A physics lesson

I was looking in my refrigerator this afternoon and noticed something that seemed strange.

I always keep a bunch of water bottles in the fridge.  When I went to pull one out I saw that a two of them were half collapsed but none of the others were.  That seemed odd.  They were all cold, so why were some of them collapsed and the others weren’t?

The answer was pretty obvious, it was basic physics.  The two bottles that were partially collapsed were only half full of water, the rest were full.  The full bottles didn’t collapse because fluids don’t expand or contract very much with temperature change.  Gases do.  Thus, a bottle that’s half full of liquid and half full of gas will be affected by the refrigeration far more than bottles completely full of liquid.

The air in the bottles contracted and the bottles half collapsed.

Isn’t physics cool?

Pun intended.

Here’s another fun physics question: if you have a glass of water completely full to the brim with water and ice… when the ice melts does the glass over flow?

A somewhat slow coworker thought he was clever when he asked me that and was very surprised when I knew the answer.

Nope, doesn’t over flow, the water level actually drops.  That’s because water has a pretty unique property, it contracts when it goes from solid to liquid.  Water expands when it goes from liquid to solid, which is why ice floats. Water is less dense as a solid than as a liquid.  Almost all the other elements react the opposite way, they get more dense because they contract when they go from liquid to solid.

So, the melting ice contracts when it turns into liquid and the water level in the cup would drop.

That property of water is also one of the reasons that life exists at all.  You see, if water contracted when it froze, the surface of a body of water would turn to ice and sink.  Then the new surface of the water would freeze and sink.  This would continue until the water was frozen top to bottom.

If that was the case, during periods of intense geological activity in the Earth’s history all life would have died, or at least frozen.  Maybe some bacteria would have survived but surely anything bigger wouldn’t have.  Instead, the surface of the water freezes and floats, insulating the rest of the water which remains liquid below.  This allowed life to continue despite otherwise hazardous conditions.

I’m not a particularly religious person, I’m “spiritual” (though I hate that term, it’s kind of a generic cop-out), but realizing that if water behaved like almost all the other known elements… I might not be here, that’s kind of a crazy thought.  Seems like kind of a big coincidence, doesn’t it?

Who doesn’t love some good physics?