Let's build a penguin hospital.  The funny little birds who look like they are wearing tuxedoes do get hurt every now and then.  It might not be a bad idea to build a space where they can come and heal.  Of course, we will have to build it in a very cold place where the penguins live.  It looks like the only place we can afford is on top of a large iceberg floating in the sea.  Will the hospital be safe?  I hope so . . . for the penguins.

Before we start building, we had better learn a few things about this iceberg.  First, it's floating in the sea, like a piece of ice floating in a glass of water.  Fluid is gas or liquid where all of its small parts are always moving.  This can be anything from air to orange juice.  When something is in a fluid, it is pressed on from all sides.  This is the ache you get in your head when you dive deep in the pool.  This is the feeling a balloon has when floating through the air.  So water is pushing at our iceberg from all sides at all times.  Why does it not sink?

This Antarctic Iceberg (B-15A) is so big it's hard to see where it ends.

NSF/Josh Landis, employee 1999-2001, Public domain, via Wikimedia Commons

Like I said, our iceberg is being pushed from all directions.  Water is pushing left, it's pushing right, it's pushing up, and it's pushing down.  The deeper something is under the water, the more the water pushes down on it.  This is why submarines are built very strong and scuba divers cannot dive too deep.  Net force is all of the pushing or pulling on something added together.  We can think of this as many arrows under the surface, pointing in many directions.  The water pushes on our ice and the ice also pushes back.  There is more pushing from the water on the bottom of our iceberg than there is on the top.  This is because there is more water above the bottom of the iceberg and that water pushes more, the deeper you go. This is good news for the penguins.

Wait a second.  Rocks do not float.  Boats with holes do not float.  What makes the iceberg different?  Ice is frozen water, which means all of its building blocks are stuck together and spaced out while water's are tighter together.  Since the ice building blocks are farther apart, the water pushes them up and they float.  The upward force is a force that pushes from below, making something rise.  It's what makes hot air balloons lift into the sky and it's what keeps our iceberg floating on the water.  This is even better news for the penguins.  

I feel like I'm floating on air. Wait a minute, I am floating on air.

Pseudopanax at English Wikipedia, Public domain, via Wikimedia Commons

Now for the bad news.  If we build on top of the ice, we will add a lot of downward force.  Even though the spread out ice is pushing up out of the water, it can only push so much.  Rubber ducks float in the bath . . . but not if you keep pushing down on their little heads.  Weight is how much something is pulled down to the earth by gravity.  Hospitals are not light!  Our building could sink the ice if the net force pushing down grows more than the force of the ice pushing up.  We would not help any penguins if we all sank into the cold water.  

I think I'm going to need a bigger bathtub.

Photo by Florientijn Hofman

Too bad.  No penguin hospital.  There are lots of arrows pushing at the ice from every direction under the water.  Since the ice's building blocks are more spread out than the water, the biggest arrow of force pushes it up so it does not sink.  If we built something heavy on top of it though, it would add weight, making an even bigger arrow pushing down.  This could put us under the waves . . .  Hey, don't be sad.  You and I do not weigh a lot.  Grab those flipper bandages and that beak medicine and a couple of warm coats for us.  Penguins like the cold anyway.

References:

Physics Classroom.  "Types of Forces"  Physics Classroom, 2011.  <http://www.physicsclassroom.com/class/newtlaws/u2l2b.cfm>

Physics 4 Kids.  "Forces of Nature"  Physics 4 Kids, 2009.  <http://www.physics4kids.com/files/motion_force.html>

Physics Classroom.  "The Meaning of Force"  Physics Classroom, 2011.  <http://www.physicsclassroom.com/class/newtlaws/u2l2a.cfm>