# Why is air pressure force not considered while weighing?

• tanmay
In summary, air pressure on Earth is 1 atm, and can be expressed as 10^5 P. The Pressure (P) equation is P = F/A, and if the small area (A) on which we are standing is taken into account, the pressure force is equal to 10^5*A. However, this force is not subtracted from our weight (W) and we only consider gravity, not air pressure force. This is because air pressure is not solely pushing down on the human body, but also pushing all around us in a balanced way. The difference in weight due to buoyancy is negligible and can be safely neglected.
tanmay
we know that air pressure on our Earth is 1 atm.
Also 1 atm = 10^5 P
Also we know Pressure(P) equation = P = F/A So, F = P*A
So if small area(A) in which we are standing is also taken then pressure force is that area(A) times 10^5(Atmospheric pressure in Pascal) i.e 10^5*A
So why don't we subtract this force from our weight(W) as W - F
Why do we only consider gravity?
Why we don't consider air pressure force?

if I have any misconceptions then tell me...
Thank You

Air pressure isn't solely pushing down on the human body. Rather, it's pushing all around us, (almost, I believe) balancing out, and so there's no increase in weight from it.

tanmay
tanmay said:
we know that air pressure on our Earth is 1 atm.
Also 1 atm = 10^5 P
Also we know Pressure(P) equation = P = F/A So, F = P*A
So if small area(A) in which we are standing is also taken then pressure force is that area(A) times 10^5(Atmospheric pressure in Pascal) i.e 10^5*A
So why don't we subtract this force from our weight(W) as W - F
Why do we only consider gravity?
Why we don't consider air pressure force?

if I have any misconceptions then tell me...
Thank You
What would the scale read if you were not standing on it?

tanmay
tanmay said:
force is that area(A) times 10^5(Atmospheric pressure in Pascal)
That only applies to flat surface elements. Otherwise force is the surface integral of pressure * surface normal, which for will yield a small buoyant force on the human body.

tanmay
As others have pointed out, it's not just the top pushing down on you, but also the bottom pushing you up, and in fact with more force from below than from above (that's how things swim).
If you view a human as a cylinder (spherical cow in a vacuum, lol) of 2m height and say 1m^2 surface area at the ends, you get:

Force on top: 1m^2 * 100kPa = 100kN

2 meters lower the pressure has increased by 24 Pascal (1.2kPa per 100m per Wikipedia)
Force at the bottom: 1m^2 * 100.024kPa = 100.024kN

So, the net force (bottom minus top) is 24N. That's worth a mass of 3kg or so. But because we're not cylinders, it's actually significantly less than that.

It's interesting to note though that, if you prevent air from pushing from the bottom (e.g. by having an airtight seal at the bottom), then you actually notice the full weight of the air column when trying to lift it. The moment you break the air seal (thus allowing the air to push from the bottom again) it becomes easy to lift it.

Last edited:
tanmay
Hello

The pressure of air is related to its density: maybe, you should take into account the corrections of bouyancy

Let's take an example: if you weigh a 1kg of Helium (contained in a ballon) in the surface, will the result be the same if you repeat the measure if the pressure of air if different?

As it's been said, it depends on what you are weighing :)

Greetings!

tanmay
Consider a 1 meter cube, sitting on and vacuum-sealed to a scale.

What is the difference between the air pressure force the scale feels with or without the object sitting on it?

tanmay
The density of water is 1000 kg/m3 while that of air at the surface of the Earth is about 1.3 kg/m3. So the buoyant force amounts to only about 0.13% of the body weight, and thus this "red herring" can safely be neglected. Now, with this realization, what is the answer to Russ' question?

Chet

tanmay
The problem of the difference between mass and weight due to buoyancy is encountered with vertical seismometers. The weight is less than the mass by the volume x density of air that is displaced. Unless the vertical seismometer is sealed in an air tight container it will bob up and down with the weather.

tanmay

## 1. Why is air pressure force not considered while weighing?

Air pressure force is not considered while weighing because it is a relatively small force compared to gravity. The force of gravity is much stronger and dominates the weighing process. Additionally, most weighing scales are designed to measure the weight of an object in a vacuum, where air pressure is not a factor.

## 2. How does air pressure affect weighing results?

Air pressure can affect weighing results if the weighing scale is not calibrated or adjusted for the specific air pressure at the location where the weighing is taking place. This is because air pressure can cause the scale to read slightly higher or lower depending on the amount of air pressure pushing down on the object being weighed.

## 3. Can air pressure affect the weight of an object?

No, air pressure does not change the weight of an object. Weight is a measure of the force of gravity acting on an object, and air pressure is a separate force that does not contribute to an object's weight. However, air pressure can affect the accuracy of weighing results if not properly accounted for.

## 4. Why is air pressure not considered in everyday weighing situations?

In most everyday situations, the effect of air pressure on weighing results is negligible and does not significantly impact the accuracy of measurements. Therefore, it is not necessary to consider air pressure in everyday weighing situations. However, in more precise scientific measurements, air pressure may need to be taken into account.

## 5. How does altitude affect air pressure and weighing?

As altitude increases, air pressure decreases due to the decrease in the amount of air molecules in the atmosphere. This decrease in air pressure can affect weighing results, as there is less force pushing down on the object being weighed. Therefore, at higher altitudes, weighing scales may need to be calibrated or adjusted to account for the lower air pressure.

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