Calculating Thrust & Initial Acceleration of Saturn V

[Amber_]

Homework Statement

The first stage of a Saturn V space vehicle
consumes fuel at the rate of 47400 kg/s, with
an exhaust speed of 2820 m/s.
The acceleration of gravity is 9.8 m/s2 .
Calculate the thrust produced by these en-
gines.
Answer in units of N.

(part 2 of 2) 10.0 points
Note: You must include the force of gravity.
Find the initial acceleration of the vehicle
on the launch pad if its initial mass is 1.1 ×
107 kg.
Answer in units of m/s2.

Homework Equations

vrel*r= ma = thrust

The Attempt at a Solution

For the first part, I got 133668000 N, which is right. For the second part I divided the answer to the first part by the given mass. I got 12.51 m/s^2. Then to account for the acceleration of gravity, I subtracted 9.81 m/s^2 and got 2.3 m/s^2. However, that's not the correct answer. What am I doing wrong here? Can I not substract accelerations like that? It seems to me, I should be able to-- They're vector quantities acting in opposite directions.

 

[rock.freak667]

You need to formulate the expression using Newton's 2nd Law. You'd need to divide by the mass of the fuel to get its acceleration.

ma=Thrust-mg, find 'a'.