What (if anything) is the difference between the displacement of a ship and the weight of a ship? If they are the same why are there two different terms?
In: Engineering
Well, let’s look back at the ancient Greeks – no, hold on, it’s actually relevant, come back here.
Archimedes was the first person to fully realize and use this principle. He was given the problem of figuring out whether a particular crown was pure gold, or whether the maker had fudged by including some silver. Without melting it down or otherwise breaking it.
He had to think about this for a while. The solution came to him while he was in the public baths – you’ll see why – and, famously, it caused him to leap OUT of the bath and go streaking through town yelling “Eureka!”, Greek for “I have got it!”
The problem boils down to finding the volume of the crown. Its weight is easy to find. He realized that if you filled a pot exaaactly full of water, and then dropped the crown in? The amount of water that came over the edge would exactly equal the VOLUME of the crown… its “displacement”, for the water volume it displaces.
Once you have the weight and the volume of the crown, its density is easy, and then compare that to pure gold’s density and voila! the answer.
So the ship’s weight is how much it weighs (duh), and usually you have values for “empty” and “fully loaded”. Its displacement is how much water it displaces for either of these, and by extension the weight of that water. If things are going well, the displaced water weighs exactly as much as the whole ship, but has a smaller volume … which means there’s ship sticking out above the surface of the water, and the ship doesn’t sink. Because only as much VOLUME of ship goes underwater as the volume of the displaced water.
Dave, ship captains don’t go streaking about the ship. usually.
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