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Homework Help: Weight on a rocket accelerating upward

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data

    "During the launch, a rocket is accelerated with 20.0m/s^2 upward. A 105.0kg astronaut is more concerned about his weight than about his safety and is standing on a scale. What is the scale reading in kg?"

    2. Relevant equations

    F=ma and also 4.45N=1lb

    3. The attempt at a solution

    So, first I solved F=ma with the information given to get 2100N, and since I don't think that N can convert to kg with an equation, I took from the front of the book that 1kg=2.20lbs where g=9.80m/s^2, so I thought that I could figure out what 1kg was with an acceleration of 20m/s^2, and I got that 4.5lbs would equal 1kg.

    I then took my previous answer of 2100N and converted it to 471.9lbs and divided by 4.51lbs, hoping to get the correct kg with the acceleration given, but I got 104.6 which seems very wrong (first, because it's almost exactly the starting weight and second, because it should be higher, not lower than the starting weight). Thoughts?
  2. jcsd
  3. Oct 21, 2007 #2
    If his scale was calibrated on the earths surface, so that 1kg =2.2lbs, using just F=ma would yield you the correct answer.
    On earth he feels a weight of mg (105*9.8) of 1029N
    When accelerating he feels a weight of ma (105*20) of 2100N
    Find the ratio and multiply by his mass.
  4. Oct 22, 2007 #3
    Oh, yeah I guess I was over-thinking it. Thanks!
  5. Oct 22, 2007 #4
    No problem
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