Bad news.  You are being chased by lions.  Worse news.  You just ran into a lake.  Worst news.  The lake is filled with crocodiles.  Lions are growling behind you.  Crocodiles are snapping in front of you.  Your only chance is to somehow float across the lake to the other side.  There is a piece of wood, a plastic cooler, and a piece of bent metal all floating by the bank.  If you get in the wrong one, you will sink.  If you do not choose fast enough, you will be eaten by lions.  Which thing will keep you afloat?

Choose wisely. . .


Wait, why are these things floating in the first place?  You know that gravity is pulling down on everything, trying to get it as close to the middle of earth as possible.  Water gets in the way though.  Buoyant force is the amount of force that pushes something up when it is in a fluid, working against gravity.  All of the things in front of you--the piece of wood, the cooler, and the piece of metal--are all floating because the force pushing from below is greater than what is pulling down.  Why do some things float in water while others do not?


A rock sinks while a boat floats even though the boat is a lot heavier than the rock.  So how can you know which of these things in front of you is going to sink if you step on it?  First you need to know a couple of things about each thing.  Volume is how much space something takes up.  All of these floating things takes up a different amount of space.  For the ones that have a space inside of them, you can fill each one with water and then pour that water into a measuring cup, to find its volume.  If you took the time to do this, you would also probably be eaten.

Yikes!


There's another way of looking at how much space something takes up.  If any of these things were to be pushed down into the lake, then the water would rise to the exact volume of the thing.  Displacement means to move into a new place, and when it comes to water it means to take up space in the water and move the water up.  We can push any of these things floating on the water down to see how much water it moves.  When they are floating, each thing is taking up space above the water.  You can step on it.  Why is that?


I know the lions are closing in, but pay attention.  The amount of space that each of these things takes up in the water is the same amount as how much the water would rise if it were to be pushed down.  Think about it this way:  Imagine each of these things is made of water.  If the thing is more heavy than the water, it will go down.  If it is less heavy than the water it moves, it will stay floating.  Archimedes' principle is the idea that if the weight of water that something moves is the same as the buoyant force that pushes up it will float.  In other words, if you step on a boat, it will start to sink until pushes up the same amount of water as its weight moved.  If it gets too heavy, you and the floating thing are going to meet some crocs . . . 

Our friend, Archimedes.


Good news.  You may be chased by lions toward a lake full of crocodiles, but because you understand how water pushes up, you might be able to float.  The volume of something floating is the same as how much the water rises if the thing sinks.  If it is lighter than the water it moves, it will float!  So which thing should you jump on?  ANY OF THEM!  Just choose one that looks the safest and jump!  There's no time for science when you are being chased by lions!  



References:

Science Kids.  "Archimedes Principle"  Science Kids, 2010.  <http://www.sciencekids.co.nz/videos/physics/archimedesprinciple.html>

Education.com.  "Buoyant Force of Water"  Education, 2012.  <http://www.education.com/science-fair/article/archimedes-principle-floats-boat/>

Nature.  "Quicksand Can't Suck You Under"  Nature, 2005.  <http://www.nature.com/news/2005/050926/full/news050926-9.html>

It's Not Magic It's Science.  "Archimedes and the Bathtub"  It's Not Magic It's Science, 2010.  <http://www.itsnotmagicitsscience.com/science.asp?newsid=381>