Pond Ice

A couple of weeks ago my wife and I took our daughters on a walk through the neighborhood.  It had been cold enough the previous few days that a few nearby ponds had frozen over.  On the day of the walk, however, the temperature outside was close to 50 degrees F.  The ice on the ponds was quickly melting, but had not finished melting yet.  We had fun throwing rocks onto the ice to see if we could break through.  We did.  We then found a couple of large sticks and starting poking the ice near the pond edges.  The cool thing is that the ice was the perfect thickness to break, but not shatter the ice into tiny pieces.  This allowed us to take out chunks of the ice from the pond.

We then threw these chunks out onto the pond ice and watched it shatter into tiny pieces.  The next cool thing we found is that some of the shattered pieces flew far distances across the pond ice.  Why?  The answer is conservation of momentum.  The large chunk of ice has some momentum when thrown.  Momentum must be conserved, disregarding friction, meaning the momentum of each tiny piece must add up to the momentum of the original big chunk.  Some of the shattered pieces moved back toward us while others barely moved at all.  To account for these zero and negative momentum pieces, there must be several pieces with large positive momentums.  These are the pieces propelled forward and travel great distances across the ice.

In the end we had a great time at the ponds.  I felt like a kid again as I cut out pieces of pond ice and threw them across the pond.  Neither of our daughters wanted to leave!  They were having a great time as well.  Fun times!