interval : 0 - 5
a = 0
v = 0
X = 0

Int : 5-10
a = 2.5
v = 12.5
X = 0 + 1/2 (2.5) (5) ^2 = 31.25 m

int : 10-15
a = 5
v = 37.5
X = (37.5)(5) +1/2*(5)(5)^2 +31.25 = 281.25 m

int : 15-20
a= -10
v = -12.5
X = (281.25) + (-12.5)(5) + (1/2)*(-10) (5)^2 = 93.75 m

I submit 93.75 as my final answer and it was wrong. Any help or ideas?

 

[tiny-tim]

welcome to pf!

hi makemefly! welcome to pf!

(try using the X² icon just above the Reply box )

in your constant acceleration equation s = v_it + 1/2 at², i think you're using v_f instead of v_i for c and d