# Simple Rocket Problem

1. Jan 30, 2012

### fisselt

I know I've done this problem a few times before but I'm just a fumbling idiot at the moment..

1. The problem statement, all variables and given/known data
Rocket has an acceleration of 12m/s^2 and at an altitude of 1km the motor shuts off. What is the maximum height?

2. Relevant equations
V^2=V_i+2a(x_f-X_i)
x_f=x_i+V_i(t)+1/2at^2

3. The attempt at a solution
V^2=0+2(12)(1000)=154.92m/s
0=154.92t-9.8t^2, t=17.213s
x_f=1000+154.95(17.213)-1/2(9.8)(17.213)^2=2214.82meters

I feel like I'm doing it wrong. 12m/s^2 is the acceleration of the motor, shouldnt their be some force from gravity on the rocket while going towards 1000m?

Thanks for the help.

2. Jan 30, 2012

### PeterO

The acceleration of the motor/rocket combination is the change noticed under the combined influence of the force of gravity, and the thrust force of the rocket motor, and presumably any frictional forces from the air. You are probably supposed to ignore the air resistance - a common approximation used with this sort of problem.

3. Jan 30, 2012

### fisselt

thanks for the help