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Homework Help: Acceleration and weight question

  1. Sep 3, 2012 #1
    1. The problem statement, all variables and given/known data

    How heavy does a 180Ib person feel if while they are accelerating at a rate of .9 m/s^2?

    2. Relevant equations



    3. The attempt at a solution
    I thought I would use F=W=ma, so .9 * 180, but it says that is wrong.
     
  2. jcsd
  3. Sep 3, 2012 #2
    you have heard of "g force" I hope, so how many "gs" is the acceleration equivalent too?
     
  4. Sep 3, 2012 #3
    I know that it is 9.8 m/s^2, but I'm not sure what gs is?
     
  5. Sep 3, 2012 #4
    "g"s experienced by fighter pilots, astronauts, riders on a roller coaster. All the result of acceleration.

    If that still doesn't help then what causes you to have a weight?
     
  6. Sep 3, 2012 #5
    Gravity causes us to have weight. Are you saying I should multiply .9 by 9.8?
     
  7. Sep 3, 2012 #6
    Gravity gives us mass. Are you saying I should multiply 9.8 by .9?
     
  8. Sep 3, 2012 #7
    I'm not going to say anything yet about how you should calculate or not.

    Gravity does not give you mass so what does gravity do?
     
  9. Sep 3, 2012 #8
    *weight. Gravity gives us weight.
     
  10. Sep 3, 2012 #9
    yes. weight is an example of a _______ . The units of gravity are the same as an ________
     
  11. Sep 3, 2012 #10
    Weight is an example of a force. The units of gravity are the same as an acceleration?
     
  12. Sep 3, 2012 #11
    yes. so now use your f = ma formula and see where each piece you have fits. Beware units.

    How much would you weigh on the moon where gravity is 1/6th that of the Earth.
     
  13. Sep 3, 2012 #12
    I'm still not sure. If I plug them in wouldn't I just get 180*.9? Or is the acceleration given just an arbitrary value and I need 9.81 instead?
     
  14. Sep 3, 2012 #13
    forget about the additional acceleration for now. You are standing on a set of scales in a stationary lift how much do you weigh, how do you know how much you weigh.

    [Physicists always have a set of scales with them in lifts :D ]
     
  15. Sep 3, 2012 #14
    You would weigh 180 * 9.81. You cancel units to determine what you end up with?
     
  16. Sep 3, 2012 #15
    So the 180lb person is standing on their bathroom scales and they look down and is says 180*9.81?
     
  17. Sep 3, 2012 #16
    Ahh! 180/9.81= 18.3 as the mass.
     
  18. Sep 3, 2012 #17
    lbs are an old unit and I suspect the question is using them to see if you are clear in your mind as to the difference between mass and weight. lbs are awkward because they are used for both mass and weight: the 1 lb weight = 1 lb mass conversion works on Earth because the acceleration on Earth is 1 gravity [or 1 "gee"]

    How much does a person who weighs 180lb on earth weigh on the Moon?
     
  19. Sep 3, 2012 #18
    The question actually says pounds, I just shortened it for the problem. If on the moon they weigh 1/6 less, then would it be (1/6)*180
     
  20. Sep 3, 2012 #19
    so same mass but less weight on the Moon. How about on a moon a little smaller than ours where the acceleration due to that moon's gravity is 0.9m/sec/sec [please start giving answers with appropriate units as the correct selection of units is at the heart of the problem]

    (I'm ignoring that 1/6 less is not the same as 1/6th)
     
  21. Sep 3, 2012 #20
    This is the part I don't know. Would it be 9.8/.9 * mass=180Ibs? So 16.5 lbs?
     
  22. Sep 3, 2012 #21
    you appear to be saying that the person's mass changes when they are on the Moon.
     
  23. Sep 3, 2012 #22
    What about...
    9.8*m=180 m=18.4
    18.4 * .9 =16.5 lbs??

    Can you please give some hints since I am really lost here.
     
  24. Sep 3, 2012 #23
  25. Sep 3, 2012 #24
    Ok, converting pounds to kg I get that the person is 81.65. Now would I multiply that by .9?

    I don't want to sound rude, but I've been at this one problem for over an hour and it really seems as though it should be pretty straight forward.
     
  26. Sep 3, 2012 #25
    it is straightforward. but only once you can see the difference between mass and weight. You should then be able to see if the units in the problem are consistent or if some conversion is necessary.

    For myself I kept the lbs weight and converted acceleration into earth gravities aka "gee"s The numbers in the question look a lot like what I might see if I took my bathroom scales into a lift and selected to go up or maybe down.
     
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