Newton's Third law problem Astronaut and Force

[sweetheart901]

The question states:

How much force does an 81.0 kg astronaut exert on his chair while sitting at rest on the launch pad?

the answer that I got was 794 N
using...
W=m*g and
Fnet=w+fchair=0

But I cannot seem to get the right answer for this second question:

How much force does the astronaut exert on his chair while accelerating straight up at 18.0 m/s^2?


I know that Fnet= w+ Fchair
and also Fnet= m*a
so 81 kg * 18m/s^2= 794 + Fchair
the answer I get for Fchair is 664 but its not the correct answer.
what am I doing wrong??

 

[Nabeshin]

Recall forces have directions associated with them, and thus signs. Be careful with your signs.

 

[sweetheart901]

Thank you! I guess that was my fault for making a silly error >.<