[opcon]

Yes, google will automatically pick a winner, as explained here in the faq (http://prizes.org/faq)

How does Voting work? You may vote on as many contests as you wish. However, you can only vote for one entry in a given contest each day, and you may not vote for yourself. You do have the option to vote for more entries in the same contest tomorrow! If buyers don't pick a winner when the contest ends, winners will be chosen by popular vote.