Kinematics: Rocket/Free Fall Problem

[beauch1322]

Homework Statement

A one stage booster rocket has a Xo of 0m and a Vo of 0 m/s. The rocket takes off with a constant acceleration until 5 seconds when the boosters turn off and goes into free fall. The rocket is set to explode the instant it hits it's max height. The max height is 1200m.

What is the acceleration for the first 5 seconds?
What is the Vf for the first 5 seconds?
What is the total time of flight?

Homework Equations

Xf=Xo+Vo(t)+(.5)a(t^2)
Vf=Vo+at
(Vf^2)=(Vo^2)+2a(Xf-Xo)

The Attempt at a Solution

1st Part
Xo1=0m
Xf1=Xo2=
Vo1=0m/s
Vf1=Vo2=
a1=
t1=5s

Part 2
Xo2=Xf1=
Xf2=
Vo2=Vf1=
Vf2=0
a2=-9.8 m/s^2
t2=

Also we know: Xf1+Xf2=1200m

I kept trying to get to an equation where I could boil it down to one variable but I can't seem to do it... I have used a lot of substitution, please help...


THANK YOU FOR YOUR TIME!

 

[Mindscrape]

You've got a good start. For part 1 you can simply say vf=a*t. Now the problem is really what I'm sure you've done over and over, just a kinematic problem with an initial velocity and height. Use that "final velocity" as an initial velocity in problem two. Then in problem two you can use the distance kinematic equation, and you can get time from the velocity kinematic equation. So on and so forth. Try it out with some confidence and come back if you're stuck.

 

[beauch1322]

got it. I used a m/s vs t graph and solved for Vf1... 1200= (5*Vf1)/2 + (Vf1*(Vf1/9.8))/2