How to calculate force and acceleration in a space shuttle launch?

[runningirl]

Homework Statement

Right after clearing the launch tower, the space shuttle is accelerating upward at 6 m/s/s.

a) Calculate the net force on an 80 kg astronaut during this acceleration. \
b) Calculate the force that the seat must exert on the 80 kg astronaut during this acceleration.
c) A few seconds later in the flight, the seat is exerting a force of 2080 N on the astronaut. At what rate is the space shuttle accelerating?

Homework Equations

f=ma
x=vot+.5(at^2)

The Attempt at a Solution

a) f=ma
f=80(6)
f=480 N

i did not know how to solve parts b and c.

 

[HallsofIvy]

b) Without any acceleration at all, the seat exerts 80g N= 80(9.81) N on the astronaut. Add that to 480 N.

c) Subtract off 80g N and set that equal to ma. Solve for a.

 

[runningirl]

so far part a, i should do:

80(9.81) and then add that to 480
and why would i subtract 80 from 2080 N in part c?thanks so much!