Water Displacement
5. While we are still on the topic of water, lets talk about water displacement. I remember a post once where someone asked if, since water displaces the weight of an object, does that mean that rocks weigh less? Well, the short answer is no.
Archimedes principle states that an object displaces its weight in water if it is floating (and only floating). Basically if something that is 10 lbs is floating, then it displaces 10 lbs of water. The volume of the water displaced is the same volume of the amount of the object under water. That is why ice is mostly under water when it floats (its just slightly lighter than water). When an object doesn’t float, its weight is being supported by the bottom of the tank, and not the water, so it displaces its volume, not its weight. Thus rocks weigh the same in the tank as they do outside of it.
An explanation of #5, with a few more details. The answer that the true mass of the object is fixed, is correct. If you were to take a 1000g steel weight with a density of 8.0g/cm3 and add it to an aquarium that was sitting on top of a scale the weight of the aquarium would increase by 1000g. However if you were to place the scale inside the aquarium and put the 1000g weight on the scale you will find that the scale would indicate slightly less than 1000g. The buoyancy effect is still there even if the object sinks. The downward force of the 1000g weight on the submerged scale would be decreased by the weight of the water that it is displacing.
1000g / 8g/cm3 = 125 cm3 (volume of 1000g weight)
The volume of the 1000g weight is equal to the volume of the water that it is displacing. Water has a density of 1.0g/cm3
125cm3 * 1.0g/cm3 = 125g (weight of displaced water)
1000g - 125g = 875g (value indicated on the scale)
The scale submerged in the aquarium would indicate that the 1000g weight was only 875g. This is because the pan of the scale would be supporting 875g of the total weight and the water would be supporting the other 125g.
Being a metrologist I have to compensate for the buoyancy effect in air when making precision mass measurements. The math works the same only the density of air is approx 0.0012g/cm3.
As for what floats, any thing with a density less than water (<1.0g/cm3), and any object with a density greater than water will sink.