# The Naked Scientists Forum

### Author Topic: Is a sealed, water-filled can of water denser in water than a similar empty can?  (Read 1598 times)

#### gti_fly_bye

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##### Is a sealed, water-filled can of water denser in water than a similar empty can?
« on: 03/04/2013 23:28:51 »
My friends and I watched a magic show where a magician tied him self to a jerry can full of water and jumped in to a lake.
I said that as the can contained only water and was been smudged in water the only 'real' weight to it was the can its self as the water inside has the same density as the water around it?
My friends think as it's a sealed can it is a single mass, like any 25kg weight and would sink to the bottom like a rock?
I obviously know the can would sink but I think only due to the the Wight of the can not the water inside?
Who is correct???
« Last Edit: 01/05/2013 08:35:24 by chris »

#### bizerl

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##### Re: Please help settle a bet on fluid density?!
« Reply #1 on: 04/04/2013 04:38:04 »
I'm not a scientist but I'm pretty sure you're right. When the density inside the container is the same as the outside, it's weight cancels out.

Another way of thinking about it is if you have a balloon full of air, it doesn't become heavier in air simply because it is sealed.

I know someone will pick me up on the intracacies of mass and weight and how technically, a balloon full of air DOES weigh more. I think I mean bouyency rather than "weight", but I'm not sure...

#### CliffordK

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##### Re: Please help settle a bet on fluid density?!
« Reply #2 on: 04/04/2013 07:25:33 »
Yes, exactly.
If it is a 5 gallon Jerry can, it would displace 5 gallons of water.  Then you could calculate buoyancy based on the weight of the can and the weight of the water displaced, with the net difference being very small depending on air in the can, and the weight/density of the metal.

If there is an air gap at the top of the can, it may actually float.  Likewise, if you filled it with a lower density liquid such as an alcohol, or oil, then it may also float.

Magicians often try to mislead the audience.  Perhaps his next experiment should be wearing a 50 lb lead diving belt in fresh water.

#### Lmnre

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##### Re: Please help settle a bet on fluid density?!
« Reply #3 on: 06/04/2013 14:14:20 »
In the 1950s and 1960s, the Trieste bathyscaphe used ballast tanks filled with [incompressible, lighter-than-water] gasoline to provide positive (ie, upward) ballast. The gasoline ballast allowed the Trieste to rise  after descending almost 7 miles into the deep.

source
« Last Edit: 06/04/2013 14:19:35 by Lmnre »

#### Bored chemist

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##### Re: Please help settle a bet on fluid density?!
« Reply #4 on: 06/04/2013 21:13:36 »
A plastic jerry can might float, even if it was full of water.

#### The Naked Scientists Forum

##### Re: Please help settle a bet on fluid density?!
« Reply #4 on: 06/04/2013 21:13:36 »