Finding Constant Acceleration of Train: Liz's Homework Solution

[atypical]

Homework Statement

Liz rushes down onto a subway platform to find her train already
departing. She stops and watches the cars go by. Each
car is 8.60 m long. The first moves past her in 1.50 s and the
second in 1.10 s. Find the constant acceleration of the train.

Homework Equations

change in velocity=a(t)

The Attempt at a Solution

My answer is different from the book. I found the velocity of each cart given the length and time to pass. From there I took the difference of the two velocities to get my change in velocity.
Then, from the equation i found the total time from both cars to pass then divided to solve for a.

change in velocity=a(t)
therefore: 2.11m/s=a(1.5s+1.1s)
a=0.8m/s^2
Book is getting 1.6m/s^2
...so I am off by a product of 2?

 

[tiny-tim]

hi atypical!

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

I found the velocity of each cart given the length and time to pass.

nooo … v = s/t only works for constant velocity …

call the initial velocity v, and get two equations for a and v, from which you can eliminate v to find a