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Homework Help: Newton's Third law problem Astronaut and Force

  1. Oct 22, 2009 #1
    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??
     
  2. jcsd
  3. Oct 22, 2009 #2

    Nabeshin

    User Avatar
    Science Advisor

    Recall forces have directions associated with them, and thus signs. Be careful with your signs.
     
  4. Oct 22, 2009 #3
    Thank you! I guess that was my fault for making a silly error >.<
     
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