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B Where does the sense of weight come from?

  1. Dec 24, 2016 #1
    The weight act on me and the normal force act on me should be cancelled out, so where does the sense of weight come from?
     
  2. jcsd
  3. Dec 24, 2016 #2
    The sense of weight comes from the sense of normal force. The way our body sensors are made, we cannot feel directly the gravitational pull, however we can feel the normal force (which normal force usually is equal and opposite to the (gravitational pull=weight)) via the sensors we have in our feet.

    Also the reason we cannot feel directly the gravitational pull might have to do with general relativity theory, might have to wait for someone good in relativity to explain this to you.
     
  4. Dec 24, 2016 #3
    As normal force points upward, why do we feel a force downward?
     
  5. Dec 24, 2016 #4
    I don't think we literally feel a downward force, we feel an upward force. BUT our brain does internally or should i say subconsciously, a thought "I feel an upward force from the floor so there must be something else pushing me downwards, cause our brain considers the floor to be a "passive" object which means it cannot exert a force unless ofcourse first it is acted by an opposite and equal force."
     
    Last edited: Dec 24, 2016
  6. Dec 24, 2016 #5
    Why don't the sense of force(simply equal to weight) include the centripetal force which have a sense of pulling outward?
     
  7. Dec 24, 2016 #6
    First of all the centripetal force is the force that is applied on us when we are in circular orbit , it is a force that is inward towards the centre of the orbit. However from Newton's 3rd law since something is applying an inward force on us and keep us in circular orbit, then we apply an opposite (hence outwards) and equal force to that something, and this force is called centrifugal. It depends what we can feel here, the inward force that is applied on us, or the outward force that we apply to? I believe we can feel both.

    But what centripetal force you had in mind, the centripetal force that makes us rotate together with earth as earth rotates around its axis? Or the centripetal force in say a carousel?
     
  8. Dec 24, 2016 #7
    For the case with the Earth, I think the reason we can't feel the pull is that the centripetal force is much smaller than the gravitational force, therefore there is a net normal force acting upward.
    For the case of a carousel, we don't feel the inward pull at all but only the fictitious force. For the case of a person in an accelerating car we don't feel the forward force but the backward fictitious force again. Is it related to relativity?
     
  9. Dec 24, 2016 #8
    When a person is accelerating upward in a lift. The sense of downward push is due to the fictitious force or the increased normal force?
     
  10. Dec 24, 2016 #9

    Drakkith

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    It is cancelled, but weight is not evenly distributed throughout your body. You have 95% or more of your weight pressing down on your ankles and feet, but only 10% or so of it pressing down on your neck. The normal force is likewise divided. 100% of it is pressing up on the soles of your feet, but only about 10% is pressing upwards on your head.

    You sense weight because your body has to work to keep your head upright, your limbs from falling straight down, to keep you from falling over, etc.

    You don't need to look to GR to explain this. It's because gravitation is solely attractive, so every part of your body is being accelerated towards the Earth at approximately the same rate. If there were a large difference in acceleration between your feet and your head you would feel it.
     
  11. Dec 24, 2016 #10

    Drakkith

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    Both really. The "fictitious force" arises because of the increased normal force.

    The centripetal force is the force of gravity.
     
  12. Dec 24, 2016 #11
    If the centripetal force is gravity, then why the normal force cancelled out the weight?
    Then why the gravity on any point on the earth is the same, however the centripetal force varies as mw^2 r?(r is the distance from the rotational axis)
     
  13. Dec 24, 2016 #12

    Drakkith

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    The normal force doesn't exactly cancel out the force of gravity. You actually weigh slightly less than you would if the Earth were not rotating. If you add up the normal force and the force of gravity you'll find that there's only a very small net force, which is what is causing you to move in a curved path as you and the Earth rotate. This is the centripetal force. It's still gravity, though. It's just that gravity's downward pull is being mostly counteracted by the upward normal force.
     
  14. Dec 24, 2016 #13
    Please help me on #7 thanks
     
  15. Dec 24, 2016 #14
    The centripetal force is indeed much smaller than the gravitational force, but the net force is downwards (the normal force is just abit smaller than the gravitational pull). BUT it then again depends what you have in mind, seems you say centripetal force but you actually mean the centrifugal force.
    I have a major disagree here. My view is that we do NOT REALLY feel the fictitious force , but it is the subconscious interpretation that our brain does that make us feel the fictitious force. For example when we are in an accelerating car, what we really feel is the backseat pushing us forward but our brain works in a subconscious level and thinks something like "the seat cannot be pushing me forwards on its own, there must be something (the fictitious force) pushing me onto the seat".
     
  16. Dec 24, 2016 #15

    And the last force that is explained there is called the Tidal force. It is the same force that you would feel in a small black hole.
     
  17. Dec 24, 2016 #16

    Drakkith

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    I'm in agreement with Delta. I can't see how you can feel a fictitious force since it doesn't actually exist. But that doesn't stop your body from thinking it does!
     
  18. Dec 24, 2016 #17
    In this context, one thing especially that our nervous system represents to us can be called "pressure." Thus for example, if you are standing, you feel a general pressure in the soles of your feet. This we can upon consideration tell ourselves is due to our "weight," a.k.a. our mass in a gravitational field, etc. It does not matter whether we say it's the floor pressing up on our feet or our weight pressing our feet down into the floor; the sensation we call "pressure" would feel the same. Likewise if we lay on our back with our legs outstretched and someone pressed a flat board against the soles of our feet: Gravity would not be directly involved (we could do this in a spaceship very far from any large mass such as a planet or star) and yet if the person pressed the board hard enough, the sensations in our feet (though perhaps not elsewhere in our body) would be identical.
     
    Last edited: Dec 24, 2016
  19. Dec 24, 2016 #18
    . . . And of course many other sensations get involved in representing the position and alignment of our body in relation to gravity, e.g. balance, proprioception, etc. As has been said, being language users we start assigning words to all this, so as to be able to inform the rest of the tribe as to our status. But the base representations are purely physiological.

    https://en.m.wikipedia.org/wiki/Somatosensory_system

    I will add (though this is getting pretty far off-topic from the OP's original query) that quite a bit of what our nervous system reports to us about body alignment and status is in some ways misleading - e.g. in some cases much more to do with preventing us from damaging ourselves, etc. My point being that the nervous system would be not merely an inaccurate but a deceptive instrument for exploring concepts of force and motion in physics: https://www.bettermovement.org/blog/2008/the-central-nervous-system
     
    Last edited: Dec 24, 2016
  20. Dec 24, 2016 #19
    If a fictitious force doesn't exist do you mean centrifugal force doesn't exist?
     
  21. Dec 24, 2016 #20

    Drakkith

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    Indeed. But that doesn't stop the concept of a centrifugal force from being useful in physics.
     
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