Answer:
You forgot to account for the acceleration due to the gravity of the earth (9.8 m/s²).
The net force must equal the force from thrust minus force of gravity because thrust pushes upward, but gravity pushes downward. The leftover force is the net force where the rocket will be going.
Rearrange the equation to find for F_thrust.
F_net = F_Thrust - F_gravity
F_thrust = F_Net + F_gravity
We're given the net acceleration of the rocket, 1.4m/s, so we can use that to calculate the net force.
F = ma
F = (12000)(1.4)
F = 16800
Then there's the acceleration due to gravity. So the force of that is.
F = ma
F = (12000)(9.8)
F = 117600
Add them up and you get:
F_thrust = F_Net + F_gravity
F_thrust = 16800 + 117600
F_thrust = 134400 N
