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Buoyancy and Archimedes’ Principle.

Bouyancy of a partially Immersed container

Buoyancy of a partially Immersed container

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Tonight I ask the question: “How well will a container float in water?” Said a better way: “How much will a container sink in water?” The answer is: “It will sink as much as it needs to.” Obviously, that is a flippant answer so I will do a little better job. If you look at the picture, I show a container partially immersed in water. There is a force pointing downward. That force is the weight of the container and any additional force. In the video the additional force is my hand pushing the bottle down. However, if we were making a small raft, that additional force would also include the weight of the raft and any load on it.

The upward force is the force due to buoyancy and from Archimedes Principle it is known to be equal to the weight of the liquid being displaced by the container. When those two forces are equal the container stops moving downward or upward and reaches equilibrium. So truthfully, the container sinks exactly as much as it needs to have the buoyancy force equal to the applied force and the weight of the container.

Since we like numbers and we will be using a lot of them soon, lets immerse ourselves in a few.  (Yeah.. bad pun, I know.)
The empty container weighed 9.7 grams so the water displaced was exactly 9.7 grams of water.
When I filled the container as well as I could the container weighed 277 grams and when the container was placed in the water it displaced 277 grams of water and the top of the container still floated because of a little air bubble in the container.
When I placed some coins in the container, it weighed 302.4 grams and the container could not displace enough water to equal that weight and the container went to the bottom.

The density of water is 1 gram per cubic centimetre.(cm3) and there are 0.06104 cubic inches (in3) per cm3.

This means the empty container displaced 9.7 cm3 or 0.592 in3.  (That is not much water.)

The complete container displaced between 277 cm3  and 302 cm3. This is between 16.9 and 18.43 in3.

When the container was heavy enough to sink it weighted much less when it was completely immersed under the water than it did above the water.   For example it the true amount of displacement was 285 cm3.  The force under the water with the coins would be 302.4g – 285g or 17.4 g.   But above the water it would pull toward earth with a weight the full 302.4 g.

For your viewing pleasure, my little kitchen experiment on buoyancy.

Gary

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“Buoyancy and Archimedes’ Principle.” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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