Why do some objects float and some sink?
18th December 2017
When we think about floating and sinking first we assume bigger objects will sink and smaller objects will float. But some really big objects like boats float and some tiny objects like your hot wheels car will sink!
Whether or not something will float depends partly on it’s density.
What is density?
Density is how full of stuff an object is. Look at this cool illustration from inspiration laboratories to explain density. Which box do you think is more dense?
The blue box is more packed up that is why we say it is more dense.
Everything that we see around us is made up of particles called molecules( like the black dots filling the box) The more the particles the more dense it is.
Some objects have lots of particles that are packed up together. Others have fewer particles that are packed more loosely. This is density.
Objects with tightly packed particles are denser and sink. A gold ring, rocks, clothes are all denser than water and so they sink in water.
Objects with less particles are less dense and float. Wood,paper,leaves are less dense than water and float. ( Read more at easyscienceforkids)
Even liquids have different densities ! Oil floats on water because it is less dense than water. Try below experiment at home and see what a difference 2 spoons of salt makes to the density of water.
Fill two cups about two-thirds full of water. Add two tablespoons of salt to one of the cups.
Gently put an egg in each cup.
The density of an egg is slightly more than fresh water, so the egg will sink. If it is put in salty water, it will float because the salt-water solution is denser than the egg!
Watch this video that explains density in a fun and interesting way.
Question Your answer: Correct answer:
Take this Quiz on density!
See list of children’s books about sinking and floating.