I wrote on this subject for my grandchildren, as I find the subject very interesting

                                      Buoyancy and Flotation                 22-9-13

alanmcp@xtra.co.nz Author A McPherson

Half fill a sink with water, take a cooking pot by the handle, and press down with the bottom of the pot on the water surface. As you push down, the water must move out of the way, it has no option but to rise, now here lies the key to buoyancy.

If you cause water to rise, then you must be exerting a force, [ pot pushed down, water pushed up.] but if you relax the downward force, the weight of the water you have lifted, will push back on the pot and will attempt to force the pot out again. [ water pushing down, Pot pushed up.]
Do you recall the axiom which applies perfectly here? “For every action, there is an equal and opposite reaction”.

It is this backward force of the lifted water, trying, by the force of gravity, to get back to where it was, that makes objects lighter when immersed in water, and in some cases, makes them float. To float, the object, on entering the water pushes down by the force of gravity, and raises the water height until the weight of this lifted water equals the weight of the object, providing the object is only partially submerged at this time, then the weight of the object is in balance with the weight of the displaced or pushed up water, and the object will now float, this action is like a set of balances with equal weights on each side.

Now imagine the cooking pot in our first scenario is now a block of solid metal, and is lowered into the sink under it’s own weight. Because this pot is much heavier, it will sink much further and lift more water, it continues to sink until fully submerged and heads for the bottom. Why did it sink ? it sunk because the weight of the raised water never equaled the weight of the submerged object to support it. It’s the surface area of the object pushing against the water, relative to the weight of the object which governs whether the object will sink or float, the greater the area pushing against the water, the more water will be displaced, and the greater the weight for the object to balance against, and the greater the chance for the object to float.

Now let’s imagine we take this solid block of metal and spread it out thinly into the shape of a canoe, it will be the same weight as the solid block, but because the surface area of this canoe pushing down on the water is now much greater it is forced to lift a lot more water, which will reach equilibrium with the weight of the canoe, and will float well before submerging.

So remember, it is the weight of the object relative to it’s surface area pressing against the water which governs the different levels of buoyancy. Any object which enters the domain of a gas or liquid, becomes lighter and gains buoyancy equal to the weight of gas or liquid displaced.

Although the solid block metal pot with the small surface area went to the bottom, it didn’t float, but it did gain some buoyancy, and become lighter, while under water, by the value of the weight of the water displaced. even the rocks and pebbles in the rivers and sea have acquired some buoyancy by being immersed.

Next time you are in the bath stretch out your arm and lower it slowly into the water, watch the level rise and note how the weight of your arm diminishes, now lift it slowly out and feel that weight return..........That was buoyancy.

A dolphin leaping out of the sea to reach amazing heights is greatly assisted by buoyancy, I imagine a dolphin could displace 40 gallons of water, that’s about 340 pounds of force helping to rocket that dolphin into space.

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