# Homework Help: Kinematics problem

1. Sep 24, 2011

### shawli

1. The problem statement, all variables and given/known data

I've attempted this problem for just Laura so far.

t1 = 2.00s
t2 = 10.4s - 2.00s = 8.40s

vi = 0.0m/s
vf at the end of 2.00s = vmax

For her displacement I have a little diagram:

|------------------|---------------------------|
........x..............xf = 100 - x.....

where the split between the two sections occurs at whatever x is after 2.00s.

a=? for the initial time interval from 0.00s to 2.00s.
a=0 from 2.00s to 10.4s.

2. Relevant equations

For constant velocity: xf = xi + v*t

For constant acceleration:

3. The attempt at a solution

From t=0.00 to t=2.00:

(using eq'n 2.15, where xf=x, xi=0, vxi=0, and t1 = 2.00)

x = 0.5 * v * t2
x = 0.5 *v*2.00
x = v (???)

From t=2.00 to t=10.4:

(using constant velocity equation, where xi=0, and xf = 100 - x, and t2=8.40)

100 - x= v * t2
100 - x = v*8.40

And basically when I equate the two equations I don't get the right answer (which is a=5.32m/s...). Help!

2. Sep 24, 2011

### Delphi51

All your work looks correct - I don't know how you missed the final answer. You have that the distances for the two parts are v and 8.4*v.
So v + 8.4*v = 100. That works out to v = 10.6.
a = Δv/Δt = 10.6/2 = 5.3 m/s².

3. Sep 25, 2011

### shawli

God Da-...Ugghh.

Well then. Frustrating, but I guess it feels good to know I had it all along haha.

Thank you :)