What is a statement of purpose?

What is a purpose statement and should I use one, or should I use an if-then statement?

• I am doing a lab report and the instructions tell me that "If the lab is one of finding a value then a PURPOSE statement should be used in place of a hypothesis". The lab was to determine/verify the optimum launch angle for a projectile to produce a maximum distance (obviously 45 degrees). We shot metal balls out of a spring loaded launcher and landed them on carbon paper, and measured the resulting distances. knowing this, should my hypothesis be an if/then statement, or a purpose statement, and if it is in fact a purpose statement, what should it look like?

• Answer:

  Purpose statement: The purpose of this experiment is to determine the optimum launch angle for a projectile to produce a maximum travel distance... Hypothesis: The angle of maximum travel distance should be 45 degrees... By the way, if you found 45 degrees to be that maximum angle you probably fudged the data or your measurement method was too imprecise...