# Weight of fluids on a submerged person?

• aspodkfpo
In summary, the weight of fluids on a submerged person is determined by the density of the fluid and the volume of the person. The greater the density of the fluid, the more weight the person will experience. This weight is known as buoyant force and it counteracts the weight of the person, making them feel lighter while submerged. The volume of the person also plays a role, as a larger volume will displace more fluid and result in a greater buoyant force. This concept is important in understanding the mechanics of swimming and other water activities.
aspodkfpo
Pg 9 Q 11 A)

Was wondering why there isn't a weight of water in the force diagram? The boy is submerged, as I understand.

Do we normally include the weight of a fluid on someone or how does this work?
For instance, is there a weight of air on us when we stand on land? What is this quantity of air? Is it a negligible amount that collides with our head at any given time, or a rectangular prism of air stretching from the surface of our head to the atmosphere?

"Draw a free body diagram showing the forces acting on the boy when his lungs are empty after breathing out slowly. Indicate the magnitude of the forces acting on the boy by the size of the arrows you draw to represent the forces."

We should only be analyzing the external forces acting on the body of the boy.

https://www.physicsforums.com/media/free-body-diagrams.98/

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

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

The substances inside the body are not important, only its average density, which interacts with the density of the fluid around it.
More or less air in his lungs means less or more dense body, which affects the buoyancy effect.

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aspodkfpo said:
Do we normally include the weight of a fluid on someone or how does this work?
No. Fluids can flow sideways, so there is no simple relationship between the weight of a parcel of fluid and the force exerted on an object directly below. Instead, think in terms of pressure.
As this is a buoyancy question, the easiest way is to use Archimedes' principle. This avoids having to integrate the pressure over the surface area of the body. For more details see https://www.physicsforums.com/insights/frequently-made-errors-mechanics-hydrostatics/.

aspodkfpo
Lnewqban said:
"Draw a free body diagram showing the forces acting on the boy when his lungs are empty after breathing out slowly. Indicate the magnitude of the forces acting on the boy by the size of the arrows you draw to represent the forces."

We should only be analyzing the external forces acting on the body of the boy.

https://www.physicsforums.com/media/free-body-diagrams.98/

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

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

The substances inside the body are not important, only its average density, which interacts with the density of the fluid around it.
More or less air in his lungs means less or more dense body, which affects the buoyancy effect.
Was referring to the weight of the water on top of his body, because he is submerged (i.e. below water level).

Archimedes principle still holds true. Yes there is greater pressure on the "top" as one submerges, but there is also greater pressure on the "bottom". The net force is still Archimedes buoyancy.

Lnewqban
hutchphd said:
Archimedes principle still holds true. Yes there is greater pressure on the "top" as one submerges, but there is also greater pressure on the "bottom". The net force is still Archimedes buoyancy.

So we don't consider the weight of water, but look at is as buoyancy?

aspodkfpo said:
So we don't consider the weight of water, but look at is as buoyancy?
Yes.

How does one find buoyancy (or more precisely buoyant force)? It depends upon the weight of water displaced...which is related to the difference in pressure "below" and "above". So yes...

Lnewqban and aspodkfpo
aspodkfpo said:
Was referring to the weight of the water on top of his body, because he is submerged (i.e. below water level).

Copied from the third link of my previous post:

"Consider a cube immersed in a fluid with the upper surface horizontal.

The sides are identical in area, and have the same depth distribution, therefore they also have the same pressure distribution, and consequently the same total force resulting from hydrostatic pressure, exerted perpendicular to the plane of the surface of each side.

There are two pairs of opposing sides, therefore the resultant horizontal forces balance in both orthogonal directions, and the resultant force is zero.

The upward force on the cube is the pressure on the bottom surface integrated over its area. The surface is at constant depth, so the pressure is constant. Therefore, the integral of the pressure over the area of the horizontal bottom surface of the cube is the hydrostatic pressure at that depth multiplied by the area of the bottom surface.

Similarly, the downward force on the cube is the pressure on the top surface integrated over its area. The surface is at constant depth, so the pressure is constant. Therefore, the integral of the pressure over the area of the horizontal top surface of the cube is the hydrostatic pressure at that depth multiplied by the area of the top surface.
...
An object of any shape can be approximated as a group of cubes in contact with each other, and as the size of the cube is decreased, the precision of the approximation increases. The limiting case for infinitely small cubes is the exact equivalence.

Angled surfaces do not nullify the analogy as the resultant force can be split into orthogonal components and each dealt with in the same way."

hutchphd

## 1. How does the weight of fluids affect a submerged person?

The weight of fluids on a submerged person is determined by the density and volume of the fluid, as well as the surface area and depth of the person. The greater the volume and density of the fluid, the greater the weight will be on the person. Additionally, the deeper the person is submerged, the greater the weight will be due to the increased pressure from the fluid.

## 2. Why does a person feel lighter when submerged in water?

When a person is submerged in water, they feel lighter because the upward force of the water, known as buoyancy, counteracts the downward force of gravity. This is due to Archimedes' principle, which states that the buoyant force on an object is equal to the weight of the fluid it displaces.

## 3. How does the weight of fluids on a submerged person differ from the weight on land?

The weight of fluids on a submerged person is greater than the weight on land due to the added pressure from the fluid. This is why objects, including people, feel lighter when submerged in water compared to when they are on land.

## 4. Can a person float in any type of fluid?

Yes, a person can float in any type of fluid as long as the density of their body is less than the density of the fluid. This is why people can float in water, which has a density of 1 gram per cubic centimeter, but not in denser fluids such as oil, which has a density of 0.9 grams per cubic centimeter.

## 5. How does the weight of fluids on a submerged person impact their ability to move?

The weight of fluids on a submerged person can make it more difficult for them to move due to the added resistance from the fluid. This is why it takes more effort to move through water compared to moving through air. However, the buoyancy provided by the fluid can also help support the person's weight and make it easier for them to move in some cases.