What is the Maximum Height and Time of a Rocket's Flight?

[Garrett21]

Homework Statement

A rocket moves upward, starting from rest with an acceleration of 29.4 m/s^2 for 4s. It runs out of fuel at the end of this 4s interval and continues to move upward.
a.) how high does it rise from the ground?
b.)When the rocket will hit the ground?

The Attempt at a Solution

lets say a>b is the 4s with fuel, the rocket would travel 235.2 meters in 4s
from b>c the initial velocity is 117.6 m/s so the rocket will continue for 705.6 m before peaking 12s
from c to the ground the final velocity is 135.79 m/s 13.8564s

a.) 940.8 meters
b.) 29.8564 seconds

Is this correct?

 

[hjelmgart]

You should show us your calculations. I cannot tell if those numbers are correct, unless you show me the math you used to get those. Which kinematic sentences did you use for it? etc..

 

[Sunfire]

Notice, part of the motion is with variable mass (until it runs out of fuel) . From there onward, the mass is constant and it reduces to body with constant mass moving under the influence of gravity.

 

[BvU]

I agree with hjemgart. At the risk of becoming too personal:
Does it really help you when I state: yes, your answers are correct ?
They are, and I estimate you are quite capable to check them yourself. So why do you post ?

 

[Garrett21]

Notice, part of the motion is with variable mass (until it runs out of fuel) . From there onward, the mass is constant and it reduces to body with constant mass moving under the influence of gravity.

Yes i understand this.

I agree with hjemgart. At the risk of becoming too personal:
Does it really help you when I state: yes, your answers are correct ?
They are, and I estimate you are quite capable to check them yourself. So why do you post ?

Just to double check i suppose.
This is my first week studying physics.

 

[BvU]

You're doing fine. Physics is fun: it is great to wonder about the world around you and rewarding to (only now and then) have the feeling to understand a little bit of what's going on and how it works. Don't let the tedious investment of making exercises distract youfrom the fun part. Good luck!