Physics concept question

[byeh2004]

Hey guys, I was reading on fluid dynamics the other day and I had a few questions about the concepts of archimedes principle.

I know that archimedes principle can be expressed with the equation
Fb= density of fluid x Volume of material floating/sinking x g

How does a boat or ship carrying hundreds of lbs worth of material float while that same stuff would sink to the bottom of the ocean if dumped overboard?
- totally clueless on this one; is it because the density of the ship is lighter than the density of the water?

Will the buoyancy felt by your body in a pool be more if you stretch your body out flat or if you curl up into a ball?
- Wouldn't it not matter because your volume is always going to be the same?

Thanks!

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[Funky]

Hey guys, I was reading on fluid dynamics the other day and I had a few questions about the concepts of archimedes principle.

I know that archimedes principle can be expressed with the equation
Fb= density of fluid x Volume of material floating/sinking x g

How does a boat or ship carrying hundreds of lbs worth of material float while that same stuff would sink to the bottom of the ocean if dumped overboard?
- totally clueless on this one; is it because the density of the ship is lighter than the density of the water?

Will the buoyancy felt by your body in a pool be more if you stretch your body out flat or if you curl up into a ball?
- Wouldn't it not matter because your volume is always going to be the same?

Thanks!

to answer your first question, the density of a ship has to be less than the density of water. Density can be lowered by increasing the volume. For instance, a sheet of foil will float but crumpled up it will sink. With that same logic, a boat's material may have a higher density if the volumes of water and boat were equal but boats are constructed so the volume of the boat is large to make the density less than water's density.

the second question, i'm guessing as long as you're completely submerged underwater, the force of buoyancy will be the same. It'll only matter if when you stretch out your body and start floating you'll feel less of a buoyancy force because you would be displacing less water.

 

[Snichols]

I have an additional comment about your second question. The way I would think about it is that due to the shape of the boat, the average density (steel and air) of the ship is much lower than the density water. Therefore, very little of the boat needs to be submerged in order to displace its weight (in water) in order to float satisfying Archimedes principle. Hope that helps.

 

[killinsound]

the second question, i'm guessing as long as you're completely submerged underwater, the force of buoyancy will be the same. It'll only matter if when you stretch out your body and start floating you'll feel less of a buoyancy force because you would be displacing less water.

That's true if you are totally submerged. However, you are still increasing your density somewhat when you curl up in a ball. If you're at the surface of the water, at the very moment your skin touches the water, you are displacing less water curled up in a ball. All your weight in a ball is focused on a relatively small area, while your whole body is spread out with more surface area you would increase the bouyant force felt.

I think.