How can displacement be used to find the density of an object water?
Displacement in water can determine the volume of an irregularly shaped object.
It is a simple matter to find the mass of any object, but to calculate density, you also require the volume. If the object has an irregular shape, how to do it?
Archimedes to the rescue. He realized that when an object is placed in water, it displaces a volume of water equal to the volume of the object that below the surface of the water.
(History records that Archimedes himself was the first object tested in this way, and that the discovery of his famous principle caused him such satisfaction that he jumped out of his bath and ran naked into the streets shouting "Eureka" - "I have it!")
So, if you submerge the entire object in water, all you need do is record the amount by which the water level rises, and you will have the volume of the object. Doing this with a large graduated cylinder gives good results.
Now divide mass by volume, and you too will have it! :)