First we have to decide where to conduct the experiments.  Imagine that you are on a merry-go-round.  You throw a ball to a friend who is on the opposite side of the merry-go-round (which is spinning at a constant rate).  How would an observer standing on the ground describe the ballís path?  How would you describe the path of ball?  If you throw it at your friend, do they catch it?
 

It does not travel in a straight line.  This is because the experiment's reference frame is accelerating. 
 
 

Or how about a pendulum bob that is carried while walking. 

This is different than the pendulum being held by a person while standing still.  Again, the acceleration of the frame seems to screw up the laws of physics.
 
 

Does the earth qualify as an inertial reference frame?