Conservation of momentum in the English Premier League

So, I watched the Sunderland-Liverpool match in the English Premier League on Saturday morning.  (I like to write my student comments in front of sports on TV.)  Just a few minutes into the game, Sunderland scored a strange goal.  Watch -- there's a real physics purpose here...

[Edit:  Looks like the EPL removed this video from youtube.  I'm sure you can find it somewhere... it's fantastic.]

[To summarize:  A Sunderland striker executed a shot on goal from about 15 yards out.  The shot hit a red beach ball-type object and deflected at an angle; an easy save for the Liverpool goalkeeper turned into the deciding goal in the1-0 match.]

The commentators originally called the unusual red object on the pitch a "balloon."  I wrote some sports-related commentary on this event on my sports blog,  "Nachoman's Baseball."  But imagine the physics possibilities here...

The assignment, which I will use as an independent experiment in the spring in AP physics, is:  Determine the mass of the balloon / beach ball / whatever that caused the goal. 

GCJ