With no runners on base, the batter (B1) singles by hitting a line drive overtop the shortstop. The left fielder easily fields the ball on one hop and throws the ball to the shortstop. However, 15 feet after B1 rounds first base he collides with the first baseman (who was needlessly in the base path). B1 regains his feet and easily returns safely to first base. 

 

The offensive coach calls time and asks that obstruction be called on the first baseman and that B1 be awarded second base.

 

The defensive coach acknowledges the collision, but all agree that B1 was not attempting to stretch his single into a double, the defense was not making a play on B1, and B1 easily safely returned to first base -- so play on with no penalty.

 

What does the umpire do now?