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utilae

Plasticine floating and sinking?

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I know that a object will float if the objects density is less than the density of the water it displaces. There is an experiment with plasticine where if you roll it up into a ball, the plasticine sinks. If you flatten the plasticine it sinks. But if you flatten the plasticine and make it into a bowl shape, the plasticine floats. Why does the plasticine float and sink in those situations?

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When you make a bowl, don't forget that now the in the bowl is part of the object that displaces the water.


Here a picture

* := water
O := ball
===== := flat with water around it
\___/ := bowl

|*******|
|***O***|
|*******|
|*******|
+-------+

the ball only displaces "1" water part.


|*******|
|*******|
|*=====*|
|*******|
+-------+

the flat only displaces a littel more than the ball


|*\___/*|
|*******|
|*******|
|*******|
+-------+

know the bowl displaces "5" water parts.

but the wight is allways the same of the palstic.

Hope this helps a little.

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Quote:
Original post by utilae
I know that a object will float if the objects density is less than the density of the water it displaces.


There is an experiment with plasticine where if you roll it up into a ball, the plasticine sinks. If you flatten the plasticine it sinks. But if you flatten the plasticine and make it into a bowl shape, the plasticine floats.


Why does the plasticine float and sink in those situations?


Densitiy is mass divided by volume. When you form a bowl out of the plasticine, you preserve the mass but increase the volume (because you have to count the air inside the bowl too), and the mass of air is far less then the mass of water.

That's the same reason why ships don't sink even if they are made of steel and weight hundreds of tons.

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Another question.

A person who lies down on his back in the water will float. Yet if that person is standing up on the water, the person will sink.

Why is this?

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The main factor there is surface tension. A body laying down on the water will have more contact with the surface of the water, allowing the tension to hold it up, whereas a body standing up makes less contact with the surface, and will sink.

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when a person rests on the back offers more surface , its center of mass if above the mass displaced by the immersed body, when someone is standing , the feet offers less surfaces , and gravity pulls the center of mass down, at every point in time the immersed surface is not enough to balance the immersed volume.

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Surface tension might make a difference - I dont understand what ketek said atall. Personally, it doesn't matter what shape I make myself I just sink anyway :}

Try making your plasticine into a hollow ball - flatten it, then fold it over and seal the edges so that it traps a large bubble of air. That should float without actually being bowl shaped. It has the advantage over the bowl that if you force both shapes underwater, the bowl will sink, but the "bubble" shape will come back up.

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Guest Anonymous Poster
Quote:
Original post by SephirothEX
The main factor there is surface tension. A body laying down on the water will have more contact with the surface of the water, allowing the tension to hold it up, whereas a body standing up makes less contact with the surface, and will sink.



Surface tension has nothing to do with it. The surface ratio to weight is too small on something as the size and shape of a human. It works with razor blades, but doesnt on things much bigger than that.

Whne you are upright, your head has to be mostly out of the water for you to breath and the center of gravity of a human body puts the water surface above your mouth and nose (usually). Laying on your back puts your mouth/nose at the very top and you can maintain breathing (center of gravity is puts your face above the water - its much smaller weight out of the water than most of your head.)

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Quote:
Original post by SephirothEX
The main factor there is surface tension. A body laying down on the water will have more contact with the surface of the water, allowing the tension to hold it up, whereas a body standing up makes less contact with the surface, and will sink.


Nonsense. Surface tension in this case is negligable, especially as most of the body is underwater anyway.

Whether you lie down or stand up, the same proportion of the body mass (assuming uniform density) will remain above the surface.

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Whether you lie down or stand up, the same proportion of the body mass (assuming uniform density) will remain above the surface.

Not at all, we're talking about surfaces at contact, not overall surface above the water , this branch of physics is called fluid mechanics and idhrostatics

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