According topatryangolf.com, the average speed of a golf ball when it hits the ground is 47.5 mph. I don't know the formula to calculate the force that generates, but I can promise you that it hurts when it hits you on the side of the head.
Ask Chris Logan of West Chester, PA how it feels. On July 4th, 2010 Chris was standing in the rough of Aronimink Golf Club just outside Philadelphia watching the AT&T National golf tournament with other spectators when he was struck in the head by an errant golf shot hit by PGA golfer Sean O'Hair. Little did Chris know as he lay on the ground in pain, but that shot may have saved his life.