# Buoyant force in seawater and freshwater

• urbano
In summary, the buoyant force acting on an object is determined by the weight of the displaced fluid. While the volume of fluid displaced may vary depending on the density of the fluid, the buoyant force remains the same. This is true for an object submerged in both freshwater and seawater, while a floating boat may displace different volumes of these liquids, the buoyant force will be the same as long as the boat is not sinking.
urbano

## Homework Statement

I'm doing a problem that is likely a common one. It is comparing the buoyant force of an object in seawater to that of the same object in freshwater. which has more buoyant force acting on it?

## Homework Equations

not sure of equations at this stage, and we are just doing it real simple so we are assuming all things are equal other than the water density.

## The Attempt at a Solution

My take on it is that the object will displace more volume of freshwater than seawater and the amount displaced is determined by the volume of the object.

However if we captured the displaced water and weighed it they would have the same mass but different volumes.

I believe the buoyant force would be the same, just different volumes of the water has been displaced dependent on the water type.

Further to this, if the object was a boat for example, the mass of the water displaced would be the same as the mass of the boat.

Have I interpreted the basics of buoyant force correctly or am I off down the wrong path ?

Hi urbano
The buoyant force is determined by the weight of the displaced liquid.
Not its volume.
Of course, the weight is related to the volume, but what matters is the weight.
So if you submerge o solid of some volume V in two different liquids (seawater or freshwater), you will displace the same volume V of said liquid.
but since those liquids have different densities, for one of them, more mass, will be displaced, and the buoyancy resulting force will be stronger.

Cheers...

urbano said:
1. My take on it is that the object will displace more volume of freshwater than seawater

correct
and the amount (volume) displaced is determined by (equal to) the volume of the object under water.

However if we captured the displaced water and weighed it they would have the same mass but different volumes. I believe the buoyant force would be the same, just different volumes of the water has been displaced dependent on the water type.

Further to this, if the object was a boat for example, the mass of the water displaced would be the same as the mass of the boat.

Have I interpreted the basics of buoyant force correctly or am I off down the wrong path ?

OK with the added comment above in red.. The assumption is that the boat is floating. All of the above would be incorrect if the boat sank. However, irrespective of floating or sunk, it is always true that the buoyancy force is equal to the weight of the displaced water.

Last edited:
ohh I'm definitely confused...

oli4 makes mention of the fact that if I submerge an object in freshwater or seawater then the volume displaced will be identical. I believe this is due to the fact that the object has been submerged.

I guess it's different in the case of a boat floating ? In the case of a floating boat , I believe the boat would displace different volumes of seawater compared to freshwater (i.e less of the boat is physically in the seawater), but the mass of the displaced waters (sea and fresh) would be equal.

? is this correct thinking??

Your thinking is correct.
Let us say that an object is submerged to a volume V in a given fluid . Then the fluid applies a force on the object ,which is equal to the force on an equal volume V of the fluid due to the rest of the fluid body. This force is equal and opposite to the weight of the fluid of volume V ( assuming that the fluid body is at rest). This is the bouyant force.

Since the boat is floating , the net force on it is zero ( in the vertical direction) . What are the forces? The Earth's gravitational pull (i.e weight of the boat) and the bouyant force of the fluid. Since the weight of the boat is same in both cases the bouyant Force is also same in both cases.
What is the bouyant force ? It is equal to the weight of the displaced fluid , which is equal to the (density of fluid)*(volume of fluid displaced ) . Thus for the fluids with greater density ,lesser volume is displaced , and hence the volume submerged would be less.

urbano said:
ohh I'm definitely confused...

oli4 makes mention of the fact that if I submerge an object in freshwater or seawater then the volume displaced will be identical. I believe this is due to the fact that the object has been submerged.

I guess it's different in the case of a boat floating ? In the case of a floating boat , I believe the boat would displace different volumes of seawater compared to freshwater (i.e less of the boat is physically in the seawater), but the mass of the displaced waters (sea and fresh) would be equal.

? is this correct thinking??

All you just said is corrrect, so why are you " definitely confused"??

## What is buoyant force?

Buoyant force is the upward force exerted by a fluid on an object immersed in it. It is equal to the weight of the fluid that the object displaces.

## How does the buoyant force differ in seawater and freshwater?

The buoyant force in seawater is greater than that in freshwater because seawater is denser than freshwater. This means that an object will displace more seawater and thus experience a greater upward force.

## How is buoyant force related to an object's volume?

The amount of buoyant force experienced by an object is directly proportional to its volume. This means that the greater the volume of an object, the greater the buoyant force it will experience.

## What is the significance of buoyant force in naval architecture?

Buoyant force plays a crucial role in determining the stability and buoyancy of a ship. Engineers must carefully consider the buoyant force acting on different parts of a ship in order to ensure its safe and efficient operation.

## How does the shape of an object affect the buoyant force it experiences?

The shape of an object can greatly impact the buoyant force it experiences. Objects with a greater surface area will experience a greater buoyant force because they displace more fluid. Additionally, objects with a streamlined shape will experience less resistance from the fluid and thus may experience less buoyant force.

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