How Do You Calculate the Final Speed of a Rocket Car on an Inclined Ramp?

[canucks81]

Homework Statement

A 1200 kg rocket car is placed at the bottom of a 100 meter long ramp inclined at 10°. The rocket is turned on and it exerts a force of 8300 N for 5.70 seconds, and then it shuts down. Find the speed with which the rocket car leaves the ramp (assume frictionless).

Homework Equations

http://img98.imageshack.us/img98/9425/54fl1.png

The Attempt at a Solution

http://i.imgur.com/MGDp7Rf.png?1

I didn't show all of my solution but I need to know if that part is correct. What I did next is I assumed vi = 0 and the used the formula vf = vi + at which gave me an answer of 29.7 m/s, however, the correct answer is 28.8 m/s.

 

[haruspex]

You've calculated the max speed. Did you use the ramp length?

 

[canucks81]

What do I do with the ramp length? I didn't use it.

 

[haruspex]

You're asked for the speed at the end of the ramp, not the speed when the engine shuts down.

 

[canucks81]

So would I calculate the distance traveled up to when the engine shuts down, then subtract it from the length of the ramp to find the remaining distance?

 

[haruspex]

That'll work.

 

[canucks81]

I'm not sure how to get the new acceleration from when the engine shuts down. I know that it has to be negative because the rocket will slow down.