# Where does the sense of weight come from?

• Clara Chung
In summary, the sense of weight comes from the sense of normal force due to the way our body sensors are made. We do not directly feel the gravitational pull, but we can feel the normal force through the sensors in our feet. This may be related to general relativity theory. The sense of weight is not evenly distributed throughout the body, as different parts of the body experience different amounts of normal force. This is why we can sense weight even when the weight and normal force are technically cancelled out. In situations where there is an increased normal force and/or acceleration, such as in a lift or during circular motion, the sense of weight may be affected by the increased normal force or fictitious forces. However, the force of gravity is still present
Clara Chung
The weight act on me and the normal force act on me should be canceled out, so where does the sense of weight come from?

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.

Clara Chung
Delta² said:
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.

As normal force points upward, why do we feel a force downward?

Clara Chung said:
As normal force points upward, why do we feel a force downward?
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."

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Clara Chung
Delta² said:
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."
Why don't the sense of force(simply equal to weight) include the centripetal force which have a sense of pulling outward?

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?

Clara Chung
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?

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?

Clara Chung said:
The weight act on me and the normal force act on me should be canceled out, so where does the sense of weight come from?

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.

Delta² said:
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.

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.

Clara Chung said:
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?

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

Clara Chung said:
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.

The centripetal force is the force of gravity.

Clara Chung
If the centripetal force is gravity, then why the normal force canceled 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)

Clara Chung said:
If the centripetal force is gravity, then why the normal force canceled out the weight?

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.

Clara Chung

Clara Chung said:
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.
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.
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?
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".

Clara Chung
Drakkith said:
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.
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.

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!

Delta² said:
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."

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.

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Drakkith
. . . 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

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Drakkith said:
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!
If a fictitious force doesn't exist do you mean centrifugal force doesn't exist?

Clara Chung said:
If a fictitious force doesn't exist do you mean centrifugal force doesn't exist?

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

AstroChris
Drakkith said:
Indeed. But that doesn't stop the concept of a centrifugal force from being useful in physics.
Sorry I don't understand. If it doesn't exist why are there tools like centrifuge that utilize this force? It makes more sense if centripetal force doesn't exist respect to an inertia frame view.

Clara Chung said:
Sorry I don't understand. If it doesn't exist why are there tools like centrifuge that utilize this force? It makes more sense if centripetal force doesn't exist respect to an inertia frame view.

That's exactly my point. The concept of a centrifugal force that forces things away from the axis of rotation is a useful concept that makes certain problems easier to analyze or more intuitive to understand. But it's still a fictitious force that doesn't exist. That's what "fictitious" means.

UsableThought said:
What might be some comparable fictions? E.g. how about the useful convention in analyzing electrical circuits of assigning a direction to current rather than just sign?

The first one that comes to mind is from geometric optics where reflection is treated as refraction but with the index of refraction as negative for the medium the light is entering. The refractive index of the medium certainly isn't negative, but it's useful to consider it negative since it allows you to use the same equations without having to remember more than one sign change.

Drakkith said:
The first one that comes to mind is from geometric optics where reflection is treated as refraction but with the index of refraction as negative for the medium the light is entering. The refractive index of the medium certainly isn't negative, but it's useful to consider it negative since it allows you to use the same equations without having to remember more than one sign change.

Thanks. Sorry to have deleted the question before I saw your answer - it's interesting & probably there could be some very long lists of conventions - but on further reflection I decided I didn't want to distract from the OP's concern.

Clara Chung said:
As normal force points upward, why do we feel a force downward?

You feel an upward force on the soles of your feet (or whatever is in contact with the ground/chair) and a downward force on the rest of you.

Clara Chung said:
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.

That's a somewhat confused way to see things...

Remember the centripetal force is the force that causes an object to move in a curve so it acts towards the centre of rotation. In the case of a person standing on the Earth it is gravity that provides the centripetal force.

The centripetal force required to move in a circle is given by mv2/r. However on the surface of the Earth gravity provides much more centripetal force than mv2/r. If it wasn't for the normal force produced by the ground this excess centripetal force would cause the person to move in a circle with a much smaller radius than the radius of the earth. You can prove that by throwing a stone into a pond. The water can't provide the same normal force as the ground so the stone accelerates towards to the centre of the Earth and it's radius of motion reduces.

cnh1995

## 1. What is the scientific explanation for the sense of weight?

The sense of weight is the result of the force of gravity acting on an object. Gravity is a fundamental force of nature that causes all objects with mass to be attracted to each other. The strength of gravity depends on the mass of the objects and the distance between them.

## 2. How do our bodies perceive weight?

Our bodies perceive weight through a combination of sensory information from various systems. These include the vestibular system, which is responsible for balance and spatial orientation, and the proprioceptive system, which provides information about body position and movement. Additionally, our muscles and joints also play a role in sensing weight.

## 3. Can the sense of weight be influenced by other factors?

Yes, the sense of weight can be influenced by factors such as altitude, temperature, and even emotions. For example, at higher altitudes, the force of gravity is slightly weaker, so objects may feel lighter. Changes in temperature can also affect the density of objects, making them feel heavier or lighter. Emotions and perceptions can also play a role in how we perceive weight, as our mental state can influence our physical sensations.

## 4. How does the sense of weight differ from the sense of mass?

The sense of weight and mass are often used interchangeably, but they are actually two distinct concepts. Mass is a measure of the amount of matter in an object, while weight is a measure of the force exerted on an object by gravity. Therefore, an object may have the same mass, but its weight can vary depending on the strength of gravity.

## 5. Is there a relationship between the sense of weight and the sense of balance?

Yes, there is a relationship between the sense of weight and the sense of balance. Our sense of balance is closely connected to our perception of weight and is essential for maintaining an upright posture and stable movement. When our sense of weight is altered, such as when carrying a heavy load, it can affect our balance and coordination. Similarly, when our balance is compromised, it can impact our perception of weight.

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