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Pilots, diving and apparent weight

  1. Jun 17, 2013 #1
    1. The problem statement, all variables and given/known data
    While pulling out of a dive, does a pilot's apparent weight increase or decrease?


    2. Relevant equations



    3. The attempt at a solution
    My instinct for this question says increase but only because of the feeling that I remember from being in planes. I'm trying to grasp the physics-based/conceptual explanation but don't really get it too much. And I'm not sure what equations would be relevant. I only understand apparent weight in the context of buoyancy...
     
  2. jcsd
  3. Jun 18, 2013 #2

    NascentOxygen

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    You can think of apparent weight here as being equal§ to the force that the pilot's seat must exert on him to cause his body to follow the same trajectory as his plane.

    § equal but opposite to
     
  4. Jun 18, 2013 #3
    Or, to make it easier, you can assume that the pilot is standing on a weighing machine inside the plane. When the pilot is moving upwards, what are the forces acting on him?
     
  5. Jun 18, 2013 #4
    So wouldn't that just be the normal force which is equal to mg? Mathematically, it seems that his weight would decrease because gravity is acting on him pulling him downwards (mg) and the plane is pulling him up faster than gravity is pulling him down (mg + x). So his apparent weight would be mg - mg+x, like in hydrostatics...
     
  6. Jun 18, 2013 #5

    rcgldr

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    Assume that pulling out of a dive means accelerating upwards.
     
  7. Jun 18, 2013 #6
    So if you're accelerating upwards, wouldn't your apparent weight be less?
     
  8. Jun 18, 2013 #7

    CWatters

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    No, the plane is not pulling him up. It's pushing him up by the seat of his pants. Unlike gravity the planes upward acceleration does not act uniformly on his whole body.

    When you sit in a chair you don't feel gravity pulling you down, you feel the reaction force from the chair pushing you up.

    Perhaps consider how spring scales work.. There are two plates with a spring between them. A scale displays the amount that the spring is compressed. It doesn't matter if the top plate (with pilot on) is accelerated downwards by gravity or if the bottom plate is accelerated upwards by the plane. Both cause the spring to be compressed.
     
  9. Jun 18, 2013 #8
    So, as anything increases in height above the ground, its apparent weight increases? For example, if I lifted an 8 pound weight, it would feel like it was heavier the higher I lifted it?
     
  10. Jun 18, 2013 #9

    CWatters

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    No it's not (necessarily) "increasing height" it's accelerating upwards.

    It's possible for something to accelerate in one direction while still travelling in the other.

    For example consider a car travelling at 50mph. It slams on the brakes and decelerates. Deceleration in the forward direction is the same as acceleration in the backward direction. In both cases the driver is thrown forwards in the seat right?

    The plane is initially in a dive so it has velocity downwards. To pull out it has to first reduce it's vertical velocity to zero =deceleration = acceleration upwards.

    At some point it's vertical velocity will reduce to zero and it will start to climb again but as soon as it starts to decelerate it's actually accelerating upwards.

    It's the upward acceleration that makes the pilot feel heavier.
     
    Last edited: Jun 18, 2013
  11. Jun 18, 2013 #10

    NascentOxygen

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    Definitely not.

    No. But if you replace the word "higher" with "more energetically" you would be spot on. (We are not talking about velocity, as such. The appropriate physics concept here is acceleration, and that's concerned with the changing of velocity.)
     
  12. Jun 18, 2013 #11

    PeterO

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    I suggest you take a set of bathroom scales to a lift in a tall building.

    Enter the lift at the ground floor and press, say, levels 7, 14 and 20.

    Now watch the the reading on the scales as you complete your journey up the level 20.

    Once there, press ground, 8 and 15 and watch the scales during the return trip.

    The scales will at all times be showing your apparent weight (which will be your actual weight at most times in those trips above - but will increase and decrease at various times).
     
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